List of referenced programming language syntax designs
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Bash:
#!/usr/bin/env bash
# First line of the script is the shebang which tells the system how to execute
# the script: https://en.wikipedia.org/wiki/Shebang_(Unix)
# As you already figured, comments start with #. Shebang is also a comment.
# Simple hello world example:
echo "Hello world!" # => Hello world!
# Each command starts on a new line, or after a semicolon:
echo "This is the first command"; echo "This is the second command"
# => This is the first command
# => This is the second command
# Declaring a variable looks like this:
variable="Some string"
# But not like this:
variable = "Some string" # => returns error "variable: command not found"
# Bash will decide that `variable` is a command it must execute and give an error
# because it can't be found.
# Nor like this:
variable= "Some string" # => returns error: "Some string: command not found"
# Bash will decide that "Some string" is a command it must execute and give an
# error because it can't be found. In this case the "variable=" part is seen
# as a variable assignment valid only for the scope of the "Some string"
# command.
# Using the variable:
echo "$variable" # => Some string
echo '$variable' # => $variable
# When you use a variable itself — assign it, export it, or else — you write
# its name without $. If you want to use the variable's value, you should use $.
# Note that ' (single quote) won't expand variables!
# You can write variables without surrounding double quotes but it's not
# recommended due to how Bash handles variables with spaces in them.
# Parameter expansion ${...}:
echo "${variable}" # => Some string
# This is a simple usage of parameter expansion such as two examples above.
# Parameter expansion gets a value from a variable.
# It "expands" or prints the value.
# During the expansion time the value or parameter can be modified.
# Below are other modifications that add onto this expansion.
# String substitution in variables:
echo "${variable/Some/A}" # => A string
# This will substitute the first occurrence of "Some" with "A".
# Substring from a variable:
length=7
echo "${variable:0:length}" # => Some st
# This will return only the first 7 characters of the value
echo "${variable: -5}" # => tring
# This will return the last 5 characters (note the space before -5).
# The space before minus is mandatory here.
# String length:
echo "${#variable}" # => 11
# Indirect expansion:
other_variable="variable"
echo ${!other_variable} # => Some string
# This will expand the value of `other_variable`.
# The default value for variable:
echo "${foo:-"DefaultValueIfFooIsMissingOrEmpty"}"
# => DefaultValueIfFooIsMissingOrEmpty
# This works for null (foo=) and empty string (foo=""); zero (foo=0) returns 0.
# Note that it only returns default value and doesn't change variable value.
# Declare an array with 6 elements:
array=(one two three four five six)
# Print the first element:
echo "${array[0]}" # => "one"
# Print all elements:
echo "${array[@]}" # => "one two three four five six"
# Print the number of elements:
echo "${#array[@]}" # => "6"
# Print the number of characters in third element
echo "${#array[2]}" # => "5"
# Print 2 elements starting from fourth:
echo "${array[@]:3:2}" # => "four five"
# Print all elements each of them on new line.
for item in "${array[@]}"; do
echo "$item"
done
# Built-in variables:
# There are some useful built-in variables, like:
echo "Last program's return value: $?"
echo "Script's PID: $$"
echo "Number of arguments passed to script: $#"
echo "All arguments passed to script: $@"
echo "Script's arguments separated into different variables: $1 $2..."
# Brace Expansion {...}
# used to generate arbitrary strings:
echo {1..10} # => 1 2 3 4 5 6 7 8 9 10
echo {a..z} # => a b c d e f g h i j k l m n o p q r s t u v w x y z
# This will output the range from the start value to the end value.
# Note that you can't use variables here:
from=1
to=10
echo {$from..$to} # => {$from..$to}
# Now that we know how to echo and use variables,
# let's learn some of the other basics of Bash!
# Our current directory is available through the command `pwd`.
# `pwd` stands for "print working directory".
# We can also use the built-in variable `$PWD`.
# Observe that the following are equivalent:
echo "I'm in $(pwd)" # execs `pwd` and interpolates output
echo "I'm in $PWD" # interpolates the variable
# If you get too much output in your terminal, or from a script, the command
# `clear` clears your screen:
clear
# Ctrl-L also works for clearing output.
# Reading a value from input:
echo "What's your name?"
read name
# Note that we didn't need to declare a new variable.
echo "Hello, $name!"
# We have the usual if structure.
# Condition is true if the value of $name is not equal to the current user's login username:
if [[ "$name" != "$USER" ]]; then
echo "Your name isn't your username"
else
echo "Your name is your username"
fi
# To use && and || with if statements, you need multiple pairs of square brackets:
read age
if [[ "$name" == "Steve" ]] && [[ "$age" -eq 15 ]]; then
echo "This will run if $name is Steve AND $age is 15."
fi
if [[ "$name" == "Daniya" ]] || [[ "$name" == "Zach" ]]; then
echo "This will run if $name is Daniya OR Zach."
fi
# There are other comparison operators for numbers listed below:
# -ne - not equal
# -lt - less than
# -gt - greater than
# -le - less than or equal to
# -ge - greater than or equal to
# There is also the `=~` operator, which tests a string against the Regex pattern:
email=me@example.com
if [[ "$email" =~ [a-z]+@[a-z]{2,}\.(com|net|org) ]]
then
echo "Valid email!"
fi
# There is also conditional execution
echo "Always executed" || echo "Only executed if first command fails"
# => Always executed
echo "Always executed" && echo "Only executed if first command does NOT fail"
# => Always executed
# => Only executed if first command does NOT fail
# A single ampersand & after a command runs it in the background. A background command's
# output is printed to the terminal, but it cannot read from the input.
sleep 30 &
# List background jobs
jobs # => [1]+ Running sleep 30 &
# Bring the background job to the foreground
fg
# Ctrl-C to kill the process, or Ctrl-Z to pause it
# Resume a background process after it has been paused with Ctrl-Z
bg
# Kill job number 2
kill %2
# %1, %2, etc. can be used for fg and bg as well
# Redefine command `ping` as alias to send only 5 packets
alias ping='ping -c 5'
# Escape the alias and use command with this name instead
\ping 192.168.1.1
# Print all aliases
alias -p
# Expressions are denoted with the following format:
echo $(( 10 + 5 )) # => 15
# Unlike other programming languages, bash is a shell so it works in the context
# of a current directory. You can list files and directories in the current
# directory with the ls command:
ls # Lists the files and subdirectories contained in the current directory
# This command has options that control its execution:
ls -l # Lists every file and directory on a separate line
ls -t # Sorts the directory contents by last-modified date (descending)
ls -R # Recursively `ls` this directory and all of its subdirectories
# Results (stdout) of the previous command can be passed as input (stdin) to the next command
# using a pipe |. Commands chained in this way are called a "pipeline", and are run concurrently.
# The `grep` command filters the input with provided patterns.
# That's how we can list .txt files in the current directory:
ls -l | grep "\.txt"
# Use `cat` to print files to stdout:
cat file.txt
# We can also read the file using `cat`:
Contents=$(cat file.txt)
# "\n" prints a new line character
# "-e" to interpret the newline escape characters as escape characters
echo -e "START OF FILE\n$Contents\nEND OF FILE"
# => START OF FILE
# => [contents of file.txt]
# => END OF FILE
# Use `cp` to copy files or directories from one place to another.
# `cp` creates NEW versions of the sources,
# so editing the copy won't affect the original (and vice versa).
# Note that it will overwrite the destination if it already exists.
cp srcFile.txt clone.txt
cp -r srcDirectory/ dst/ # recursively copy
# Look into `scp` or `sftp` if you plan on exchanging files between computers.
# `scp` behaves very similarly to `cp`.
# `sftp` is more interactive.
# Use `mv` to move files or directories from one place to another.
# `mv` is similar to `cp`, but it deletes the source.
# `mv` is also useful for renaming files!
mv s0urc3.txt dst.txt # sorry, l33t hackers...
# Since bash works in the context of a current directory, you might want to
# run your command in some other directory. We have cd for changing location:
cd ~ # change to home directory
cd # also goes to home directory
cd .. # go up one directory
# (^^say, from /home/username/Downloads to /home/username)
cd /home/username/Documents # change to specified directory
cd ~/Documents/.. # now in home directory (if ~/Documents exists)
cd - # change to last directory
# => /home/username/Documents
# Use subshells to work across directories
(echo "First, I'm here: $PWD") && (cd someDir; echo "Then, I'm here: $PWD")
pwd # still in first directory
# Use `mkdir` to create new directories.
mkdir myNewDir
# The `-p` flag causes new intermediate directories to be created as necessary.
mkdir -p myNewDir/with/intermediate/directories
# if the intermediate directories didn't already exist, running the above
# command without the `-p` flag would return an error
# You can redirect command input and output (stdin, stdout, and stderr)
# using "redirection operators". Unlike a pipe, which passes output to a command,
# a redirection operator has a command's input come from a file or stream, or
# sends its output to a file or stream.
# Read from stdin until ^EOF$ and overwrite hello.py with the lines
# between "EOF" (which are called a "here document"):
cat > hello.py << EOF
#!/usr/bin/env python
from __future__ import print_function
import sys
print("#stdout", file=sys.stdout)
print("#stderr", file=sys.stderr)
for line in sys.stdin:
print(line, file=sys.stdout)
EOF
# Variables will be expanded if the first "EOF" is not quoted
# Run the hello.py Python script with various stdin, stdout, and
# stderr redirections:
python hello.py < "input.in" # pass input.in as input to the script
python hello.py > "output.out" # redirect output from the script to output.out
python hello.py 2> "error.err" # redirect error output to error.err
python hello.py > "output-and-error.log" 2>&1
# redirect both output and errors to output-and-error.log
# &1 means file descriptor 1 (stdout), so 2>&1 redirects stderr (2) to the current
# destination of stdout (1), which has been redirected to output-and-error.log.
python hello.py > /dev/null 2>&1
# redirect all output and errors to the black hole, /dev/null, i.e., no output
# The output error will overwrite the file if it exists,
# if you want to append instead, use ">>":
python hello.py >> "output.out" 2>> "error.err"
# Overwrite output.out, append to error.err, and count lines:
info bash 'Basic Shell Features' 'Redirections' > output.out 2>> error.err
wc -l output.out error.err
# Run a command and print its file descriptor (e.g. /dev/fd/123)
# see: man fd
echo <(echo "#helloworld")
# Overwrite output.out with "#helloworld":
cat > output.out <(echo "#helloworld")
echo "#helloworld" > output.out
echo "#helloworld" | cat > output.out
echo "#helloworld" | tee output.out >/dev/null
# Cleanup temporary files verbosely (add '-i' for interactive)
# WARNING: `rm` commands cannot be undone
rm -v output.out error.err output-and-error.log
rm -r tempDir/ # recursively delete
# You can install the `trash-cli` Python package to have `trash`
# which puts files in the system trash and doesn't delete them directly
# see https://pypi.org/project/trash-cli/ if you want to be careful
# Commands can be substituted within other commands using $( ):
# The following command displays the number of files and directories in the
# current directory.
echo "There are $(ls | wc -l) items here."
# The same can be done using backticks `` but they can't be nested -
# the preferred way is to use $( ).
echo "There are `ls | wc -l` items here."
# Bash uses a `case` statement that works similarly to switch in Java and C++:
case "$Variable" in
# List patterns for the conditions you want to meet
0) echo "There is a zero.";;
1) echo "There is a one.";;
*) echo "It is not null.";; # match everything
esac
# `for` loops iterate for as many arguments given:
# The contents of $Variable is printed three times.
for Variable in {1..3}
do
echo "$Variable"
done
# => 1
# => 2
# => 3
# Or write it the "traditional for loop" way:
for ((a=1; a <= 3; a++))
do
echo $a
done
# => 1
# => 2
# => 3
# They can also be used to act on files..
# This will run the command `cat` on file1 and file2
for Variable in file1 file2
do
cat "$Variable"
done
# ..or the output from a command
# This will `cat` the output from `ls`.
for Output in $(ls)
do
cat "$Output"
done
# Bash can also accept patterns, like this to `cat`
# all the Markdown files in current directory
for Output in ./*.markdown
do
cat "$Output"
done
# while loop:
while [ true ]
do
echo "loop body here..."
break
done
# => loop body here...
# You can also define functions
# Definition:
function foo ()
{
echo "Arguments work just like script arguments: $@"
echo "And: $1 $2..."
echo "This is a function"
returnValue=0 # Variable values can be returned
return $returnValue
}
# Call the function `foo` with two arguments, arg1 and arg2:
foo arg1 arg2
# => Arguments work just like script arguments: arg1 arg2
# => And: arg1 arg2...
# => This is a function
# Return values can be obtained with $?
resultValue=$?
# More than 9 arguments are also possible by using braces, e.g. ${10}, ${11}, ...
# or simply
bar ()
{
echo "Another way to declare functions!"
return 0
}
# Call the function `bar` with no arguments:
bar # => Another way to declare functions!
# Calling your function
foo "My name is" $Name
# There are a lot of useful commands you should learn:
# prints last 10 lines of file.txt
tail -n 10 file.txt
# prints first 10 lines of file.txt
head -n 10 file.txt
# print file.txt's lines in sorted order
sort file.txt
# report or omit repeated lines, with -d it reports them
uniq -d file.txt
# prints only the first column before the ',' character
cut -d ',' -f 1 file.txt
# replaces every occurrence of 'okay' with 'great' in file.txt
# (regex compatible)
sed -i 's/okay/great/g' file.txt
# be aware that this -i flag means that file.txt will be changed
# -i or --in-place erase the input file (use --in-place=.backup to keep a back-up)
# print to stdout all lines of file.txt which match some regex
# The example prints lines which begin with "foo" and end in "bar"
grep "^foo.*bar$" file.txt
# pass the option "-c" to instead print the number of lines matching the regex
grep -c "^foo.*bar$" file.txt
# Other useful options are:
grep -r "^foo.*bar$" someDir/ # recursively `grep`
grep -n "^foo.*bar$" file.txt # give line numbers
grep -rI "^foo.*bar$" someDir/ # recursively `grep`, but ignore binary files
# perform the same initial search, but filter out the lines containing "baz"
grep "^foo.*bar$" file.txt | grep -v "baz"
# if you literally want to search for the string,
# and not the regex, use `fgrep` (or `grep -F`)
fgrep "foobar" file.txt
# The `trap` command allows you to execute a command whenever your script
# receives a signal. Here, `trap` will execute `rm` if it receives any of the
# three listed signals.
trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM
# `sudo` is used to perform commands as the superuser
# usually it will ask interactively the password of superuser
NAME1=$(whoami)
NAME2=$(sudo whoami)
echo "Was $NAME1, then became more powerful $NAME2"
# Read Bash shell built-ins documentation with the bash `help` built-in:
help
help help
help for
help return
help source
help .
# Read Bash manpage documentation with `man`
apropos bash
man 1 bash
man bash
# Read info documentation with `info` (`?` for help)
apropos info | grep '^info.*('
man info
info info
info 5 info
# Read bash info documentation:
info bash
info bash 'Bash Features'
info bash 6
info --apropos bash
C:
// Single-line comments start with // - only available in C99 and later.
/*
Multi-line comments look like this. They work in C89 as well.
*/
/*
Multi-line comments don't nest /* Be careful */ // comment ends on this line...
*/ // ...not this one!
// Constants: #define <keyword>
// Constants are written in all-caps out of convention, not requirement
#define DAYS_IN_YEAR 365
// Enumeration constants are also ways to declare constants.
// All statements must end with a semicolon
enum days {SUN, MON, TUE, WED, THU, FRI, SAT};
// SUN gets 0, MON gets 1, TUE gets 2, etc.
// Enumeration values can also be specified
enum days {SUN = 1, MON, TUE, WED = 99, THU, FRI, SAT};
// MON gets 2 automatically, TUE gets 3, etc.
// WED get 99, THU gets 100, FRI gets 101, etc.
// Import headers with #include
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
// File names between <angle brackets> tell the compiler to look in your system
// libraries for the headers.
// For your own headers, use double quotes instead of angle brackets, and
// provide the path:
#include "my_header.h" // local file
#include "../my_lib/my_lib_header.h" //relative path
// Declare function signatures in advance in a .h file, or at the top of
// your .c file.
void function_1();
int function_2(void);
// At a minimum, you must declare a 'function prototype' before its use in any function.
// Normally, prototypes are placed at the top of a file before any function definition.
int add_two_ints(int x1, int x2); // function prototype
// although `int add_two_ints(int, int);` is also valid (no need to name the args),
// it is recommended to name arguments in the prototype as well for easier inspection
// Function prototypes are not necessary if the function definition comes before
// any other function that calls that function. However, it's standard practice to
// always add the function prototype to a header file (*.h) and then #include that
// file at the top. This prevents any issues where a function might be called
// before the compiler knows of its existence, while also giving the developer a
// clean header file to share with the rest of the project.
// Your program's entry point is a function called "main". The return type can
// be anything, however most operating systems expect a return type of `int` for
// error code processing.
int main(void) {
// your program
}
// The command line arguments used to run your program are also passed to main
// argc being the number of arguments - your program's name counts as 1
// argv is an array of character arrays - containing the arguments themselves
// argv[0] = name of your program, argv[1] = first argument, etc.
int main (int argc, char** argv)
{
// print output using printf, for "print formatted"
// %d is an integer, \n is a newline
printf("%d\n", 0); // => Prints 0
// take input using scanf
// '&' is used to define the location
// where we want to store the input value
int input;
scanf("%d", &input);
///////////////////////////////////////
// Types
///////////////////////////////////////
// Compilers that are not C99-compliant require that variables MUST be
// declared at the top of the current block scope.
// Compilers that ARE C99-compliant allow declarations near the point where
// the value is used.
// For the sake of the tutorial, variables are declared dynamically under
// C99-compliant standards.
// ints are usually 4 bytes (use the `sizeof` operator to check)
int x_int = 0;
// shorts are usually 2 bytes (use the `sizeof` operator to check)
short x_short = 0;
// chars are defined as the smallest addressable unit for a processor.
// This is usually 1 byte, but for some systems it can be more (ex. for TMS320 from TI it's 2 bytes).
char x_char = 0;
char y_char = 'y'; // Char literals are quoted with ''
// longs are often 4 to 8 bytes; long longs are guaranteed to be at least
// 8 bytes
long x_long = 0;
long long x_long_long = 0;
// floats are usually 32-bit floating point numbers
float x_float = 0.0f; // 'f' suffix here denotes floating point literal
// doubles are usually 64-bit floating-point numbers
double x_double = 0.0; // real numbers without any suffix are doubles
// integer types may be unsigned (greater than or equal to zero)
unsigned short ux_short;
unsigned int ux_int;
unsigned long long ux_long_long;
// chars inside single quotes are integers in machine's character set.
'0'; // => 48 in the ASCII character set.
'A'; // => 65 in the ASCII character set.
// sizeof(T) gives you the size of a variable with type T in bytes
// sizeof(obj) yields the size of the expression (variable, literal, etc.).
printf("%zu\n", sizeof(int)); // => 4 (on most machines with 4-byte words)
// If the argument of the `sizeof` operator is an expression, then its argument
// is not evaluated (except VLAs (see below)).
// The value it yields in this case is a compile-time constant.
int a = 1;
// size_t is an unsigned integer type of at least 2 bytes used to represent
// the size of an object.
size_t size = sizeof(a++); // a++ is not evaluated
printf("sizeof(a++) = %zu where a = %d\n", size, a);
// prints "sizeof(a++) = 4 where a = 1" (on a 32-bit architecture)
// Arrays must be initialized with a concrete size.
char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes
// (assuming 4-byte words)
// You can initialize an array of twenty ints that all equal 0 thusly:
int my_array[20] = {0};
// where the "{0}" part is called an "array initializer".
// All elements (if any) past the ones in the initializer are initialized to 0:
int my_array[5] = {1, 2};
// So my_array now has five elements, all but the first two of which are 0:
// [1, 2, 0, 0, 0]
// NOTE that you get away without explicitly declaring the size
// of the array IF you initialize the array on the same line:
int my_array[] = {0};
// NOTE that, when not declaring the size, the size of the array is the number
// of elements in the initializer. With "{0}", my_array is now of size one: [0]
// To evaluate the size of the array at run-time, divide its byte size by the
// byte size of its element type:
size_t my_array_size = sizeof(my_array) / sizeof(my_array[0]);
// WARNING You should evaluate the size *before* you begin passing the array
// to functions (see later discussion) because arrays get "downgraded" to
// raw pointers when they are passed to functions (so the statement above
// will produce the wrong result inside the function).
// Indexing an array is like other languages -- or,
// rather, other languages are like C
my_array[0]; // => 0
// Arrays are mutable; it's just memory!
my_array[1] = 2;
printf("%d\n", my_array[1]); // => 2
// In C99 (and as an optional feature in C11), variable-length arrays (VLAs)
// can be declared as well. The size of such an array need not be a compile
// time constant:
printf("Enter the array size: "); // ask the user for an array size
int array_size;
fscanf(stdin, "%d", &array_size);
int var_length_array[array_size]; // declare the VLA
printf("sizeof array = %zu\n", sizeof var_length_array);
// Example:
// > Enter the array size: 10
// > sizeof array = 40
// Strings are just arrays of chars terminated by a NULL (0x00) byte,
// represented in strings as the special character '\0'.
// (We don't have to include the NULL byte in string literals; the compiler
// inserts it at the end of the array for us.)
char a_string[20] = "This is a string";
printf("%s\n", a_string); // %s formats a string
printf("%d\n", a_string[16]); // => 0
// i.e., byte #17 is 0 (as are 18, 19, and 20)
// If we have characters between single quotes, that's a character literal.
// It's of type `int`, and *not* `char` (for historical reasons).
int cha = 'a'; // fine
char chb = 'a'; // fine too (implicit conversion from int to char)
// Multi-dimensional arrays:
int multi_array[2][5] = {
{1, 2, 3, 4, 5},
{6, 7, 8, 9, 0}
};
// access elements:
int array_int = multi_array[0][2]; // => 3
///////////////////////////////////////
// Operators
///////////////////////////////////////
// Shorthands for multiple declarations:
int i1 = 1, i2 = 2;
float f1 = 1.0, f2 = 2.0;
int b, c;
b = c = 0;
// Arithmetic is straightforward
i1 + i2; // => 3
i2 - i1; // => 1
i2 * i1; // => 2
i1 / i2; // => 0 (0.5, but truncated towards 0)
// You need to cast at least one integer to float to get a floating-point result
(float)i1 / i2; // => 0.5f
i1 / (double)i2; // => 0.5 // Same with double
f1 / f2; // => 0.5, plus or minus epsilon
// Floating-point numbers are defined by IEEE 754, thus cannot store perfectly
// exact values. For instance, the following does not produce expected results
// because 0.1 might actually be 0.099999999999 inside the computer, and 0.3
// might be stored as 0.300000000001.
(0.1 + 0.1 + 0.1) != 0.3; // => 1 (true)
// and it is NOT associative due to reasons mentioned above.
1 + (1e123 - 1e123) != (1 + 1e123) - 1e123; // => 1 (true)
// this notation is scientific notations for numbers: 1e123 = 1*10^123
// It is important to note that most all systems have used IEEE 754 to
// represent floating points. Even python, used for scientific computing,
// eventually calls C which uses IEEE 754. It is mentioned this way not to
// indicate that this is a poor implementation, but instead as a warning
// that when doing floating point comparisons, a little bit of error (epsilon)
// needs to be considered.
// Modulo is there as well, but be careful if arguments are negative
11 % 3; // => 2 as 11 = 2 + 3*x (x=3)
(-11) % 3; // => -2, as one would expect
11 % (-3); // => 2 and not -2, and it's quite counter intuitive
// Comparison operators are probably familiar, but
// there is no Boolean type in C. We use ints instead.
// (C99 introduced the _Bool type provided in stdbool.h)
// 0 is false, anything else is true. (The comparison
// operators always yield 0 or 1.)
3 == 2; // => 0 (false)
3 != 2; // => 1 (true)
3 > 2; // => 1
3 < 2; // => 0
2 <= 2; // => 1
2 >= 2; // => 1
// C is not Python - comparisons do NOT chain.
// Warning: The line below will compile, but it means `(0 < a) < 2`.
// This expression is always true, because (0 < a) could be either 1 or 0.
// In this case it's 1, because (0 < 1).
int between_0_and_2 = 0 < a < 2;
// Instead use:
int between_0_and_2 = 0 < a && a < 2;
// Logic works on ints
!3; // => 0 (Logical not)
!0; // => 1
1 && 1; // => 1 (Logical and)
0 && 1; // => 0
0 || 1; // => 1 (Logical or)
0 || 0; // => 0
// Conditional ternary expression ( ? : )
int e = 5;
int f = 10;
int z;
z = (e > f) ? e : f; // => 10 "if e > f return e, else return f."
// Increment and decrement operators:
int j = 0;
int s = j++; // Return j THEN increase j. (s = 0, j = 1)
s = ++j; // Increase j THEN return j. (s = 2, j = 2)
// same with j-- and --j
// Bitwise operators!
~0x0F; // => 0xFFFFFFF0 (bitwise negation, "1's complement", example result for 32-bit int)
0x0F & 0xF0; // => 0x00 (bitwise AND)
0x0F | 0xF0; // => 0xFF (bitwise OR)
0x04 ^ 0x0F; // => 0x0B (bitwise XOR)
0x01 << 1; // => 0x02 (bitwise left shift (by 1))
0x02 >> 1; // => 0x01 (bitwise right shift (by 1))
// Be careful when shifting signed integers - the following are undefined:
// - shifting into the sign bit of a signed integer (int a = 1 << 31)
// - left-shifting a negative number (int a = -1 << 2)
// - shifting by an offset which is >= the width of the type of the LHS:
// int a = 1 << 32; // UB if int is 32 bits wide
///////////////////////////////////////
// Control Structures
///////////////////////////////////////
if (0) {
printf("I am never run\n");
} else if (0) {
printf("I am also never run\n");
} else {
printf("I print\n");
}
// While loops exist
int ii = 0;
while (ii < 10) { //ANY value less than ten is true.
printf("%d, ", ii++); // ii++ increments ii AFTER using its current value.
} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
printf("\n");
int kk = 0;
do {
printf("%d, ", kk);
} while (++kk < 10); // ++kk increments kk BEFORE using its current value.
// => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
printf("\n");
// For loops too
int jj;
for (jj=0; jj < 10; jj++) {
printf("%d, ", jj);
} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
printf("\n");
// *****NOTES*****:
// Loops and Functions MUST have a body. If no body is needed:
int i;
for (i = 0; i <= 5; i++) {
; // use semicolon to act as the body (null statement)
}
// Or
for (i = 0; i <= 5; i++);
// branching with multiple choices: switch()
switch (a) {
case 0: // labels need to be integral *constant* expressions (such as enums)
printf("Hey, 'a' equals 0!\n");
break; // if you don't break, control flow falls over labels
case 1:
printf("Huh, 'a' equals 1!\n");
break;
// Be careful - without a "break", execution continues until the
// next "break" is reached.
case 3:
case 4:
printf("Look at that.. 'a' is either 3, or 4\n");
break;
default:
// if `some_integral_expression` didn't match any of the labels
fputs("Error!\n", stderr);
exit(-1);
break;
}
/*
Using "goto" in C
*/
typedef enum { false, true } bool;
// for C don't have bool as data type before C99 :(
bool disaster = false;
int i, j;
for(i=0; i<100; ++i)
for(j=0; j<100; ++j)
{
if((i + j) >= 150)
disaster = true;
if(disaster)
goto error; // exit both for loops
}
error: // this is a label that you can "jump" to with "goto error;"
printf("Error occurred at i = %d & j = %d.\n", i, j);
/*
https://ideone.com/GuPhd6
this will print out "Error occurred at i = 51 & j = 99."
*/
/*
it is generally considered bad practice to do so, except if
you really know what you are doing. See
https://en.wikipedia.org/wiki/Spaghetti_code#Meaning
*/
///////////////////////////////////////
// Typecasting
///////////////////////////////////////
// Every value in C has a type, but you can cast one value into another type
// if you want (with some constraints).
int x_hex = 0x01; // You can assign vars with hex literals
// binary is not in the standard, but allowed by some
// compilers (x_bin = 0b0010010110)
// Casting between types will attempt to preserve their numeric values
printf("%d\n", x_hex); // => Prints 1
printf("%d\n", (short) x_hex); // => Prints 1
printf("%d\n", (char) x_hex); // => Prints 1
// If you assign a value greater than a types max val, it will rollover
// without warning.
printf("%d\n", (unsigned char) 257); // => 1 (Max char = 255 if char is 8 bits long)
// For determining the max value of a `char`, a `signed char` and an `unsigned char`,
// respectively, use the CHAR_MAX, SCHAR_MAX and UCHAR_MAX macros from <limits.h>
// Integral types can be cast to floating-point types, and vice-versa.
printf("%f\n", (double) 100); // %f always formats a double...
printf("%f\n", (float) 100); // ...even with a float.
printf("%d\n", (char)100.0);
///////////////////////////////////////
// Pointers
///////////////////////////////////////
// A pointer is a variable declared to store a memory address. Its declaration will
// also tell you the type of data it points to. You can retrieve the memory address
// of your variables, then mess with them.
int x = 0;
printf("%p\n", (void *)&x); // Use & to retrieve the address of a variable
// (%p formats an object pointer of type void *)
// => Prints some address in memory;
// Pointers start with * in their declaration
int *px, not_a_pointer; // px is a pointer to an int
px = &x; // Stores the address of x in px
printf("%p\n", (void *)px); // => Prints some address in memory
printf("%zu, %zu\n", sizeof(px), sizeof(not_a_pointer));
// => Prints "8, 4" on a typical 64-bit system
// To retrieve the value at the address a pointer is pointing to,
// put * in front to dereference it.
// Note: yes, it may be confusing that '*' is used for _both_ declaring a
// pointer and dereferencing it.
printf("%d\n", *px); // => Prints 0, the value of x
// You can also change the value the pointer is pointing to.
// We'll have to wrap the dereference in parenthesis because
// ++ has a higher precedence than *.
(*px)++; // Increment the value px is pointing to by 1
printf("%d\n", *px); // => Prints 1
printf("%d\n", x); // => Prints 1
// Arrays are a good way to allocate a contiguous block of memory
int x_array[20]; //declares array of size 20 (cannot change size)
int xx;
for (xx = 0; xx < 20; xx++) {
x_array[xx] = 20 - xx;
} // Initialize x_array to 20, 19, 18,... 2, 1
// Declare a pointer of type int and initialize it to point to x_array
int* x_ptr = x_array;
// x_ptr now points to the first element in the array (the integer 20).
// This works because arrays often decay into pointers to their first element.
// For example, when an array is passed to a function or is assigned to a pointer,
// it decays into (implicitly converted to) a pointer.
// Exceptions: when the array is the argument of the `&` (address-of) operator:
int arr[10];
int (*ptr_to_arr)[10] = &arr; // &arr is NOT of type `int *`!
// It's of type "pointer to array" (of ten `int`s).
// or when the array is a string literal used for initializing a char array:
char otherarr[] = "foobarbazquirk";
// or when it's the argument of the `sizeof` or `alignof` operator:
int arraythethird[10];
int *ptr = arraythethird; // equivalent with int *ptr = &arr[0];
printf("%zu, %zu\n", sizeof(arraythethird), sizeof(ptr));
// probably prints "40, 4" or "40, 8"
// Pointers are incremented and decremented based on their type
// (this is called pointer arithmetic)
printf("%d\n", *(x_ptr + 1)); // => Prints 19
printf("%d\n", x_array[1]); // => Prints 19
// You can also dynamically allocate contiguous blocks of memory with the
// standard library function malloc, which takes one argument of type size_t
// representing the number of bytes to allocate (usually from the heap, although this
// may not be true on e.g. embedded systems - the C standard says nothing about it).
int *my_ptr = malloc(sizeof(*my_ptr) * 20);
for (xx = 0; xx < 20; xx++) {
*(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx
} // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
// Be careful passing user-provided values to malloc! If you want
// to be safe, you can use calloc instead (which, unlike malloc, also zeros out the memory)
int* my_other_ptr = calloc(20, sizeof(int));
// Note that there is no standard way to get the length of a
// dynamically allocated array in C. Because of this, if your arrays are
// going to be passed around your program a lot, you need another variable
// to keep track of the number of elements (size) of an array. See the
// functions section for more info.
size_t size = 10;
int *my_arr = calloc(size, sizeof(int));
// Add an element to the array
size++;
my_arr = realloc(my_arr, sizeof(int) * size);
if (my_arr == NULL) {
//Remember to check for realloc failure!
return
}
my_arr[10] = 5;
// Dereferencing memory that you haven't allocated gives
// "unpredictable results" - the program is said to invoke "undefined behavior"
printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what? It may even crash.
// When you're done with a malloc'd block of memory, you need to free it,
// or else no one else can use it until your program terminates
// (this is called a "memory leak"):
free(my_ptr);
// Strings are arrays of char, but they are usually represented as a
// pointer-to-char (which is a pointer to the first element of the array).
// It's good practice to use `const char *' when referring to a string literal,
// since string literals shall not be modified (i.e. "foo"[0] = 'a' is ILLEGAL.)
const char *my_str = "This is my very own string literal";
printf("%c\n", *my_str); // => 'T'
// This is not the case if the string is an array
// (potentially initialized with a string literal)
// that resides in writable memory, as in:
char foo[] = "foo";
foo[0] = 'a'; // this is legal, foo now contains "aoo"
function_1();
} // end main function
///////////////////////////////////////
// Functions
///////////////////////////////////////
// Function declaration syntax:
// <return type> <function name>(<args>)
int add_two_ints(int x1, int x2)
{
return x1 + x2; // Use return to return a value
}
/*
Functions are call by value. When a function is called, the arguments passed to
the function are copies of the original arguments (except arrays). Anything you
do to the arguments in the function do not change the value of the original
argument where the function was called.
Use pointers if you need to edit the original argument values (arrays are always
passed in as pointers).
Example: in-place string reversal
*/
// A void function returns no value
void str_reverse(char *str_in)
{
char tmp;
size_t ii = 0;
size_t len = strlen(str_in); // `strlen()` is part of the c standard library
// NOTE: length returned by `strlen` DOESN'T
// include the terminating NULL byte ('\0')
// in C99 and newer versions, you can directly declare loop control variables
// in the loop's parentheses. e.g., `for (size_t ii = 0; ...`
for (ii = 0; ii < len / 2; ii++) {
tmp = str_in[ii];
str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
str_in[len - ii - 1] = tmp;
}
}
//NOTE: string.h header file needs to be included to use strlen()
/*
char c[] = "This is a test.";
str_reverse(c);
printf("%s\n", c); // => ".tset a si sihT"
*/
/*
as we can return only one variable
to change values of more than one variables we use call by reference
*/
void swapTwoNumbers(int *a, int *b)
{
int temp = *a;
*a = *b;
*b = temp;
}
/*
int first = 10;
int second = 20;
printf("first: %d\nsecond: %d\n", first, second);
swapTwoNumbers(&first, &second);
printf("first: %d\nsecond: %d\n", first, second);
// values will be swapped
*/
// Return multiple values.
// C does not allow for returning multiple values with the return statement. If
// you would like to return multiple values, then the caller must pass in the
// variables where they would like the returned values to go. These variables must
// be passed in as pointers such that the function can modify them.
int return_multiple( int *array_of_3, int *ret1, int *ret2, int *ret3)
{
if(array_of_3 == NULL)
return 0; //return error code (false)
//de-reference the pointer so we modify its value
*ret1 = array_of_3[0];
*ret2 = array_of_3[1];
*ret3 = array_of_3[2];
return 1; //return error code (true)
}
/*
With regards to arrays, they will always be passed to functions
as pointers. Even if you statically allocate an array like `arr[10]`,
it still gets passed as a pointer to the first element in any function calls.
Again, there is no standard way to get the size of a dynamically allocated
array in C.
*/
// Size must be passed!
// Otherwise, this function has no way of knowing how big the array is.
void printIntArray(int *arr, size_t size) {
int i;
for (i = 0; i < size; i++) {
printf("arr[%d] is: %d\n", i, arr[i]);
}
}
/*
int my_arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int size = 10;
printIntArray(my_arr, size);
// will print "arr[0] is: 1" etc
*/
// if referring to external variables outside function, you should use the extern keyword.
int i = 0;
void testFunc() {
extern int i; //i here is now using external variable i
}
// make external variables private to source file with static:
static int j = 0; //other files using testFunc2() cannot access variable j
void testFunc2() {
extern int j;
}
// The static keyword makes a variable inaccessible to code outside the
// compilation unit. (On almost all systems, a "compilation unit" is a .c
// file.) static can apply both to global (to the compilation unit) variables,
// functions, and function-local variables. When using static with
// function-local variables, the variable is effectively global and retains its
// value across function calls, but is only accessible within the function it
// is declared in. Additionally, static variables are initialized to 0 if not
// declared with some other starting value.
//**You may also declare functions as static to make them private**
///////////////////////////////////////
// User-defined types and structs
///////////////////////////////////////
// Typedefs can be used to create type aliases
typedef int my_type;
my_type my_type_var = 0;
// Structs are just collections of data, the members are allocated sequentially,
// in the order they are written:
struct rectangle {
int width;
int height;
};
// It's not generally true that
// sizeof(struct rectangle) == sizeof(int) + sizeof(int)
// due to potential padding between the structure members (this is for alignment
// reasons). [1]
void function_1()
{
struct rectangle my_rec = { 1, 2 }; // Fields can be initialized immediately
// Access struct members with .
my_rec.width = 10;
my_rec.height = 20;
// You can declare pointers to structs
struct rectangle *my_rec_ptr = &my_rec;
// Use dereferencing to set struct pointer members...
(*my_rec_ptr).width = 30;
// ... or even better: prefer the -> shorthand for the sake of readability
my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
}
// You can apply a typedef to a struct for convenience
typedef struct rectangle rect;
int area(rect r)
{
return r.width * r.height;
}
// Typedefs can also be defined right during struct definition
typedef struct {
int width;
int height;
} rect;
// Like before, doing this means one can type
rect r;
// instead of having to type
struct rectangle r;
// if you have large structs, you can pass them "by pointer" to avoid copying
// the whole struct:
int areaptr(const rect *r)
{
return r->width * r->height;
}
///////////////////////////////////////
// Function pointers
///////////////////////////////////////
/*
At run time, functions are located at known memory addresses. Function pointers are
much like any other pointer (they just store a memory address), but can be used
to invoke functions directly, and to pass handlers (or callback functions) around.
However, definition syntax may be initially confusing.
Example: use str_reverse from a pointer
*/
void str_reverse_through_pointer(char *str_in) {
// Define a function pointer variable, named f.
void (*f)(char *); // Signature should exactly match the target function.
f = &str_reverse; // Assign the address for the actual function (determined at run time)
// f = str_reverse; would work as well - functions decay into pointers, similar to arrays
(*f)(str_in); // Just calling the function through the pointer
// f(str_in); // That's an alternative but equally valid syntax for calling it.
}
/*
As long as function signatures match, you can assign any function to the same pointer.
Function pointers are usually typedef'd for simplicity and readability, as follows:
*/
typedef void (*my_fnp_type)(char *);
// Then used when declaring the actual pointer variable:
// ...
// my_fnp_type f;
/////////////////////////////
// Printing characters with printf()
/////////////////////////////
//Special characters:
/*
'\a'; // alert (bell) character
'\n'; // newline character
'\t'; // tab character (left justifies text)
'\v'; // vertical tab
'\f'; // new page (form feed)
'\r'; // carriage return
'\b'; // backspace character
'\0'; // NULL character. Usually put at end of strings in C.
// hello\n\0. \0 used by convention to mark end of string.
'\\'; // backslash
'\?'; // question mark
'\''; // single quote
'\"'; // double quote
'\xhh'; // hexadecimal number. Example: '\xb' = vertical tab character
'\0oo'; // octal number. Example: '\013' = vertical tab character
//print formatting:
"%d"; // integer
"%3d"; // integer with minimum of length 3 digits (right justifies text)
"%s"; // string
"%f"; // float
"%ld"; // long
"%3.2f"; // minimum 3 digits left and 2 digits right decimal float
"%7.4s"; // (can do with strings too)
"%c"; // char
"%p"; // pointer. NOTE: need to (void *)-cast the pointer, before passing
// it as an argument to `printf`.
"%x"; // hexadecimal
"%o"; // octal
"%%"; // prints %
*/
///////////////////////////////////////
// Order of Evaluation
///////////////////////////////////////
// From top to bottom, top has higher precedence
//---------------------------------------------------//
// Operators | Associativity //
//---------------------------------------------------//
// () [] -> . | left to right //
// ! ~ ++ -- + = *(type) sizeof | right to left //
// * / % | left to right //
// + - | left to right //
// << >> | left to right //
// < <= > >= | left to right //
// == != | left to right //
// & | left to right //
// ^ | left to right //
// | | left to right //
// && | left to right //
// || | left to right //
// ?: | right to left //
// = += -= *= /= %= &= ^= |= <<= >>= | right to left //
// , | left to right //
//---------------------------------------------------//
/******************************* Header Files **********************************
Header files are an important part of C as they allow for the connection of C
source files and can simplify code and definitions by separating them into
separate files.
Header files are syntactically similar to C source files but reside in ".h"
files. They can be included in your C source file by using the preprocessor
directive #include "example.h", given that example.h exists in the same directory
as the C file.
*/
/* A safe guard to prevent the header from being defined too many times. This */
/* happens in the case of circle dependency, the contents of the header is */
/* already defined. */
#ifndef EXAMPLE_H /* if EXAMPLE_H is not yet defined. */
#define EXAMPLE_H /* Define the macro EXAMPLE_H. */
/* Other headers can be included in headers and therefore transitively */
/* included into files that include this header. */
#include <string.h>
/* Like for c source files, macros can be defined in headers */
/* and used in files that include this header file. */
#define EXAMPLE_NAME "Dennis Ritchie"
/* Function macros can also be defined. */
#define ADD(a, b) ((a) + (b))
/* Notice the parenthesis surrounding the arguments -- this is important to */
/* ensure that a and b don't get expanded in an unexpected way (e.g. consider */
/* MUL(x, y) (x * y); MUL(1 + 2, 3) would expand to (1 + 2 * 3), yielding an */
/* incorrect result) */
/* Structs and typedefs can be used for consistency between files. */
typedef struct Node
{
int val;
struct Node *next;
} Node;
/* So can enumerations. */
enum traffic_light_state {GREEN, YELLOW, RED};
/* Function prototypes can also be defined here for use in multiple files, */
/* but it is bad practice to define the function in the header. Definitions */
/* should instead be put in a C file. */
Node createLinkedList(int *vals, int len);
/* Beyond the above elements, other definitions should be left to a C source */
/* file. Excessive includes or definitions should also not be contained in */
/* a header file but instead put into separate headers or a C file. */
#endif /* End of the if preprocessor directive. */
C#:
// Single-line comments start with //
/*
Multi-line comments look like this
*/
/// <summary>
/// This is an XML documentation comment which can be used to generate external
/// documentation or provide context help within an IDE
/// </summary>
/// <param name="firstParam">This is some parameter documentation for firstParam</param>
/// <returns>Information on the returned value of a function</returns>
public void MethodOrClassOrOtherWithParsableHelp(string firstParam) { }
// Specify the namespaces this source code will be using
// The namespaces below are all part of the standard .NET Framework Class Library
using System;
using System.Collections.Generic;
using System.Dynamic;
using System.Linq;
using System.Net;
using System.Threading.Tasks;
using System.IO;
// But this one is not:
using System.Data.Entity;
// In order to be able to use it, you need to add a dll reference
// This can be done with the NuGet package manager: `Install-Package EntityFramework`
// Namespaces define scope to organize code into "packages" or "modules"
// Using this code from another source file: using Learning.CSharp;
// You can also do this in C# 10, it is called file-scoped namespaces.
// namespace Learning.CSharp;
namespace Learning.CSharp
{
// Each .cs file should at least contain a class with the same name as the file.
// You're allowed to do otherwise, but shouldn't for sanity.
public class LearnCSharp
{
// BASIC SYNTAX - skip to INTERESTING FEATURES if you have used Java or C++ before
public static void Syntax()
{
// Use Console.WriteLine to print lines
Console.WriteLine("Hello World");
Console.WriteLine(
"Integer: " + 10 +
" Double: " + 3.14 +
" Boolean: " + true);
// To print without a new line, use Console.Write
Console.Write("Hello ");
Console.Write("World");
///////////////////////////////////////////////////
// Types & Variables
//
// Declare a variable using <type> <name>
///////////////////////////////////////////////////
// Sbyte - Signed 8-bit integer
// (-128 <= sbyte <= 127)
sbyte fooSbyte = 100;
// Byte - Unsigned 8-bit integer
// (0 <= byte <= 255)
byte fooByte = 100;
// Short - 16-bit integer
// Signed - (-32,768 <= short <= 32,767)
// Unsigned - (0 <= ushort <= 65,535)
short fooShort = 10000;
ushort fooUshort = 10000;
// Integer - 32-bit integer
int fooInt = 1; // (-2,147,483,648 <= int <= 2,147,483,647)
uint fooUint = 1; // (0 <= uint <= 4,294,967,295)
// Long - 64-bit integer
long fooLong = 100000L; // (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
ulong fooUlong = 100000L; // (0 <= ulong <= 18,446,744,073,709,551,615)
// Numbers default to being int or uint depending on size.
// L is used to denote that this variable value is of type long or ulong
// Double - Double-precision 64-bit IEEE 754 Floating Point
double fooDouble = 123.4; // Precision: 15-16 digits
// Float - Single-precision 32-bit IEEE 754 Floating Point
float fooFloat = 234.5f; // Precision: 7 digits
// f is used to denote that this variable value is of type float
// Decimal - a 128-bits data type, with more precision than other floating-point types,
// suited for financial and monetary calculations
decimal fooDecimal = 150.3m;
// Boolean - true & false
bool fooBoolean = true; // or false
// Char - A single 16-bit Unicode character
char fooChar = 'A';
// Strings -- unlike the previous base types which are all value types,
// a string is a reference type. That is, you can set it to null
string fooString = "\"escape\" quotes and add \n (new lines) and \t (tabs)";
Console.WriteLine(fooString);
// You can access each character of the string with an indexer:
char charFromString = fooString[1]; // => 'e'
// Strings are immutable: you can't do fooString[1] = 'X';
// Compare strings with current culture, ignoring case
string.Compare(fooString, "x", StringComparison.CurrentCultureIgnoreCase);
// Formatting, based on sprintf
string fooFs = string.Format("Check Check, {0} {1}, {0} {1:0.0}", 1, 2);
// Dates & Formatting
DateTime fooDate = DateTime.Now;
Console.WriteLine(fooDate.ToString("hh:mm, dd MMM yyyy"));
// Verbatim String
// You can use the @ symbol before a string literal to escape all characters in the string
string path = "C:\\Users\\User\\Desktop";
string verbatimPath = @"C:\Users\User\Desktop";
Console.WriteLine(path == verbatimPath); // => true
// You can split a string over two lines with the @ symbol. To escape " use ""
string bazString = @"Here's some stuff
on a new line! ""Wow!"", the masses cried";
// Use const or read-only to make a variable immutable
// const values are calculated at compile time
const int HoursWorkPerWeek = 9001;
///////////////////////////////////////////////////
// Data Structures
///////////////////////////////////////////////////
// Arrays - zero indexed
// The array size must be decided upon declaration
// The format for declaring an array is
// <datatype>[] <var name> = new <datatype>[<array size>];
int[] intArray = new int[10];
// Another way to declare & initialize an array
int[] y = { 9000, 1000, 1337 };
// Indexing an array - Accessing an element
Console.WriteLine("intArray @ 0: " + intArray[0]);
// Arrays are mutable.
intArray[1] = 1;
// Lists
// Lists are used more frequently than arrays as they are more flexible
// The format for declaring a list is
// List<datatype> <var name> = new List<datatype>();
List<int> intList = new List<int>();
List<string> stringList = new List<string>();
List<int> z = new List<int> { 9000, 1000, 1337 }; // initialize
// The <> are for generics - Check out the cool stuff section
// Lists don't default to a value;
// A value must be added before accessing the index
intList.Add(1);
Console.WriteLine("intList at 0: " + intList[0]);
// Other data structures to check out:
// Stack/Queue
// Dictionary (an implementation of a hash map)
// HashSet
// Read-only Collections
// Tuple (.NET 4+)
///////////////////////////////////////
// Operators
///////////////////////////////////////
Console.WriteLine("\n->Operators");
int i1 = 1, i2 = 2; // Shorthand for multiple declarations
// Arithmetic is straightforward
Console.WriteLine(i1 + i2 - i1 * 3 / 7); // => 3
// Modulo
Console.WriteLine("11%3 = " + (11 % 3)); // => 2
// Comparison operators
Console.WriteLine("3 == 2? " + (3 == 2)); // => false
Console.WriteLine("3 != 2? " + (3 != 2)); // => true
Console.WriteLine("3 > 2? " + (3 > 2)); // => true
Console.WriteLine("3 < 2? " + (3 < 2)); // => false
Console.WriteLine("2 <= 2? " + (2 <= 2)); // => true
Console.WriteLine("2 >= 2? " + (2 >= 2)); // => true
// Bitwise operators!
/*
~ Unary bitwise complement
<< Signed left shift
>> Signed right shift
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise inclusive OR
*/
// Incrementing
int i = 0;
Console.WriteLine("\n->Inc/Dec-rement");
Console.WriteLine(i++); //Prints "0", i = 1. Post-Increment
Console.WriteLine(++i); //Prints "2", i = 2. Pre-Increment
Console.WriteLine(i--); //Prints "2", i = 1. Post-Decrement
Console.WriteLine(--i); //Prints "0", i = 0. Pre-Decrement
///////////////////////////////////////
// Control Structures
///////////////////////////////////////
Console.WriteLine("\n->Control Structures");
// If statements are C-like
int j = 10;
if (j == 10)
{
Console.WriteLine("I get printed");
}
else if (j > 10)
{
Console.WriteLine("I don't");
}
else
{
Console.WriteLine("I also don't");
}
// Ternary operators
// A simple if/else can be written as follows
// <condition> ? <true> : <false>
int toCompare = 17;
string isTrue = toCompare == 17 ? "True" : "False";
// While loop
int fooWhile = 0;
while (fooWhile < 100)
{
// Iterated 100 times, fooWhile 0->99
fooWhile++;
}
// Do While Loop
int fooDoWhile = 0;
do
{
// Start iteration 100 times, fooDoWhile 0->99
if (false)
continue; // skip the current iteration
fooDoWhile++;
if (fooDoWhile == 50)
break; // breaks from the loop completely
} while (fooDoWhile < 100);
// for loop structure => for(<start_statement>; <conditional>; <step>)
for (int fooFor = 0; fooFor < 10; fooFor++)
{
// Iterated 10 times, fooFor 0->9
}
// For Each Loop
// foreach loop structure => foreach(<iteratorType> <iteratorName> in <enumerable>)
// The foreach loop loops over any object implementing IEnumerable or IEnumerable<T>
// All the collection types (Array, List, Dictionary...) in the .NET framework
// implement one or both of these interfaces.
// (The ToCharArray() could be removed, because a string also implements IEnumerable)
foreach (char character in "Hello World".ToCharArray())
{
// Iterated over all the characters in the string
}
// Switch Case
// A switch works with byte, short, char, and int data types.
// It also works with enumerated types (discussed in Enum Types),
// the String class, and a few special classes that wrap
// primitive types: Character, Byte, Short, and Integer.
int month = 3;
string monthString;
switch (month)
{
case 1:
monthString = "January";
break;
case 2:
monthString = "February";
break;
case 3:
monthString = "March";
break;
// You can assign more than one case to an action
// But you can't add an action without a break before another case
// (if you want to do this, you would have to explicitly add a goto case x)
case 6:
case 7:
case 8:
monthString = "Summer time!!";
break;
default:
monthString = "Some other month";
break;
}
///////////////////////////////////////
// Converting Data Types And Typecasting
///////////////////////////////////////
// Converting data
// Convert String To Integer
// this will throw a FormatException on failure
int.Parse("123"); // returns an integer version of "123"
// TryParse will default to the type's default value on failure
// in this case 0
int tryInt;
if (int.TryParse("123", out tryInt)) // Function is boolean
Console.WriteLine(tryInt); // 123
// Convert Integer To String
// The Convert class has a number of methods to facilitate conversions
// String to int
// Better
bool result = int.TryParse(string, out var integer)
int.Parse(string);
// Not recommended
Convert.ToString(123);
// Int to string
tryInt.ToString();
// Casting
// Cast decimal 15 to an int
// and then implicitly cast to long
long x = (int) 15M;
}
///////////////////////////////////////
// CLASSES - see definitions at end of file
///////////////////////////////////////
public static void Classes()
{
// See Declaration of objects at end of file
// Use new to instantiate a class
Bicycle trek = new Bicycle();
// Call object methods
trek.SpeedUp(3); // You should always use setter and getter methods
trek.Cadence = 100;
// ToString is a convention to display the value of this Object.
Console.WriteLine("trek info: " + trek.Info());
// Instantiate a new Penny Farthing
PennyFarthing funbike = new PennyFarthing(1, 10);
Console.WriteLine("funbike info: " + funbike.Info());
Console.Read();
} // End main method
// Available in C# 9 and later, this is basically syntactic sugar for a class. Records are immutable*.
public record ARecord(string Csharp);
// CONSOLE ENTRY - A console application must have a main method as an entry point
public static void Main(string[] args)
{
OtherInterestingFeatures();
}
//
// INTERESTING FEATURES
//
// DEFAULT METHOD SIGNATURES
public // Visibility
static // Allows for direct call on class without object
int // Return Type,
MethodSignatures(
int maxCount, // First variable, expects an int
int count = 0, // will default the value to 0 if not passed in
int another = 3,
params string[] otherParams // captures all other parameters passed to method
)
{
return -1;
}
// Methods can have the same name, as long as the signature is unique
// A method that differs only in return type is not unique
public static void MethodSignatures(
ref int maxCount, // Pass by reference
out int count)
{
// the argument passed in as 'count' will hold the value of 15 outside of this function
count = 15; // out param must be assigned before control leaves the method
}
// GENERICS
// The classes for TKey and TValue is specified by the user calling this function.
// This method emulates Python's dict.setdefault()
public static TValue SetDefault<TKey, TValue>(
IDictionary<TKey, TValue> dictionary,
TKey key,
TValue defaultItem)
{
TValue result;
if (!dictionary.TryGetValue(key, out result))
return dictionary[key] = defaultItem;
return result;
}
// You can narrow down the objects that are passed in
public static void IterateAndPrint<T>(T toPrint) where T: IEnumerable<int>
{
// We can iterate, since T is a IEnumerable
foreach (var item in toPrint)
// Item is an int
Console.WriteLine(item.ToString());
}
// YIELD
// Usage of the "yield" keyword indicates that the method it appears in is an Iterator
// (this means you can use it in a foreach loop)
public static IEnumerable<int> YieldCounter(int limit = 10)
{
for (var i = 0; i < limit; i++)
yield return i;
}
// which you would call like this :
public static void PrintYieldCounterToConsole()
{
foreach (var counter in YieldCounter())
Console.WriteLine(counter);
}
// you can use more than one "yield return" in a method
public static IEnumerable<int> ManyYieldCounter()
{
yield return 0;
yield return 1;
yield return 2;
yield return 3;
}
// you can also use "yield break" to stop the Iterator
// this method would only return half of the values from 0 to limit.
public static IEnumerable<int> YieldCounterWithBreak(int limit = 10)
{
for (var i = 0; i < limit; i++)
{
if (i > limit/2) yield break;
yield return i;
}
}
public static void OtherInterestingFeatures()
{
// OPTIONAL PARAMETERS
MethodSignatures(3, 1, 3, "Some", "Extra", "Strings");
MethodSignatures(3, another: 3); // explicitly set a parameter, skipping optional ones
// BY REF AND OUT PARAMETERS
int maxCount = 0, count; // ref params must have value
MethodSignatures(ref maxCount, out count);
// EXTENSION METHODS
int i = 3;
i.Print(); // Defined below
// NULLABLE TYPES - great for database interaction / return values
// any value type (i.e. not a class) can be made nullable by suffixing a ?
// <type>? <var name> = <value>
int? nullable = null; // short hand for Nullable<int>
Console.WriteLine("Nullable variable: " + nullable);
bool hasValue = nullable.HasValue; // true if not null
// ?? is syntactic sugar for specifying default value (coalesce)
// in case variable is null
int notNullable = nullable ?? 0; // 0
// ?. is an operator for null-propagation - a shorthand way of checking for null
nullable?.Print(); // Use the Print() extension method if nullable isn't null
// IMPLICITLY TYPED VARIABLES - you can let the compiler work out what the type is:
var magic = "magic is a string, at compile time, so you still get type safety";
// magic = 9; will not work as magic is a string, not an int
// GENERICS
//
var phonebook = new Dictionary<string, string>() {
{"Sarah", "212 555 5555"} // Add some entries to the phone book
};
// Calling SETDEFAULT defined as a generic above
Console.WriteLine(SetDefault<string,string>(phonebook, "Shaun", "No Phone")); // No Phone
// nb, you don't need to specify the TKey and TValue since they can be
// derived implicitly
Console.WriteLine(SetDefault(phonebook, "Sarah", "No Phone")); // 212 555 5555
// LAMBDA EXPRESSIONS - allow you to write code in line
Func<int, int> square = (x) => x * x; // Last T item is the return value
Console.WriteLine(square(3)); // 9
// ERROR HANDLING - coping with an uncertain world
try
{
var funBike = PennyFarthing.CreateWithGears(6);
// will no longer execute because CreateWithGears throws an exception
string some = "";
if (true) some = null;
some.ToLower(); // throws a NullReferenceException
}
catch (NotSupportedException)
{
Console.WriteLine("Not so much fun now!");
}
catch (Exception ex) // catch all other exceptions
{
throw new ApplicationException("It hit the fan", ex);
// throw; // A rethrow that preserves the callstack
}
// catch { } // catch-all without capturing the Exception
finally
{
// executes after try or catch
}
// DISPOSABLE RESOURCES MANAGEMENT - let you handle unmanaged resources easily.
// Most of objects that access unmanaged resources (file handle, device contexts, etc.)
// implement the IDisposable interface. The using statement takes care of
// cleaning those IDisposable objects for you.
using (StreamWriter writer = new StreamWriter("log.txt"))
{
writer.WriteLine("Nothing suspicious here");
// At the end of scope, resources will be released.
// Even if an exception is thrown.
}
// PARALLEL FRAMEWORK
// https://devblogs.microsoft.com/csharpfaq/parallel-programming-in-net-framework-4-getting-started/
var words = new List<string> {"dog", "cat", "horse", "pony"};
Parallel.ForEach(words,
new ParallelOptions() { MaxDegreeOfParallelism = 4 },
word =>
{
Console.WriteLine(word);
}
);
// Running this will produce different outputs
// since each thread finishes at different times.
// Some example outputs are:
// cat dog horse pony
// dog horse pony cat
// DYNAMIC OBJECTS (great for working with other languages)
dynamic student = new ExpandoObject();
student.FirstName = "First Name"; // No need to define class first!
// You can even add methods (returns a string, and takes in a string)
student.Introduce = new Func<string, string>(
(introduceTo) => string.Format("Hey {0}, this is {1}", student.FirstName, introduceTo));
Console.WriteLine(student.Introduce("Beth"));
// IQUERYABLE<T> - almost all collections implement this, which gives you a lot of
// very useful Map / Filter / Reduce style methods
var bikes = new List<Bicycle>();
bikes.Sort(); // Sorts the array
bikes.Sort((b1, b2) => b1.Wheels.CompareTo(b2.Wheels)); // Sorts based on wheels
var result = bikes
.Where(b => b.Wheels > 3) // Filters - chainable (returns IQueryable of previous type)
.Where(b => b.IsBroken && b.HasTassles)
.Select(b => b.ToString()); // Map - we only this selects, so result is a IQueryable<string>
var sum = bikes.Sum(b => b.Wheels); // Reduce - sums all the wheels in the collection
// Create a list of IMPLICIT objects based on some parameters of the bike
var bikeSummaries = bikes.Select(b=>new { Name = b.Name, IsAwesome = !b.IsBroken && b.HasTassles });
// Hard to show here, but you get type ahead completion since the compiler can implicitly work
// out the types above!
foreach (var bikeSummary in bikeSummaries.Where(b => b.IsAwesome))
Console.WriteLine(bikeSummary.Name);
// ASPARALLEL
// And this is where things get wicked - combine linq and parallel operations
var threeWheelers = bikes.AsParallel().Where(b => b.Wheels == 3).Select(b => b.Name);
// this will happen in parallel! Threads will automagically be spun up and the
// results divvied amongst them! Amazing for large datasets when you have lots of
// cores
// LINQ - maps a store to IQueryable<T> objects, with delayed execution
// e.g. LinqToSql - maps to a database, LinqToXml maps to an xml document
var db = new BikeRepository();
// execution is delayed, which is great when querying a database
var filter = db.Bikes.Where(b => b.HasTassles); // no query run
if (42 > 6) // You can keep adding filters, even conditionally - great for "advanced search" functionality
filter = filter.Where(b => b.IsBroken); // no query run
var query = filter
.OrderBy(b => b.Wheels)
.ThenBy(b => b.Name)
.Select(b => b.Name); // still no query run
// Now the query runs, but opens a reader, so only populates as you iterate through
foreach (string bike in query)
Console.WriteLine(result);
}
} // End LearnCSharp class
// You can include other classes in a .cs file
public static class Extensions
{
// EXTENSION METHODS
public static void Print(this object obj)
{
Console.WriteLine(obj.ToString());
}
}
// DELEGATES AND EVENTS
public class DelegateTest
{
public static int count = 0;
public static int Increment()
{
// increment count then return it
return ++count;
}
// A delegate is a reference to a method.
// To reference the Increment method,
// first declare a delegate with the same signature,
// i.e. takes no arguments and returns an int
public delegate int IncrementDelegate();
// An event can also be used to trigger delegates
// Create an event with the delegate type
public static event IncrementDelegate MyEvent;
static void Main(string[] args)
{
// Refer to the Increment method by instantiating the delegate
// and passing the method itself in as an argument
IncrementDelegate inc = new IncrementDelegate(Increment);
Console.WriteLine(inc()); // => 1
// Delegates can be composed with the + operator
IncrementDelegate composedInc = inc;
composedInc += inc;
composedInc += inc;
// composedInc will run Increment 3 times
Console.WriteLine(composedInc()); // => 4
// Subscribe to the event with the delegate
MyEvent += new IncrementDelegate(Increment);
MyEvent += new IncrementDelegate(Increment);
// Trigger the event
// ie. run all delegates subscribed to this event
Console.WriteLine(MyEvent()); // => 6
}
}
// Class Declaration Syntax:
// <public/private/protected/internal> class <class name>{
// //data fields, constructors, functions all inside.
// //functions are called as methods in Java.
// }
public class Bicycle
{
// Bicycle's Fields/Variables
public int Cadence // Public: Can be accessed from anywhere
{
get // get - define a method to retrieve the property
{
return _cadence;
}
set // set - define a method to set a property
{
_cadence = value; // Value is the value passed in to the setter
}
}
private int _cadence;
protected virtual int Gear // Protected: Accessible from the class and subclasses
{
get; // creates an auto property so you don't need a member field
set;
}
internal int Wheels // Internal: Accessible from within the assembly
{
get;
private set; // You can set modifiers on the get/set methods
}
int _speed; // Everything is private by default: Only accessible from within this class.
// can also use keyword private
public string Name { get; set; }
// Properties also have a special syntax for when you want a readonly property
// that simply returns the result of an expression
public string LongName => Name + " " + _speed + " speed";
// Enum is a value type that consists of a set of named constants
// It is really just mapping a name to a value (an int, unless specified otherwise).
// The approved types for an enum are byte, sbyte, short, ushort, int, uint, long, or ulong.
// An enum can't contain the same value twice.
public enum BikeBrand
{
AIST,
BMC,
Electra = 42, //you can explicitly set a value to a name
Gitane // 43
}
// We defined this type inside a Bicycle class, so it is a nested type
// Code outside of this class should reference this type as Bicycle.BikeBrand
public BikeBrand Brand; // After declaring an enum type, we can declare the field of this type
// Decorate an enum with the FlagsAttribute to indicate that multiple values can be switched on
// Any class derived from Attribute can be used to decorate types, methods, parameters etc
// Bitwise operators & and | can be used to perform and/or operations
[Flags]
public enum BikeAccessories
{
None = 0,
Bell = 1,
MudGuards = 2, // need to set the values manually!
Racks = 4,
Lights = 8,
FullPackage = Bell | MudGuards | Racks | Lights
}
// Usage: aBike.Accessories.HasFlag(Bicycle.BikeAccessories.Bell)
// Before .NET 4: (aBike.Accessories & Bicycle.BikeAccessories.Bell) == Bicycle.BikeAccessories.Bell
public BikeAccessories Accessories { get; set; }
// Static members belong to the type itself rather than specific object.
// You can access them without a reference to any object:
// Console.WriteLine("Bicycles created: " + Bicycle.bicyclesCreated);
public static int BicyclesCreated { get; set; }
// readonly values are set at run time
// they can only be assigned upon declaration or in a constructor
readonly bool _hasCardsInSpokes = false; // read-only private
// Constructors are a way of creating classes
// This is a default constructor
public Bicycle()
{
this.Gear = 1; // you can access members of the object with the keyword this
Cadence = 50; // but you don't always need it
_speed = 5;
Name = "Bontrager";
Brand = BikeBrand.AIST;
BicyclesCreated++;
}
// This is a specified constructor (it contains arguments)
public Bicycle(int startCadence, int startSpeed, int startGear,
string name, bool hasCardsInSpokes, BikeBrand brand)
: base() // calls base first
{
Gear = startGear;
Cadence = startCadence;
_speed = startSpeed;
Name = name;
_hasCardsInSpokes = hasCardsInSpokes;
Brand = brand;
}
// Constructors can be chained
public Bicycle(int startCadence, int startSpeed, BikeBrand brand) :
this(startCadence, startSpeed, 0, "big wheels", true, brand)
{
}
// Function Syntax:
// <public/private/protected> <return type> <function name>(<args>)
// classes can implement getters and setters for their fields
// or they can implement properties (this is the preferred way in C#)
// Method parameters can have default values.
// In this case, methods can be called with these parameters omitted
public void SpeedUp(int increment = 1)
{
_speed += increment;
}
public void SlowDown(int decrement = 1)
{
_speed -= decrement;
}
// properties get/set values
// when only data needs to be accessed, consider using properties.
// properties may have either get or set, or both
private bool _hasTassles; // private variable
public bool HasTassles // public accessor
{
get { return _hasTassles; }
set { _hasTassles = value; }
}
// You can also define an automatic property in one line
// this syntax will create a backing field automatically.
// You can set an access modifier on either the getter or the setter (or both)
// to restrict its access:
public bool IsBroken { get; private set; }
// Properties can be auto-implemented
public int FrameSize
{
get;
// you are able to specify access modifiers for either get or set
// this means only Bicycle class can call set on Framesize
private set;
}
// It's also possible to define custom Indexers on objects.
// Although this is not entirely useful in this example, you
// could do bicycle[0] which returns "chris" to get the first passenger or
// bicycle[1] = "lisa" to set the passenger. (of this apparent quattrocycle)
private string[] passengers = { "chris", "phil", "darren", "regina" };
public string this[int i]
{
get {
return passengers[i];
}
set {
passengers[i] = value;
}
}
// Method to display the attribute values of this Object.
public virtual string Info()
{
return "Gear: " + Gear +
" Cadence: " + Cadence +
" Speed: " + _speed +
" Name: " + Name +
" Cards in Spokes: " + (_hasCardsInSpokes ? "yes" : "no") +
"\n------------------------------\n"
;
}
// Methods can also be static. It can be useful for helper methods
public static bool DidWeCreateEnoughBicycles()
{
// Within a static method, we only can reference static class members
return BicyclesCreated > 9000;
} // If your class only needs static members, consider marking the class itself as static.
} // end class Bicycle
// PennyFarthing is a subclass of Bicycle
class PennyFarthing : Bicycle
{
// (Penny Farthings are those bicycles with the big front wheel.
// They have no gears.)
// calling parent constructor
public PennyFarthing(int startCadence, int startSpeed) :
base(startCadence, startSpeed, 0, "PennyFarthing", true, BikeBrand.Electra)
{
}
protected override int Gear
{
get
{
return 0;
}
set
{
throw new InvalidOperationException("You can't change gears on a PennyFarthing");
}
}
public static PennyFarthing CreateWithGears(int gears)
{
var penny = new PennyFarthing(1, 1);
penny.Gear = gears; // Oops, can't do this!
return penny;
}
public override string Info()
{
string result = "PennyFarthing bicycle ";
result += base.ToString(); // Calling the base version of the method
return result;
}
}
// Interfaces only contain signatures of the members, without the implementation.
interface IJumpable
{
void Jump(int meters); // all interface members are implicitly public
}
interface IBreakable
{
bool Broken { get; } // interfaces can contain properties as well as methods & events
}
// Classes can inherit only one other class, but can implement any amount of interfaces,
// however the base class name must be the first in the list and all interfaces follow
class MountainBike : Bicycle, IJumpable, IBreakable
{
int damage = 0;
public void Jump(int meters)
{
damage += meters;
}
public bool Broken
{
get
{
return damage > 100;
}
}
}
/// <summary>
/// Used to connect to DB for LinqToSql example.
/// EntityFramework Code First is awesome (similar to Ruby's ActiveRecord, but bidirectional)
/// https://docs.microsoft.com/ef/ef6/modeling/code-first/workflows/new-database
/// </summary>
public class BikeRepository : DbContext
{
public BikeRepository()
: base()
{
}
public DbSet<Bicycle> Bikes { get; set; }
}
// Classes can be split across multiple .cs files
// A1.cs
public partial class A
{
public static void A1()
{
Console.WriteLine("Method A1 in class A");
}
}
// A2.cs
public partial class A
{
public static void A2()
{
Console.WriteLine("Method A2 in class A");
}
}
// Program using the partial class "A"
public class Program
{
static void Main()
{
A.A1();
A.A2();
}
}
// String interpolation by prefixing the string with $
// and wrapping the expression you want to interpolate with { braces }
// You can also combine both interpolated and verbatim strings with $@
public class Rectangle
{
public int Length { get; set; }
public int Width { get; set; }
}
class Program
{
static void Main(string[] args)
{
Rectangle rect = new Rectangle { Length = 5, Width = 3 };
Console.WriteLine($"The length is {rect.Length} and the width is {rect.Width}");
string username = "User";
Console.WriteLine($@"C:\Users\{username}\Desktop");
}
}
// New C# 6 features
class GlassBall : IJumpable, IBreakable
{
// Autoproperty initializers
public int Damage { get; private set; } = 0;
// Autoproperty initializers on getter-only properties
public string Name { get; } = "Glass ball";
// Getter-only autoproperty that is initialized in constructor
public string GenieName { get; }
public GlassBall(string genieName = null)
{
GenieName = genieName;
}
public void Jump(int meters)
{
if (meters < 0)
// New nameof() expression; compiler will check that the identifier exists
// nameof(x) == "x"
// Prevents e.g. parameter names changing but not updated in error messages
throw new ArgumentException("Cannot jump negative amount!", nameof(meters));
Damage += meters;
}
// Expression-bodied properties ...
public bool Broken
=> Damage > 100;
// ... and methods
public override string ToString()
// Interpolated string
=> $"{Name}. Damage taken: {Damage}";
public string SummonGenie()
// Null-conditional operators
// x?.y will return null immediately if x is null; y is not evaluated
=> GenieName?.ToUpper();
}
static class MagicService
{
private static bool LogException(Exception ex)
{
// log exception somewhere
return false;
}
public static bool CastSpell(string spell)
{
try
{
// Pretend we call API here
throw new MagicServiceException("Spell failed", 42);
// Spell succeeded
return true;
}
// Only catch if Code is 42 i.e. spell failed
catch(MagicServiceException ex) when (ex.Code == 42)
{
// Spell failed
return false;
}
// Other exceptions, or MagicServiceException where Code is not 42
catch(Exception ex) when (LogException(ex))
{
// Execution never reaches this block
// The stack is not unwound
}
return false;
// Note that catching a MagicServiceException and rethrowing if Code
// is not 42 or 117 is different, as then the final catch-all block
// will not catch the rethrown exception
}
}
public class MagicServiceException : Exception
{
public int Code { get; }
public MagicServiceException(string message, int code) : base(message)
{
Code = code;
}
}
public static class PragmaWarning {
// Obsolete attribute
[Obsolete("Use NewMethod instead", false)]
public static void ObsoleteMethod()
{
// obsolete code
}
public static void NewMethod()
{
// new code
}
public static void Main()
{
ObsoleteMethod(); // CS0618: 'ObsoleteMethod is obsolete: Use NewMethod instead'
#pragma warning disable CS0618
ObsoleteMethod(); // no warning
#pragma warning restore CS0618
ObsoleteMethod(); // CS0618: 'ObsoleteMethod is obsolete: Use NewMethod instead'
}
}
} // End Namespace
using System;
// C# 6, static using
using static System.Math;
namespace Learning.More.CSharp
{
class StaticUsing
{
static void Main()
{
// Without a static using statement..
Console.WriteLine("The square root of 4 is {}.", Math.Sqrt(4));
// With one
Console.WriteLine("The square root of 4 is {}.", Sqrt(4));
}
}
}
// New C# 7 Feature
// Install Microsoft.Net.Compilers Latest from Nuget
// Install System.ValueTuple Latest from Nuget
using System;
namespace Csharp7
{
// TUPLES, DECONSTRUCTION AND DISCARDS
class TuplesTest
{
public (string, string) GetName()
{
// Fields in tuples are by default named Item1, Item2...
var names1 = ("Peter", "Parker");
Console.WriteLine(names1.Item2); // => Parker
// Fields can instead be explicitly named
// Type 1 Declaration
(string FirstName, string LastName) names2 = ("Peter", "Parker");
// Type 2 Declaration
var names3 = (First:"Peter", Last:"Parker");
Console.WriteLine(names2.FirstName); // => Peter
Console.WriteLine(names3.Last); // => Parker
return names3;
}
public string GetLastName() {
var fullName = GetName();
// Tuples can be deconstructed
(string firstName, string lastName) = fullName;
// Fields in a deconstructed tuple can be discarded by using _
var (_, last) = fullName;
return last;
}
// Any type can be deconstructed in the same way by
// specifying a Deconstruct method
public int randomNumber = 4;
public int anotherRandomNumber = 10;
public void Deconstruct(out int randomNumber, out int anotherRandomNumber)
{
randomNumber = this.randomNumber;
anotherRandomNumber = this.anotherRandomNumber;
}
static void Main(string[] args)
{
var tt = new TuplesTest();
(int num1, int num2) = tt;
Console.WriteLine($"num1: {num1}, num2: {num2}"); // => num1: 4, num2: 10
Console.WriteLine(tt.GetLastName());
}
}
// PATTERN MATCHING
class PatternMatchingTest
{
public static (string, int)? CreateLogMessage(object data)
{
switch(data)
{
// Additional filtering using when
case System.Net.Http.HttpRequestException h when h.Message.Contains("404"):
return (h.Message, 404);
case System.Net.Http.HttpRequestException h when h.Message.Contains("400"):
return (h.Message, 400);
case Exception e:
return (e.Message, 500);
case string s:
return (s, s.Contains("Error") ? 500 : 200);
case null:
return null;
default:
return (data.ToString(), 500);
}
}
}
// REFERENCE LOCALS
// Allow you to return a reference to an object instead of just its value
class RefLocalsTest
{
// note ref in return
public static ref string FindItem(string[] arr, string el)
{
for(int i=0; i<arr.Length; i++)
{
if(arr[i] == el) {
// return the reference
return ref arr[i];
}
}
throw new Exception("Item not found");
}
public static void SomeMethod()
{
string[] arr = {"this", "is", "an", "array"};
// note refs everywhere
ref string item = ref FindItem(arr, "array");
item = "apple";
Console.WriteLine(arr[3]); // => apple
}
}
// LOCAL FUNCTIONS
class LocalFunctionTest
{
private static int _id = 0;
public int id;
public LocalFunctionTest()
{
id = generateId();
// This local function can only be accessed in this scope
int generateId()
{
return _id++;
}
}
public static void AnotherMethod()
{
var lf1 = new LocalFunctionTest();
var lf2 = new LocalFunctionTest();
Console.WriteLine($"{lf1.id}, {lf2.id}"); // => 0, 1
int id = generateId();
// error CS0103: The name 'generateId' does not exist in the current context
}
}
}
C++:
//////////////////
// Comparison to C
//////////////////
// C++ is almost a superset of C and shares its basic syntax for
// variable declarations, primitive types, and functions.
// Just like in C, your program's entry point is a function called
// main with an integer return type.
// This value serves as the program's exit status.
// See https://en.wikipedia.org/wiki/Exit_status for more information.
int main(int argc, char** argv)
{
// Command line arguments are passed in by argc and argv in the same way
// they are in C.
// argc indicates the number of arguments,
// and argv is an array of C-style strings (char*)
// representing the arguments.
// The first argument is the name by which the program was called.
// argc and argv can be omitted if you do not care about arguments,
// giving the function signature of int main()
// An exit status of 0 indicates success.
return 0;
}
// However, C++ varies in some of the following ways:
// In C++, character literals are chars, therefore the size is 1
sizeof('c') == sizeof(char)
// In C, character literals are ints, therefore the size is 4
sizeof('c') == sizeof(int)
// C++ has strict prototyping
void func(); // function which accepts no arguments
void func(void); // same as earlier
// In C
void func(); // function which may accept any number of arguments with unknown type
void func(void); // function which accepts no arguments
// Use nullptr instead of NULL in C++
int* ip = nullptr;
// Most C standard headers are available in C++.
// C headers generally end with .h, while
// C++ headers are prefixed with "c" and have no ".h" suffix.
// The C++ standard version:
#include <cstdio>
// The C standard version:
#include <stdio.h>
int main()
{
printf("Hello, world!\n");
return 0;
}
///////////////////////
// Function overloading
///////////////////////
// C++ supports function overloading
// provided each function takes different parameters.
void print(char const* myString)
{
printf("String %s\n", myString);
}
void print(int myInt)
{
printf("My int is %d\n", myInt);
}
int main()
{
print("Hello"); // Resolves to void print(const char*)
print(15); // Resolves to void print(int)
}
/////////////////////////////
// Default function arguments
/////////////////////////////
// You can provide default arguments for a function
// if they are not provided by the caller.
void doSomethingWithInts(int a = 1, int b = 4)
{
// Do something with the ints here
}
int main()
{
doSomethingWithInts(); // a = 1, b = 4
doSomethingWithInts(20); // a = 20, b = 4
doSomethingWithInts(20, 5); // a = 20, b = 5
}
// Default arguments must be at the end of the arguments list.
void invalidDeclaration(int a = 1, int b) // Error!
{
}
/////////////
// Namespaces
/////////////
// Namespaces provide separate scopes for variable, function,
// and other declarations.
// Namespaces can be nested.
namespace First {
namespace Nested {
void foo()
{
printf("This is First::Nested::foo\n");
}
} // end namespace Nested
} // end namespace First
namespace Second {
void foo()
{
printf("This is Second::foo\n");
}
void bar()
{
printf("This is Second::bar\n");
}
}
void foo()
{
printf("This is global foo\n");
}
int main()
{
// Includes all symbols from namespace Second into the current scope. Note
// that while bar() works, simply using foo() no longer works, since it is
// now ambiguous whether we're calling the foo in namespace Second or the
// top level.
using namespace Second;
bar(); // prints "This is Second::bar"
Second::foo(); // prints "This is Second::foo"
First::Nested::foo(); // prints "This is First::Nested::foo"
::foo(); // prints "This is global foo"
}
///////////////
// Input/Output
///////////////
// C++ input and output uses streams
// cin, cout, and cerr represent stdin, stdout, and stderr.
// << is the insertion operator and >> is the extraction operator.
#include <iostream> // Include for I/O streams
int main()
{
int myInt;
// Prints to stdout (or terminal/screen)
// std::cout referring the access to the std namespace
std::cout << "Enter your favorite number:\n";
// Takes in input
std::cin >> myInt;
// cout can also be formatted
std::cout << "Your favorite number is " << myInt << '\n';
// prints "Your favorite number is <myInt>"
std::cerr << "Used for error messages";
// flush string stream buffer with new line
std::cout << "I flushed it away" << std::endl;
}
//////////
// Strings
//////////
// Strings in C++ are objects and have many member functions
#include <string>
std::string myString = "Hello";
std::string myOtherString = " World";
// + is used for concatenation.
std::cout << myString + myOtherString; // "Hello World"
std::cout << myString + " You"; // "Hello You"
// C++ string length can be found from either string::length() or string::size()
cout << myString.length() + myOtherString.size(); // Outputs 11 (= 5 + 6).
// C++ strings are mutable.
myString.append(" Dog");
std::cout << myString; // "Hello Dog"
// C++ can handle C-style strings with related functions using cstrings
#include <cstring>
char myOldString[10] = "Hello CPP";
cout << myOldString;
cout << "Length = " << strlen(myOldString); // Length = 9
/////////////
// References
/////////////
// In addition to pointers like the ones in C,
// C++ has _references_.
// These are pointer types that cannot be reassigned once set
// and cannot be null.
// They also have the same syntax as the variable itself:
// No * is needed for dereferencing and
// & (address of) is not used for assignment.
std::string foo = "I am foo";
std::string bar = "I am bar";
std::string& fooRef = foo; // This creates a reference to foo.
fooRef += ". Hi!"; // Modifies foo through the reference
std::cout << fooRef; // Prints "I am foo. Hi!"
std::cout << &fooRef << '\n'; // Prints the address of foo
// Doesn't reassign "fooRef". This is the same as "foo = bar", and
// foo == "I am bar"
// after this line.
fooRef = bar;
std::cout << &fooRef << '\n'; // Still prints the address of foo
std::cout << fooRef << '\n'; // Prints "I am bar"
// The address of fooRef remains the same, i.e. it is still referring to foo.
const std::string& barRef = bar; // Create a const reference to bar.
// Like C, const values (and pointers and references) cannot be modified.
barRef += ". Hi!"; // Error, const references cannot be modified.
// Sidetrack: Before we talk more about references, we must introduce a concept
// called a temporary object. Suppose we have the following code:
std::string tempObjectFun() { ... }
std::string retVal = tempObjectFun();
// What happens in the second line is actually:
// - a string object is returned from tempObjectFun
// - a new string is constructed with the returned object as argument to the
// constructor
// - the returned object is destroyed
// The returned object is called a temporary object. Temporary objects are
// created whenever a function returns an object, and they are destroyed at the
// end of the evaluation of the enclosing expression (Well, this is what the
// standard says, but compilers are allowed to change this behavior. Look up
// "return value optimization" if you're into these kinds of details). So in
// this code:
foo(bar(tempObjectFun()))
// assuming foo and bar exist, the object returned from tempObjectFun is
// passed to bar, and it is destroyed before foo is called.
// Now back to references. The exception to the "at the end of the enclosing
// expression" rule is if a temporary object is bound to a const reference, in
// which case its life gets extended to the current scope:
void constReferenceTempObjectFun() {
// constRef gets the temporary object, and it is valid until the end of this
// function.
const std::string& constRef = tempObjectFun();
...
}
// Another kind of reference introduced in C++11 is specifically for temporary
// objects. You cannot have a variable of its type, but it takes precedence in
// overload resolution:
void someFun(std::string& s) { ... } // Regular reference
void someFun(std::string&& s) { ... } // Reference to temporary object
std::string foo;
someFun(foo); // Calls the version with regular reference
someFun(tempObjectFun()); // Calls the version with temporary reference
// For example, you will see these two versions of constructors for
// std::basic_string:
std::basic_string(const basic_string& other);
std::basic_string(basic_string&& other);
// Idea being if we are constructing a new string from a temporary object (which
// is going to be destroyed soon anyway), we can have a more efficient
// constructor that "salvages" parts of that temporary string. You will see this
// concept referred to as "move semantics".
/////////////////////
// Enums
/////////////////////
// Enums are a way to assign a value to a constant most commonly used for
// easier visualization and reading of code
enum ECarTypes
{
Sedan,
Hatchback,
SUV,
Wagon
};
ECarTypes GetPreferredCarType()
{
return ECarTypes::Hatchback;
}
// As of C++11 there is an easy way to assign a type to the enum which can be
// useful in serialization of data and converting enums back-and-forth between
// the desired type and their respective constants
enum ECarTypes : uint8_t
{
Sedan, // 0
Hatchback, // 1
SUV = 254, // 254
Hybrid // 255
};
void WriteByteToFile(uint8_t InputValue)
{
// Serialize the InputValue to a file
}
void WritePreferredCarTypeToFile(ECarTypes InputCarType)
{
// The enum is implicitly converted to a uint8_t due to its declared enum type
WriteByteToFile(InputCarType);
}
// On the other hand you may not want enums to be accidentally cast to an integer
// type or to other enums so it is instead possible to create an enum class which
// won't be implicitly converted
enum class ECarTypes : uint8_t
{
Sedan, // 0
Hatchback, // 1
SUV = 254, // 254
Hybrid // 255
};
void WriteByteToFile(uint8_t InputValue)
{
// Serialize the InputValue to a file
}
void WritePreferredCarTypeToFile(ECarTypes InputCarType)
{
// Won't compile even though ECarTypes is a uint8_t due to the enum
// being declared as an "enum class"!
WriteByteToFile(InputCarType);
}
//////////////////////////////////////////
// Classes and object-oriented programming
//////////////////////////////////////////
// First example of classes
#include <iostream>
// Declare a class.
// Classes are usually declared in header (.h or .hpp) files.
class Dog {
// Member variables and functions are private by default.
std::string name;
int weight;
// All members following this are public
// until "private:" or "protected:" is found.
public:
// Default constructor
Dog();
// Member function declarations (implementations to follow)
// Note that we use std::string here instead of placing
// using namespace std;
// above.
// Never put a "using namespace" statement in a header.
void setName(const std::string& dogsName);
void setWeight(int dogsWeight);
// Functions that do not modify the state of the object
// should be marked as const.
// This allows you to call them if given a const reference to the object.
// Also note the functions must be explicitly declared as _virtual_
// in order to be overridden in derived classes.
// Functions are not virtual by default for performance reasons.
virtual void print() const;
// Functions can also be defined inside the class body.
// Functions defined as such are automatically inlined.
void bark() const { std::cout << name << " barks!\n"; }
// Along with constructors, C++ provides destructors.
// These are called when an object is deleted or falls out of scope.
// This enables powerful paradigms such as RAII
// (see below)
// The destructor should be virtual if a class is to be derived from;
// if it is not virtual, then the derived class' destructor will
// not be called if the object is destroyed through a base-class reference
// or pointer.
virtual ~Dog();
}; // A semicolon must follow the class definition.
// Class member functions are usually implemented in .cpp files.
Dog::Dog()
{
std::cout << "A dog has been constructed\n";
}
// Objects (such as strings) should be passed by reference
// if you are modifying them or const reference if you are not.
void Dog::setName(const std::string& dogsName)
{
name = dogsName;
}
void Dog::setWeight(int dogsWeight)
{
weight = dogsWeight;
}
// Notice that "virtual" is only needed in the declaration, not the definition.
void Dog::print() const
{
std::cout << "Dog is " << name << " and weighs " << weight << "kg\n";
}
Dog::~Dog()
{
std::cout << "Goodbye " << name << '\n';
}
int main() {
Dog myDog; // prints "A dog has been constructed"
myDog.setName("Barkley");
myDog.setWeight(10);
myDog.print(); // prints "Dog is Barkley and weighs 10 kg"
return 0;
} // prints "Goodbye Barkley"
// Inheritance:
// This class inherits everything public and protected from the Dog class
// as well as private but may not directly access private members/methods
// without a public or protected method for doing so
class OwnedDog : public Dog {
public:
void setOwner(const std::string& dogsOwner);
// Override the behavior of the print function for all OwnedDogs. See
// https://en.wikipedia.org/wiki/Polymorphism_(computer_science)#Subtyping
// for a more general introduction if you are unfamiliar with
// subtype polymorphism.
// The override keyword is optional but makes sure you are actually
// overriding the method in a base class.
void print() const override;
private:
std::string owner;
};
// Meanwhile, in the corresponding .cpp file:
void OwnedDog::setOwner(const std::string& dogsOwner)
{
owner = dogsOwner;
}
void OwnedDog::print() const
{
Dog::print(); // Call the print function in the base Dog class
std::cout << "Dog is owned by " << owner << '\n';
// Prints "Dog is <name> and weights <weight>"
// "Dog is owned by <owner>"
}
//////////////////////////////////////////
// Initialization and Operator Overloading
//////////////////////////////////////////
// In C++ you can overload the behavior of operators such as +, -, *, /, etc.
// This is done by defining a function which is called
// whenever the operator is used.
#include <iostream>
using namespace std;
class Point {
public:
// Member variables can be given default values in this manner.
double x = 0;
double y = 0;
// Define a default constructor which does nothing
// but initialize the Point to the default value (0, 0)
Point() { };
// The following syntax is known as an initialization list
// and is the proper way to initialize class member values
Point (double a, double b) :
x(a),
y(b)
{ /* Do nothing except initialize the values */ }
// Overload the + operator.
Point operator+(const Point& rhs) const;
// Overload the += operator
Point& operator+=(const Point& rhs);
// It would also make sense to add the - and -= operators,
// but we will skip those for brevity.
};
Point Point::operator+(const Point& rhs) const
{
// Create a new point that is the sum of this one and rhs.
return Point(x + rhs.x, y + rhs.y);
}
// It's good practice to return a reference to the leftmost variable of
// an assignment. `(a += b) == c` will work this way.
Point& Point::operator+=(const Point& rhs)
{
x += rhs.x;
y += rhs.y;
// `this` is a pointer to the object, on which a method is called.
return *this;
}
int main () {
Point up (0,1);
Point right (1,0);
// This calls the Point + operator
// Point up calls the + (function) with right as its parameter
Point result = up + right;
// Prints "Result is upright (1,1)"
std::cout << "Result is upright (" << result.x << ',' << result.y << ")\n";
return 0;
}
/////////////////////
// Templates
/////////////////////
// Templates in C++ are mostly used for generic programming, though they are
// much more powerful than generic constructs in other languages. They also
// support explicit and partial specialization and functional-style type
// classes; in fact, they are a Turing-complete functional language embedded
// in C++!
// We start with the kind of generic programming you might be familiar with. To
// define a class or function that takes a type parameter:
template<class T>
class Box {
public:
// In this class, T can be used as any other type.
void insert(const T&) { ... }
};
// During compilation, the compiler actually generates copies of each template
// with parameters substituted, so the full definition of the class must be
// present at each invocation. This is why you will see template classes defined
// entirely in header files.
// To instantiate a template class on the stack:
Box<int> intBox;
// and you can use it as you would expect:
intBox.insert(123);
// You can, of course, nest templates:
Box<Box<int> > boxOfBox;
boxOfBox.insert(intBox);
// Until C++11, you had to place a space between the two '>'s, otherwise '>>'
// would be parsed as the right shift operator.
// You will sometimes see
// template<typename T>
// instead. The 'class' keyword and 'typename' keywords are _mostly_
// interchangeable in this case. For the full explanation, see
// https://en.wikipedia.org/wiki/Typename
// (yes, that keyword has its own Wikipedia page).
// Similarly, a template function:
template<class T>
void barkThreeTimes(const T& input)
{
input.bark();
input.bark();
input.bark();
}
// Notice that nothing is specified about the type parameters here. The compiler
// will generate and then type-check every invocation of the template, so the
// above function works with any type 'T' that has a const 'bark' method!
Dog fluffy;
fluffy.setName("Fluffy")
barkThreeTimes(fluffy); // Prints "Fluffy barks" three times.
// Template parameters don't have to be classes:
template<int Y>
void printMessage() {
std::cout << "Learn C++ in " << Y << " minutes!\n";
}
// And you can explicitly specialize templates for more efficient code. Of
// course, most real-world uses of specialization are not as trivial as this.
// Note that you still need to declare the function (or class) as a template
// even if you explicitly specified all parameters.
template<>
void printMessage<10>() {
std::cout << "Learn C++ faster in only 10 minutes!\n";
}
printMessage<20>(); // Prints "Learn C++ in 20 minutes!"
printMessage<10>(); // Prints "Learn C++ faster in only 10 minutes!"
/////////////////////
// Exception Handling
/////////////////////
// The standard library provides a few exception types
// (see https://en.cppreference.com/w/cpp/error/exception)
// but any type can be thrown as an exception
#include <exception>
#include <stdexcept>
// All exceptions thrown inside the _try_ block can be caught by subsequent
// _catch_ handlers.
try {
// Do not allocate exceptions on the heap using _new_.
throw std::runtime_error("A problem occurred");
}
// Catch exceptions by const reference if they are objects
catch (const std::exception& ex)
{
std::cout << ex.what();
}
// Catches any exception not caught by previous _catch_ blocks
catch (...)
{
std::cout << "Unknown exception caught";
throw; // Re-throws the exception
}
///////
// RAII
///////
// RAII stands for "Resource Acquisition Is Initialization".
// It is often considered the most powerful paradigm in C++
// and is the simple concept that a constructor for an object
// acquires that object's resources and the destructor releases them.
// To understand how this is useful,
// consider a function that uses a C file handle:
void doSomethingWithAFile(const char* filename)
{
// To begin with, assume nothing can fail.
FILE* fh = fopen(filename, "r"); // Open the file in read mode.
if (fh == NULL) {
// Handle possible error
}
doSomethingWithTheFile(fh);
doSomethingElseWithIt(fh);
fclose(fh); // Close the file handle.
}
// Unfortunately, things are quickly complicated by error handling.
// Suppose fopen can fail, and that doSomethingWithTheFile and
// doSomethingElseWithIt return error codes if they fail.
// (Exceptions are the preferred way of handling failure,
// but some programmers, especially those with a C background,
// disagree on the utility of exceptions).
// We now have to check each call for failure and close the file handle
// if a problem occurred.
bool doSomethingWithAFile(const char* filename)
{
FILE* fh = fopen(filename, "r"); // Open the file in read mode
if (fh == nullptr) // The returned pointer is null on failure.
return false; // Report that failure to the caller.
// Assume each function returns false if it failed
if (!doSomethingWithTheFile(fh)) {
fclose(fh); // Close the file handle so it doesn't leak.
return false; // Propagate the error.
}
if (!doSomethingElseWithIt(fh)) {
fclose(fh); // Close the file handle so it doesn't leak.
return false; // Propagate the error.
}
fclose(fh); // Close the file handle so it doesn't leak.
return true; // Indicate success
}
// C programmers often clean this up a little bit using goto:
bool doSomethingWithAFile(const char* filename)
{
FILE* fh = fopen(filename, "r");
if (fh == nullptr)
return false;
if (!doSomethingWithTheFile(fh))
goto failure;
if (!doSomethingElseWithIt(fh))
goto failure;
fclose(fh); // Close the file
return true; // Indicate success
failure:
fclose(fh);
return false; // Propagate the error
}
// If the functions indicate errors using exceptions,
// things are a little cleaner, but still sub-optimal.
void doSomethingWithAFile(const char* filename)
{
FILE* fh = fopen(filename, "r"); // Open the file in shared_ptrread mode
if (fh == nullptr)
throw std::runtime_error("Could not open the file.");
try {
doSomethingWithTheFile(fh);
doSomethingElseWithIt(fh);
}
catch (...) {
fclose(fh); // Be sure to close the file if an error occurs.
throw; // Then re-throw the exception.
}
fclose(fh); // Close the file
// Everything succeeded
}
// Compare this to the use of C++'s file stream class (fstream)
// fstream uses its destructor to close the file.
// Recall from above that destructors are automatically called
// whenever an object falls out of scope.
void doSomethingWithAFile(const std::string& filename)
{
// ifstream is short for input file stream
std::ifstream fh(filename); // Open the file
// Do things with the file
doSomethingWithTheFile(fh);
doSomethingElseWithIt(fh);
} // The file is automatically closed here by the destructor
// This has _massive_ advantages:
// 1. No matter what happens,
// the resource (in this case the file handle) will be cleaned up.
// Once you write the destructor correctly,
// It is _impossible_ to forget to close the handle and leak the resource.
// 2. Note that the code is much cleaner.
// The destructor handles closing the file behind the scenes
// without you having to worry about it.
// 3. The code is exception safe.
// An exception can be thrown anywhere in the function and cleanup
// will still occur.
// All idiomatic C++ code uses RAII extensively for all resources.
// Additional examples include
// - Memory using unique_ptr and shared_ptr
// - Containers - the standard library linked list,
// vector (i.e. self-resizing array), hash maps, and so on
// all automatically destroy their contents when they fall out of scope.
// - Mutexes using lock_guard and unique_lock
/////////////////////
// Smart Pointer
/////////////////////
// Generally a smart pointer is a class which wraps a "raw pointer" (usage of "new"
// respectively malloc/calloc in C). The goal is to be able to
// manage the lifetime of the object being pointed to without ever needing to explicitly delete
// the object. The term itself simply describes a set of pointers with the
// mentioned abstraction.
// Smart pointers should be preferred over raw pointers, to prevent
// risky memory leaks, which happen if you forget to delete an object.
// Usage of a raw pointer:
Dog* ptr = new Dog();
ptr->bark();
delete ptr;
// By using a smart pointer, you don't have to worry about the deletion
// of the object anymore.
// A smart pointer describes a policy, to count the references to the
// pointer. The object gets destroyed when the last
// reference to the object gets destroyed.
// Usage of "std::shared_ptr":
void foo()
{
// It's no longer necessary to delete the Dog.
std::shared_ptr<Dog> doggo(new Dog());
doggo->bark();
}
// Beware of possible circular references!!!
// There will be always a reference, so it will be never destroyed!
std::shared_ptr<Dog> doggo_one(new Dog());
std::shared_ptr<Dog> doggo_two(new Dog());
doggo_one = doggo_two; // p1 references p2
doggo_two = doggo_one; // p2 references p1
// There are several kinds of smart pointers.
// The way you have to use them is always the same.
// This leads us to the question: when should we use each kind of smart pointer?
// std::unique_ptr - use it when you just want to hold one reference to
// the object.
// std::shared_ptr - use it when you want to hold multiple references to the
// same object and want to make sure that it's deallocated
// when all references are gone.
// std::weak_ptr - use it when you want to access
// the underlying object of a std::shared_ptr without causing that object to stay allocated.
// Weak pointers are used to prevent circular referencing.
/////////////////////
// Containers
/////////////////////
// Containers or the Standard Template Library are some predefined templates.
// They manage the storage space for its elements and provide
// member functions to access and manipulate them.
// Few containers are as follows:
// Vector (Dynamic array)
// Allow us to Define the Array or list of objects at run time
#include <vector>
std::string val;
std::vector<string> my_vector; // initialize the vector
std::cin >> val;
my_vector.push_back(val); // will push the value of 'val' into vector ("array") my_vector
my_vector.push_back(val); // will push the value into the vector again (now having two elements)
// To iterate through a vector we have 2 choices:
// Either classic looping (iterating through the vector from index 0 to its last index):
for (int i = 0; i < my_vector.size(); i++) {
std::cout << my_vector[i] << '\n'; // for accessing a vector's element we can use the operator []
}
// or using an iterator:
vector<string>::iterator it; // initialize the iterator for vector
for (it = my_vector.begin(); it != my_vector.end(); ++it) {
std::cout << *it << '\n';
}
// Set
// Sets are containers that store unique elements following a specific order.
// Set is a very useful container to store unique values in sorted order
// without any other functions or code.
#include<set>
std::set<int> ST; // Will initialize the set of int data type
ST.insert(30); // Will insert the value 30 in set ST
ST.insert(10); // Will insert the value 10 in set ST
ST.insert(20); // Will insert the value 20 in set ST
ST.insert(30); // Will insert the value 30 in set ST
// Now elements of sets are as follows
// 10 20 30
// To erase an element
ST.erase(20); // Will erase element with value 20
// Set ST: 10 30
// To iterate through Set we use iterators
std::set<int>::iterator it;
for (it = ST.begin(); it != ST.end(); it++) {
std::cout << *it << '\n';
}
// Output:
// 10
// 30
// To clear the complete container we use Container_name.clear()
ST.clear();
std::cout << ST.size(); // will print the size of set ST
// Output: 0
// NOTE: for duplicate elements we can use multiset
// NOTE: For hash sets, use unordered_set. They are more efficient but
// do not preserve order. unordered_set is available since C++11
// Map
// Maps store elements formed by a combination of a key value
// and a mapped value, following a specific order.
#include<map>
std::map<char, int> mymap; // Will initialize the map with key as char and value as int
mymap.insert(pair<char,int>('A',1));
// Will insert value 1 for key A
mymap.insert(pair<char,int>('Z',26));
// Will insert value 26 for key Z
// To iterate
std::map<char,int>::iterator it;
for (it = mymap.begin(); it != mymap.end(); ++it)
std::cout << it->first << "->" << it->second << '\n';
// Output:
// A->1
// Z->26
// To find the value corresponding to a key
it = mymap.find('Z');
std::cout << it->second;
// Output: 26
// NOTE: For hash maps, use unordered_map. They are more efficient but do
// not preserve order. unordered_map is available since C++11.
// Containers with object keys of non-primitive values (custom classes) require
// compare function in the object itself or as a function pointer. Primitives
// have default comparators, but you can override it.
class Foo {
public:
int j;
Foo(int a) : j(a) {}
};
struct compareFunction {
bool operator()(const Foo& a, const Foo& b) const {
return a.j < b.j;
}
};
// this isn't allowed (although it can vary depending on compiler)
// std::map<Foo, int> fooMap;
std::map<Foo, int, compareFunction> fooMap;
fooMap[Foo(1)] = 1;
fooMap.find(Foo(1)); //true
///////////////////////////////////////
// Lambda Expressions (C++11 and above)
///////////////////////////////////////
// lambdas are a convenient way of defining an anonymous function
// object right at the location where it is invoked or passed as
// an argument to a function.
// For example, consider sorting a vector of pairs using the second
// value of the pair
std::vector<pair<int, int> > tester;
tester.push_back(make_pair(3, 6));
tester.push_back(make_pair(1, 9));
tester.push_back(make_pair(5, 0));
// Pass a lambda expression as third argument to the sort function
// sort is from the <algorithm> header
std::sort(tester.begin(), tester.end(), [](const pair<int, int>& lhs, const pair<int, int>& rhs) {
return lhs.second < rhs.second;
});
// Notice the syntax of the lambda expression,
// [] in the lambda is used to "capture" variables
// The "Capture List" defines what from the outside of the lambda should be available inside the function body and how.
// It can be either:
// 1. a value : [x]
// 2. a reference : [&x]
// 3. any variable currently in scope by reference [&]
// 4. same as 3, but by value [=]
// Example:
std::vector<int> dog_ids;
// number_of_dogs = 3;
for (int i = 0; i < 3; i++) {
dog_ids.push_back(i);
}
int weight[3] = {30, 50, 10};
// Say you want to sort dog_ids according to the dogs' weights
// So dog_ids should in the end become: [2, 0, 1]
// Here's where lambda expressions come in handy
sort(dog_ids.begin(), dog_ids.end(), [&weight](const int &lhs, const int &rhs) {
return weight[lhs] < weight[rhs];
});
// Note we captured "weight" by reference in the above example.
// More on Lambdas in C++ : https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11
///////////////////////////////
// Range For (C++11 and above)
///////////////////////////////
// You can use a range for loop to iterate over a container
int arr[] = {1, 10, 3};
for (int elem: arr) {
cout << elem << endl;
}
// You can use "auto" and not worry about the type of the elements of the container
// For example:
for (auto elem: arr) {
// Do something with each element of arr
}
/////////////////////
// Fun stuff
/////////////////////
// Aspects of C++ that may be surprising to newcomers (and even some veterans).
// This section is, unfortunately, wildly incomplete; C++ is one of the easiest
// languages with which to shoot yourself in the foot.
// You can override private methods!
class Foo {
virtual void bar();
};
class FooSub : public Foo {
virtual void bar(); // Overrides Foo::bar!
};
// 0 == false == NULL (most of the time)!
bool* pt = new bool;
*pt = 0; // Sets the value points by 'pt' to false.
pt = 0; // Sets 'pt' to the null pointer. Both lines compile without warnings.
// nullptr is supposed to fix some of that issue:
int* pt2 = new int;
*pt2 = nullptr; // Doesn't compile
pt2 = nullptr; // Sets pt2 to null.
// There is an exception made for bools.
// This is to allow you to test for null pointers with if(!ptr),
// but as a consequence you can assign nullptr to a bool directly!
*pt = nullptr; // This still compiles, even though '*pt' is a bool!
// '=' != '=' != '='!
// Calls Foo::Foo(const Foo&) or some variant (see move semantics) copy
// constructor.
Foo f2;
Foo f1 = f2;
// Calls Foo::Foo(const Foo&) or variant, but only copies the 'Foo' part of
// 'fooSub'. Any extra members of 'fooSub' are discarded. This sometimes
// horrifying behavior is called "object slicing."
FooSub fooSub;
Foo f1 = fooSub;
// Calls Foo::operator=(Foo&) or variant.
Foo f1;
f1 = f2;
///////////////////////////////////////
// Tuples (C++11 and above)
///////////////////////////////////////
#include<tuple>
// Conceptually, Tuples are similar to old data structures (C-like structs)
// but instead of having named data members,
// its elements are accessed by their order in the tuple.
// We start with constructing a tuple.
// Packing values into tuple
auto first = make_tuple(10, 'A');
const int maxN = 1e9;
const int maxL = 15;
auto second = make_tuple(maxN, maxL);
// Printing elements of 'first' tuple
std::cout << get<0>(first) << " " << get<1>(first) << '\n'; //prints : 10 A
// Printing elements of 'second' tuple
std::cout << get<0>(second) << " " << get<1>(second) << '\n'; // prints: 1000000000 15
// Unpacking tuple into variables
int first_int;
char first_char;
tie(first_int, first_char) = first;
std::cout << first_int << " " << first_char << '\n'; // prints : 10 A
// We can also create tuple like this.
tuple<int, char, double> third(11, 'A', 3.14141);
// tuple_size returns number of elements in a tuple (as a constexpr)
std::cout << tuple_size<decltype(third)>::value << '\n'; // prints: 3
// tuple_cat concatenates the elements of all the tuples in the same order.
auto concatenated_tuple = tuple_cat(first, second, third);
// concatenated_tuple becomes = (10, 'A', 1e9, 15, 11, 'A', 3.14141)
std::cout << get<0>(concatenated_tuple) << '\n'; // prints: 10
std::cout << get<3>(concatenated_tuple) << '\n'; // prints: 15
std::cout << get<5>(concatenated_tuple) << '\n'; // prints: 'A'
///////////////////////////////////
// Logical and Bitwise operators
//////////////////////////////////
// Most of the operators in C++ are same as in other languages
// Logical operators
// C++ uses Short-circuit evaluation for boolean expressions, i.e, the second argument is executed or
// evaluated only if the first argument does not suffice to determine the value of the expression
true && false // Performs **logical and** to yield false
true || false // Performs **logical or** to yield true
! true // Performs **logical not** to yield false
// Instead of using symbols equivalent keywords can be used
true and false // Performs **logical and** to yield false
true or false // Performs **logical or** to yield true
not true // Performs **logical not** to yield false
// Bitwise operators
// **<<** Left Shift Operator
// << shifts bits to the left
4 << 1 // Shifts bits of 4 to left by 1 to give 8
// x << n can be thought as x * 2^n
// **>>** Right Shift Operator
// >> shifts bits to the right
4 >> 1 // Shifts bits of 4 to right by 1 to give 2
// x >> n can be thought as x / 2^n
~4 // Performs a bitwise not
4 | 3 // Performs bitwise or
4 & 3 // Performs bitwise and
4 ^ 3 // Performs bitwise xor
// Equivalent keywords are
compl 4 // Performs a bitwise not
4 bitor 3 // Performs bitwise or
4 bitand 3 // Performs bitwise and
4 xor 3 // Performs bitwise xor
Go:
// Single line comment
/* Multi-
line comment */
/* A build tag is a line comment starting with //go:build
and can be executed by go build -tags="foo bar" command.
Build tags are placed before the package clause near or at the top of the file
followed by a blank line or other line comments. */
//go:build prod || dev || test
// A package clause starts every source file.
// main is a special name declaring an executable rather than a library.
package main
// Import declaration declares library packages referenced in this file.
import (
"fmt" // A package in the Go standard library.
"io" // Implements some I/O utility functions.
m "math" // Math library with local alias m.
"net/http" // Yes, a web server!
_ "net/http/pprof" // Profiling library imported only for side effects
"os" // OS functions like working with the file system
"strconv" // String conversions.
)
// A function definition. Main is special. It is the entry point for the
// executable program. Love it or hate it, Go uses brace brackets.
func main() {
// Println outputs a line to stdout.
// It comes from the package fmt.
fmt.Println("Hello world!")
// Call another function within this package.
beyondHello()
}
// Functions have parameters in parentheses.
// If there are no parameters, empty parentheses are still required.
func beyondHello() {
var x int // Variable declaration. Variables must be declared before use.
x = 3 // Variable assignment.
// "Short" declarations use := to infer the type, declare, and assign.
y := 4
sum, prod := learnMultiple(x, y) // Function returns two values.
fmt.Println("sum:", sum, "prod:", prod) // Simple output.
learnTypes() // < y minutes, learn more!
}
/* <- multiline comment
Functions can have parameters and (multiple!) return values.
Here `x`, `y` are the arguments and `sum`, `prod` is the signature (what's returned).
Note that `x` and `sum` receive the type `int`.
*/
func learnMultiple(x, y int) (sum, prod int) {
return x + y, x * y // Return two values.
}
// Some built-in types and literals.
func learnTypes() {
// Short declaration usually gives you what you want.
str := "Learn Go!" // string type.
s2 := `A "raw" string literal
can include line breaks.` // Same string type.
// Non-ASCII literal. Go source is UTF-8.
g := 'Σ' // rune type, an alias for int32, holds a unicode code point.
f := 3.14159 // float64, an IEEE-754 64-bit floating point number.
c := 3 + 4i // complex128, represented internally with two float64's.
// var syntax with initializers.
var u uint = 7 // Unsigned, but implementation dependent size as with int.
var pi float32 = 22. / 7
// Conversion syntax with a short declaration.
n := byte('\n') // byte is an alias for uint8.
// Arrays have size fixed at compile time.
var a4 [4]int // An array of 4 ints, initialized to all 0.
a5 := [...]int{3, 1, 5, 10, 100} // An array initialized with a fixed size of five
// elements, with values 3, 1, 5, 10, and 100.
// Arrays have value semantics.
a4_cpy := a4 // a4_cpy is a copy of a4, two separate instances.
a4_cpy[0] = 25 // Only a4_cpy is changed, a4 stays the same.
fmt.Println(a4_cpy[0] == a4[0]) // false
// Slices have dynamic size. Arrays and slices each have advantages
// but use cases for slices are much more common.
s3 := []int{4, 5, 9} // Compare to a5. No ellipsis here.
s4 := make([]int, 4) // Allocates slice of 4 ints, initialized to all 0.
var d2 [][]float64 // Declaration only, nothing allocated here.
bs := []byte("a slice") // Type conversion syntax.
// Slices (as well as maps and channels) have reference semantics.
s3_cpy := s3 // Both variables point to the same instance.
s3_cpy[0] = 0 // Which means both are updated.
fmt.Println(s3_cpy[0] == s3[0]) // true
// Because they are dynamic, slices can be appended to on-demand.
// To append elements to a slice, the built-in append() function is used.
// First argument is a slice to which we are appending. Commonly,
// the slice variable is updated in place, as in example below.
s := []int{1, 2, 3} // Result is a slice of length 3.
s = append(s, 4, 5, 6) // Added 3 elements. Slice now has length of 6.
fmt.Println(s) // Updated slice is now [1 2 3 4 5 6]
// To append another slice, instead of list of atomic elements we can
// pass a reference to a slice or a slice literal like this, with a
// trailing ellipsis, meaning take a slice and unpack its elements,
// appending them to slice s.
s = append(s, []int{7, 8, 9}...) // Second argument is a slice literal.
fmt.Println(s) // Updated slice is now [1 2 3 4 5 6 7 8 9]
p, q := learnMemory() // Declares p, q to be type pointer to int.
fmt.Println(*p, *q) // * follows a pointer. This prints two ints.
// Maps are a dynamically growable associative array type, like the
// hash or dictionary types of some other languages.
m := map[string]int{"three": 3, "four": 4}
m["one"] = 1
// Looking up a missing key returns the zero value,
// which is 0 in this case, since it's a map[string]int
m["key not present"] // 0
// Check if a key is present in the map like this:
if val, ok := m["one"]; ok {
// Do something
}
// Unused variables are an error in Go.
// The underscore lets you "use" a variable but discard its value.
_, _, _, _, _, _, _, _, _, _ = str, s2, g, f, u, pi, n, a5, s4, bs
// Usually you use it to ignore one of the return values of a function
// For example, in a quick and dirty script you might ignore the
// error value returned from os.Create, and expect that the file
// will always be created.
file, _ := os.Create("output.txt")
fmt.Fprint(file, "This is how you write to a file, by the way")
file.Close()
// Output of course counts as using a variable.
fmt.Println(s, c, a4, s3, d2, m)
learnFlowControl() // Back in the flow.
}
// It is possible, unlike in many other languages for functions in go
// to have named return values.
// Assigning a name to the type being returned in the function declaration line
// allows us to easily return from multiple points in a function as well as to
// only use the return keyword, without anything further.
func learnNamedReturns(x, y int) (z int) {
z = x * y
return // z is implicit here, because we named it earlier.
}
// Go is fully garbage collected. It has pointers but no pointer arithmetic.
// You can make a mistake with a nil pointer, but not by incrementing a pointer.
// Unlike in C/Cpp taking and returning an address of a local variable is also safe.
func learnMemory() (p, q *int) {
// Named return values p and q have type pointer to int.
p = new(int) // Built-in function new allocates memory.
// The allocated int slice is initialized to 0, p is no longer nil.
s := make([]int, 20) // Allocate 20 ints as a single block of memory.
s[3] = 7 // Assign one of them.
r := -2 // Declare another local variable.
return &s[3], &r // & takes the address of an object.
}
// Use the aliased math library (see imports, above)
func expensiveComputation() float64 {
return m.Exp(10)
}
func learnFlowControl() {
// If statements require brace brackets, and do not require parentheses.
if true {
fmt.Println("told ya")
}
// Formatting is standardized by the command line command "go fmt".
if false {
// Pout.
} else {
// Gloat.
}
// Use switch in preference to chained if statements.
x := 42.0
switch x {
case 0:
case 1, 2: // Can have multiple matches on one case
case 42:
// Cases don't "fall through".
/*
There is a `fallthrough` keyword however, see:
https://go.dev/wiki/Switch#fall-through
*/
case 43:
// Unreached.
default:
// Default case is optional.
}
// Type switch allows switching on the type of something instead of value
var data interface{}
data = ""
switch c := data.(type) {
case string:
fmt.Println(c, "is a string")
case int64:
fmt.Printf("%d is an int64\n", c)
default:
// all other cases
}
// Like if, for doesn't use parens either.
// Variables declared in for and if are local to their scope.
for x := 0; x < 3; x++ { // ++ is a statement.
fmt.Println("iteration", x)
}
// x == 42 here.
// For is the only loop statement in Go, but it has alternate forms.
for { // Infinite loop.
break // Just kidding.
continue // Unreached.
}
// You can use range to iterate over an array, a slice, a string, a map, or a channel.
// range returns one (channel) or two values (array, slice, string and map).
for key, value := range map[string]int{"one": 1, "two": 2, "three": 3} {
// for each pair in the map, print key and value
fmt.Printf("key=%s, value=%d\n", key, value)
}
// If you only need the value, use the underscore as the key
for _, name := range []string{"Bob", "Bill", "Joe"} {
fmt.Printf("Hello, %s\n", name)
}
// As with for, := in an if statement means to declare and assign
// y first, then test y > x.
if y := expensiveComputation(); y > x {
x = y
}
// Function literals are closures.
xBig := func() bool {
return x > 10000 // References x declared above switch statement.
}
x = 99999
fmt.Println("xBig:", xBig()) // true
x = 1.3e3 // This makes x == 1300
fmt.Println("xBig:", xBig()) // false now.
// What's more is function literals may be defined and called inline,
// acting as an argument to function, as long as:
// a) function literal is called immediately (),
// b) result type matches expected type of argument.
fmt.Println("Add + double two numbers: ",
func(a, b int) int {
return (a + b) * 2
}(10, 2)) // Called with args 10 and 2
// => Add + double two numbers: 24
// When you need it, you'll love it.
goto love
love:
learnFunctionFactory() // func returning func is fun(3)(3)
learnDefer() // A quick detour to an important keyword.
learnInterfaces() // Good stuff coming up!
}
func learnFunctionFactory() {
// Next two are equivalent, with second being more practical
fmt.Println(sentenceFactory("summer")("A beautiful", "day!"))
d := sentenceFactory("summer")
fmt.Println(d("A beautiful", "day!"))
fmt.Println(d("A lazy", "afternoon!"))
}
// Decorators are common in other languages. Same can be done in Go
// with function literals that accept arguments.
func sentenceFactory(mystring string) func(before, after string) string {
return func(before, after string) string {
return fmt.Sprintf("%s %s %s", before, mystring, after) // new string
}
}
func learnDefer() (ok bool) {
// A defer statement pushes a function call onto a list. The list of saved
// calls is executed AFTER the surrounding function returns.
defer fmt.Println("deferred statements execute in reverse (LIFO) order.")
defer fmt.Println("\nThis line is being printed first because")
// Defer is commonly used to close a file, so the function closing the
// file stays near the function opening the file.
return true
}
// Define Stringer as an interface type with one method, String.
type Stringer interface {
String() string
}
// Define pair as a struct with two fields, ints named x and y.
type pair struct {
x, y int
}
// Define a method on type pair. Pair now implements Stringer because Pair has defined all the methods in the interface.
func (p pair) String() string { // p is called the "receiver"
// Sprintf is another public function in package fmt.
// Dot syntax references fields of p.
return fmt.Sprintf("(%d, %d)", p.x, p.y)
}
func learnInterfaces() {
// Brace syntax is a "struct literal". It evaluates to an initialized
// struct. The := syntax declares and initializes p to this struct.
p := pair{3, 4}
fmt.Println(p.String()) // Call String method of p, of type pair.
var i Stringer // Declare i of interface type Stringer.
i = p // Valid because pair implements Stringer
// Call String method of i, of type Stringer. Output same as above.
fmt.Println(i.String())
// Functions in the fmt package call the String method to ask an object
// for a printable representation of itself.
fmt.Println(p) // Output same as above. Println calls String method.
fmt.Println(i) // Output same as above.
learnVariadicParams("great", "learning", "here!")
}
// Functions can have variadic parameters.
func learnVariadicParams(myStrings ...any) { // any is an alias for interface{}
// Iterate each value of the variadic.
// The underscore here is ignoring the index argument of the array.
for _, param := range myStrings {
fmt.Println("param:", param)
}
// Pass variadic value as a variadic parameter.
fmt.Println("params:", fmt.Sprintln(myStrings...))
learnErrorHandling()
}
func learnErrorHandling() {
// ", ok" idiom used to tell if something worked or not.
m := map[int]string{3: "three", 4: "four"}
if x, ok := m[1]; !ok { // ok will be false because 1 is not in the map.
fmt.Println("no one there")
} else {
fmt.Print(x) // x would be the value, if it were in the map.
}
// An error value communicates not just "ok" but more about the problem.
if _, err := strconv.Atoi("non-int"); err != nil { // _ discards value
// prints 'strconv.ParseInt: parsing "non-int": invalid syntax'
fmt.Println(err)
}
// We'll revisit interfaces a little later. Meanwhile,
learnConcurrency()
}
// c is a channel, a concurrency-safe communication object.
func inc(i int, c chan int) {
c <- i + 1 // <- is the "send" operator when a channel appears on the left.
}
// We'll use inc to increment some numbers concurrently.
func learnConcurrency() {
// Same make function used earlier to make a slice. Make allocates and
// initializes slices, maps, and channels.
c := make(chan int)
// Start three concurrent goroutines. Numbers will be incremented
// concurrently, perhaps in parallel if the machine is capable and
// properly configured. All three send to the same channel.
go inc(0, c) // go is a statement that starts a new goroutine.
go inc(10, c)
go inc(-805, c)
// Read three results from the channel and print them out.
// There is no telling in what order the results will arrive!
fmt.Println(<-c, <-c, <-c) // channel on right, <- is "receive" operator.
cs := make(chan string) // Another channel, this one handles strings.
ccs := make(chan chan string) // A channel of string channels.
go func() { c <- 84 }() // Start a new goroutine just to send a value.
go func() { cs <- "wordy" }() // Again, for cs this time.
// Select has syntax like a switch statement but each case involves
// a channel operation. It selects a case at random out of the cases
// that are ready to communicate.
select {
case i := <-c: // The value received can be assigned to a variable,
fmt.Printf("it's a %T", i)
case <-cs: // or the value received can be discarded.
fmt.Println("it's a string")
case <-ccs: // Empty channel, not ready for communication.
fmt.Println("didn't happen.")
}
// At this point a value was taken from either c or cs. One of the two
// goroutines started above has completed, the other will remain blocked.
learnWebProgramming() // Go does it. You want to do it too.
}
// A single function from package http starts a web server.
func learnWebProgramming() {
// First parameter of ListenAndServe is TCP address to listen to.
// Second parameter is an interface, specifically http.Handler.
go func() {
err := http.ListenAndServe(":8080", pair{})
fmt.Println(err) // don't ignore errors
}()
requestServer()
}
// Make pair an http.Handler by implementing its only method, ServeHTTP.
func (p pair) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// Serve data with a method of http.ResponseWriter.
w.Write([]byte("You learned Go in Y minutes!"))
}
func requestServer() {
resp, err := http.Get("http://localhost:8080")
fmt.Println(err)
defer resp.Body.Close()
body, err := io.ReadAll(resp.Body)
fmt.Printf("\nWebserver said: `%s`", string(body))
}
Java:
// Single-line comments start with //
/*
Multi-line comments look like this.
*/
/**
* JavaDoc comments look like this. Used to describe the Class or various
* attributes of a Class.
* Main attributes:
*
* @author Name (and contact information such as email) of author(s).
* @version Current version of the program.
* @since When this part of the program was first added.
* @param For describing the different parameters for a method.
* @return For describing what the method returns.
* @deprecated For showing the code is outdated or shouldn't be used.
* @see Links to another part of documentation.
*/
// Import ArrayList class inside of the java.util package
import java.util.ArrayList;
// Import all classes inside of java.security package
import java.security.*;
// Java to illustrate calling of static members and methods without calling classname
import static java.lang.Math.*;
import static java.lang.System.*;
public class LearnJava {
// In order to run a java program, it must have a main method as an entry
// point.
public static void main(String[] args) {
///////////////////////////////////////
// Input/Output
///////////////////////////////////////
/*
* Output
*/
// Use System.out.println() to print lines.
System.out.println("Hello World!");
System.out.println(
"Integer: " + 10 +
" Double: " + 3.14 +
" Boolean: " + true);
// To print without a newline, use System.out.print().
System.out.print("Hello ");
System.out.print("World");
// Use System.out.printf() for easy formatted printing.
System.out.printf("pi = %.5f", Math.PI); // => pi = 3.14159
/*
* Input
*/
// use Scanner to read input
// must import java.util.Scanner;
Scanner scanner = new Scanner(System.in);
// read string input
String name = scanner.next();
// read byte input
byte numByte = scanner.nextByte();
// read int input
int numInt = scanner.nextInt();
// read long input
long numLong = scanner.nextLong();
// read float input
float numFloat = scanner.nextFloat();
// read double input
double numDouble = scanner.nextDouble();
// read boolean input
boolean bool = scanner.nextBoolean();
///////////////////////////////////////
// Variables
///////////////////////////////////////
/*
* Variable Declaration
*/
// Declare a variable using <type> <name>
int fooInt;
// Declare multiple variables of the same
// type <type> <name1>, <name2>, <name3>
int fooInt1, fooInt2, fooInt3;
/*
* Variable Initialization
*/
// Initialize a variable using <type> <name> = <val>
int barInt = 1;
// Initialize multiple variables of same type with same
// value <type> <name1>, <name2>, <name3>
// <name1> = <name2> = <name3> = <val>
int barInt1, barInt2, barInt3;
barInt1 = barInt2 = barInt3 = 1;
// Shorthand for multiple declarations
int barInt4 = 1, barInt5 = 2;
/*
* Variable types
*/
// Byte - 8-bit signed two's complement integer
// (-128 <= byte <= 127)
byte fooByte = 100;
// If you would like to interpret a byte as an unsigned integer
// then this simple operation can help
int unsignedIntLessThan256 = 0xff & fooByte;
// this contrasts a cast which can be negative.
int signedInt = (int) fooByte;
// Short - 16-bit signed two's complement integer
// (-32,768 <= short <= 32,767)
short fooShort = 10000;
// Integer - 32-bit signed two's complement integer
// (-2,147,483,648 <= int <= 2,147,483,647)
int bazInt = 1;
// Long - 64-bit signed two's complement integer
// (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
long fooLong = 100000L;
// L is used to denote that this variable value is of type Long;
// anything without is treated as integer by default.
// Note: byte, short, int and long are signed. They can have positive and negative values.
// There are no unsigned variants.
// char, however, is 16-bit unsigned.
// Float - Single-precision 32-bit IEEE 754 Floating Point
// 2^-149 <= float <= (2-2^-23) * 2^127
float fooFloat = 234.5f;
// f or F is used to denote that this variable value is of type float;
// otherwise it is treated as double.
// Double - Double-precision 64-bit IEEE 754 Floating Point
// 2^-1074 <= x <= (2-2^-52) * 2^1023
double fooDouble = 123.4;
// Boolean - true & false
boolean fooBoolean = true;
boolean barBoolean = false;
// Char - A single 16-bit Unicode character
char fooChar = 'A';
// final variables can't be reassigned,
final int HOURS_I_WORK_PER_WEEK = 9001;
// but they can be initialized later.
final double E;
E = 2.71828;
// BigInteger - Immutable arbitrary-precision integers
//
// BigInteger is a data type that allows programmers to manipulate
// integers longer than 64-bits. Integers are stored as an array of
// bytes and are manipulated using functions built into BigInteger
//
// BigInteger can be initialized using an array of bytes or a string.
BigInteger fooBigInteger = new BigInteger(fooByteArray);
// BigDecimal - Immutable, arbitrary-precision signed decimal number
//
// A BigDecimal takes two parts: an arbitrary precision integer
// unscaled value and a 32-bit integer scale
//
// BigDecimal allows the programmer complete control over decimal
// rounding. It is recommended to use BigDecimal with currency values
// and where exact decimal precision is required.
//
// BigDecimal can be initialized with an int, long, double or String
// or by initializing the unscaled value (BigInteger) and scale (int).
BigDecimal fooBigDecimal = new BigDecimal(fooBigInteger, fooInt);
// Be wary of the constructor that takes a float or double as
// the inaccuracy of the float/double will be copied in BigDecimal.
// Prefer the String constructor when you need an exact value.
BigDecimal tenCents = new BigDecimal("0.1");
// Type inference with 'var'
var x = 100; // int
var y = 1.90; // double
var z = 'a'; // char
var p = "tanu"; // String
var q = false; // boolean
// Strings
String fooString = "My String Is Here!";
// Text blocks
var textBlock = """
This is a <Text Block> in Java
""";
// \n is an escaped character that starts a new line
String barString = "Printing on a new line?\nNo Problem!";
// \t is an escaped character that adds a tab character
String bazString = "Do you want to add a tab?\tNo Problem!";
System.out.println(fooString);
System.out.println(barString);
System.out.println(bazString);
// String Building
// #1 - with plus operator
// That's the basic way to do it (optimized under the hood)
String plusConcatenated = "Strings can " + "be concatenated " + "via + operator.";
System.out.println(plusConcatenated);
// Output: Strings can be concatenated via + operator.
// #2 - with StringBuilder
// This way doesn't create any intermediate strings. It just stores the string pieces, and ties them together
// when toString() is called.
// Hint: This class is not thread safe. A thread-safe alternative (with some impact on performance) is StringBuffer.
StringBuilder builderConcatenated = new StringBuilder();
builderConcatenated.append("You ");
builderConcatenated.append("can use ");
builderConcatenated.append("the StringBuilder class.");
System.out.println(builderConcatenated.toString()); // only now is the string built
// Output: You can use the StringBuilder class.
// StringBuilder is efficient when the fully constructed String is not required until the end of some processing.
StringBuilder stringBuilder = new StringBuilder();
String inefficientString = "";
for (int i = 0 ; i < 10; i++) {
stringBuilder.append(i).append(" ");
inefficientString += i + " ";
}
System.out.println(inefficientString);
System.out.println(stringBuilder.toString());
// inefficientString requires a lot more work to produce, as it generates a String on every loop iteration.
// Simple concatenation with + is compiled to a StringBuilder and toString()
// Avoid string concatenation in loops.
// #3 - with String formatter
// Another alternative way to create strings. Fast and readable.
String.format("%s may prefer %s.", "Or you", "String.format()");
// Output: Or you may prefer String.format().
// Arrays
// The array size must be decided upon instantiation
// The following formats work for declaring an array
// <datatype>[] <var name> = new <datatype>[<array size>];
// <datatype> <var name>[] = new <datatype>[<array size>];
int[] intArray = new int[10];
String[] stringArray = new String[1];
boolean boolArray[] = new boolean[100];
// Another way to declare & initialize an array
int[] y = {9000, 1000, 1337};
String names[] = {"Bob", "John", "Fred", "Juan Pedro"};
boolean bools[] = {true, false, false};
// Indexing an array - Accessing an element
System.out.println("intArray @ 0: " + intArray[0]);
// Arrays are zero-indexed and mutable.
intArray[1] = 1;
System.out.println("intArray @ 1: " + intArray[1]); // => 1
// Other data types worth checking out
// ArrayLists - Like arrays except more functionality is offered, and
// the size is mutable.
// LinkedLists - Implementation of doubly-linked list. All of the
// operations perform as could be expected for a
// doubly-linked list.
// Maps - A mapping of key Objects to value Objects. Map is
// an interface and therefore cannot be instantiated.
// The type of keys and values contained in a Map must
// be specified upon instantiation of the implementing
// class. Each key may map to only one corresponding value,
// and each key may appear only once (no duplicates).
// HashMaps - This class uses a hashtable to implement the Map
// interface. This allows the execution time of basic
// operations, such as get and insert element, to remain
// constant-amortized even for large sets.
// TreeMap - A Map that is sorted by its keys. Each modification
// maintains the sorting defined by either a Comparator
// supplied at instantiation, or comparisons of each Object
// if they implement the Comparable interface.
// Failure of keys to implement Comparable combined with failure to
// supply a Comparator will throw ClassCastExceptions.
// Insertion and removal operations take O(log(n)) time
// so avoid using this data structure unless you are taking
// advantage of the sorting.
///////////////////////////////////////
// Operators
///////////////////////////////////////
System.out.println("\n->Operators");
int i1 = 1, i2 = 2;
// Arithmetic is straightforward
System.out.println("1+2 = " + (i1 + i2)); // => 3
System.out.println("2-1 = " + (i2 - i1)); // => 1
System.out.println("2*1 = " + (i2 * i1)); // => 2
System.out.println("1/2 = " + (i1 / i2)); // => 0 (int/int returns int)
System.out.println("1/2.0 = " + (i1 / (double)i2)); // => 0.5
// Modulo
System.out.println("11%3 = " + (11 % 3)); // => 2
// Comparison operators
System.out.println("3 == 2? " + (3 == 2)); // => false
System.out.println("3 != 2? " + (3 != 2)); // => true
System.out.println("3 > 2? " + (3 > 2)); // => true
System.out.println("3 < 2? " + (3 < 2)); // => false
System.out.println("2 <= 2? " + (2 <= 2)); // => true
System.out.println("2 >= 2? " + (2 >= 2)); // => true
// Boolean operators
System.out.println("3 > 2 && 2 > 3? " + ((3 > 2) && (2 > 3))); // => false
System.out.println("3 > 2 || 2 > 3? " + ((3 > 2) || (2 > 3))); // => true
System.out.println("!(3 == 2)? " + (!(3 == 2))); // => true
// Bitwise operators!
/*
~ Unary bitwise complement
<< Signed left shift
>> Signed/Arithmetic right shift
>>> Unsigned/Logical right shift
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise inclusive OR
*/
// Increment operators
int i = 0;
System.out.println("\n->Inc/Dec-rementation");
// The ++ and -- operators increment and decrement by 1 respectively.
// If they are placed before the variable, they increment then return;
// after the variable they return then increment.
System.out.println(i++); // i = 1, prints 0 (post-increment)
System.out.println(++i); // i = 2, prints 2 (pre-increment)
System.out.println(i--); // i = 1, prints 2 (post-decrement)
System.out.println(--i); // i = 0, prints 0 (pre-decrement)
///////////////////////////////////////
// Control Structures
///////////////////////////////////////
System.out.println("\n->Control Structures");
// If statements are c-like
int j = 10;
if (j == 10) {
System.out.println("I get printed");
} else if (j > 10) {
System.out.println("I don't");
} else {
System.out.println("I also don't");
}
// While loop
int fooWhile = 0;
while (fooWhile < 100) {
System.out.println(fooWhile);
// Increment the counter
// Iterated 100 times, fooWhile 0,1,2...99
fooWhile++;
}
System.out.println("fooWhile Value: " + fooWhile);
// Do While Loop
int fooDoWhile = 0;
do {
System.out.println(fooDoWhile);
// Increment the counter
// Iterated 100 times, fooDoWhile 0->99
fooDoWhile++;
} while (fooDoWhile < 100);
System.out.println("fooDoWhile Value: " + fooDoWhile);
// For Loop
// for loop structure => for(<start_statement>; <conditional>; <step>)
for (int fooFor = 0; fooFor < 10; fooFor++) {
System.out.println(fooFor);
// Iterated 10 times, fooFor 0->9
}
System.out.println("fooFor Value: " + fooFor);
// Nested For Loop Exit with Label
outer:
for (int i = 0; i < 10; i++) {
for (int j = 0; j < 10; j++) {
if (i == 5 && j ==5) {
break outer;
// breaks out of outer loop instead of only the inner one
}
}
}
// For Each Loop
// The for loop is also able to iterate over arrays as well as objects
// that implement the Iterable interface.
int[] fooList = {1, 2, 3, 4, 5, 6, 7, 8, 9};
// for each loop structure => for (<object> : <iterable>)
// reads as: for each element in the iterable
// note: the object type must match the element type of the iterable.
for (int bar : fooList) {
System.out.println(bar);
//Iterates 9 times and prints 1-9 on new lines
}
// Switch Case
// A switch works with the byte, short, char, and int data types.
// It also works with enumerated types (discussed in Enum Types), the
// String class, and a few special classes that wrap primitive types:
// Character, Byte, Short, and Integer.
// Starting in Java 7 and above, we can also use the String type.
// Note: Do remember that, not adding "break" at end any particular case ends up in
// executing the very next case(given it satisfies the condition provided) as well.
int month = 3;
String monthString;
switch (month) {
case 1: monthString = "January";
break;
case 2: monthString = "February";
break;
case 3: monthString = "March";
break;
default: monthString = "Some other month";
break;
}
System.out.println("Switch Case Result: " + monthString);
// Try-with-resources (Java 7+)
// Try-catch-finally statements work as expected in Java but in Java 7+
// the try-with-resources statement is also available. Try-with-resources
// simplifies try-catch-finally statements by closing resources
// automatically.
// In order to use a try-with-resources, include an instance of a class
// in the try statement. The class must implement java.lang.AutoCloseable.
try (BufferedReader br = new BufferedReader(new FileReader("foo.txt"))) {
// You can attempt to do something that could throw an exception.
System.out.println(br.readLine());
// In Java 7, the resource will always be closed, even if it throws
// an Exception.
} catch (IOException | SQLException ex) {
// Java 7+ Multi catch block handle both exceptions
} catch (Exception ex) {
//The resource will be closed before the catch statement executes.
System.out.println("readLine() failed.");
}
// No need for a finally statement in this case, the BufferedReader is
// already closed. This can be used to avoid certain edge cases where
// a finally statement might not be called.
// To learn more:
// https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
// Conditional Shorthand
// You can use the '?' operator for quick assignments or logic forks.
// Reads as "If (statement) is true, use <first value>, otherwise, use
// <second value>"
int foo = 5;
String bar = (foo < 10) ? "A" : "B";
System.out.println("bar : " + bar); // Prints "bar : A", because the
// statement is true.
// Or simply
System.out.println("bar : " + (foo < 10 ? "A" : "B"));
////////////////////////////////////////
// Converting Data Types
////////////////////////////////////////
// Converting data
// Convert String To Integer
Integer.parseInt("123");//returns an integer version of "123"
// Convert Integer To String
Integer.toString(123);//returns a string version of 123
// For other conversions check out the following classes:
// Double
// Long
// String
///////////////////////////////////////
// Classes And Functions
///////////////////////////////////////
System.out.println("\n->Classes & Functions");
// (definition of the Bicycle class follows)
// Use new to instantiate a class
Bicycle trek = new Bicycle();
// Call object methods
trek.speedUp(3); // You should always use setter and getter methods
trek.setCadence(100);
// toString returns this Object's string representation.
System.out.println("trek info: " + trek.toString());
} // End main method
private static class TestInitialization {
// Double Brace Initialization
// Before Java 11, the Java Language had no syntax for how to create
// static Collections in an easy way. Usually you end up like this:
private static final Set<String> COUNTRIES = new HashSet<String>();
static {
COUNTRIES.add("DENMARK");
COUNTRIES.add("SWEDEN");
COUNTRIES.add("FINLAND");
}
// There's a nifty way to achieve the same thing,
// by using something that is called Double Brace Initialization.
private static final Set<String> COUNTRIES_DOUBLE_BRACE =
new HashSet<String>() {{
add("DENMARK");
add("SWEDEN");
add("FINLAND");
}}
// The first brace is creating a new AnonymousInnerClass and the
// second one declares an instance initializer block. This block
// is called when the anonymous inner class is created.
// This does not only work for Collections, it works for all
// non-final classes.
// Another option was to initialize the Collection from an array,
// using Arrays.asList() method:
private static final List<String> COUNTRIES_AS_LIST =
Arrays.asList("SWEDEN", "DENMARK", "NORWAY");
// This has one catch: the list we get is internally backed by the array,
// and since arrays can't change their size, the list backed by the array
// is not resizeable, which means we can't add new elements to it:
public static void main(String[] args) {
COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException!
// However, we can replace elements by index, just like in array:
COUNTRIES.set(1, "FINLAND");
System.out.println(COUNTRIES); // prints [SWEDEN, FINLAND, NORWAY]
}
// The resizing problem can be circumvented
// by creating another Collection from the List:
private static final Set<String> COUNTRIES_SET =
new HashSet<>(Arrays.asList("SWEDEN", "DENMARK", "NORWAY"));
// It's perfectly fine to add anything to the Set of COUNTRIES now.
} // End TestInitialization class
private static class TestJava11Initialization {
// Since Java 11, there is a convenient option to initialize Collections:
// Set.of() and List.of() methods.
private static final Set<String> COUNTRIES =
Set.of("SWEDEN", "DENMARK", "NORWAY");
// There is a massive catch, though: Lists and Sets initialized like this
// 1) are immutable
// 2) can't contain null elements (even check for null elements fails)!
public static void main(String[] args) {
COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException
COUNTRIES.remove("NORWAY"); // throws UnsupportedOperationException
COUNTRIES.contains(null); // throws NullPointerException
}
private static final Set<String> COUNTRIES_WITH_NULL =
Set.of("SWEDEN", null, "NORWAY"); // throws NullPointerException
} // End TestJava11Initialization class
} // End LearnJava class
// You can include other, non-public outer-level classes in a .java file,
// but it is not good practice. Instead split classes into separate files.
// Class Declaration Syntax:
// <public/private/protected> class <class name> {
// // data fields, constructors, functions all inside.
// // functions are called as methods in Java.
// }
class Bicycle {
// Bicycle's Fields/Variables
public int cadence; // Public: Can be accessed from anywhere
private int speed; // Private: Only accessible from within the class
protected int gear; // Protected: Accessible from the class and subclasses
String name; // default: Only accessible from within this package
static String className; // Static class variable
// Static block
// Java has no implementation of static constructors, but
// has a static block that can be used to initialize class variables
// (static variables).
// This block will be called when the class is loaded.
static {
className = "Bicycle";
}
// Constructors are a way of creating classes
// This is a constructor
public Bicycle() {
// You can also call another constructor:
// this(1, 50, 5, "Bontrager");
gear = 1;
cadence = 50;
speed = 5;
name = "Bontrager";
}
// This is a constructor that takes arguments
public Bicycle(int startCadence, int startSpeed, int startGear,
String name) {
this.gear = startGear;
this.cadence = startCadence;
this.speed = startSpeed;
this.name = name;
}
// Method Syntax:
// <public/private/protected> <return type> <function name>(<args>)
// Java classes often implement getters and setters for their fields
// Method declaration syntax:
// <access modifier> <return type> <method name>(<args>)
public int getCadence() {
return cadence;
}
// void methods require no return statement
public void setCadence(int newValue) {
cadence = newValue;
}
public void setGear(int newValue) {
gear = newValue;
}
public void speedUp(int increment) {
speed += increment;
}
public void slowDown(int decrement) {
speed -= decrement;
}
public void setName(String newName) {
name = newName;
}
public String getName() {
return name;
}
//Method to display the attribute values of this Object.
@Override // Inherited from the Object class.
public String toString() {
return "gear: " + gear + " cadence: " + cadence + " speed: " + speed +
" name: " + name;
}
} // end class Bicycle
// PennyFarthing is a subclass of Bicycle
class PennyFarthing extends Bicycle {
// (Penny Farthings are those bicycles with the big front wheel.
// They have no gears.)
public PennyFarthing(int startCadence, int startSpeed) {
// Call the parent constructor with super
super(startCadence, startSpeed, 0, "PennyFarthing");
}
// You should mark a method you're overriding with an @annotation.
// To learn more about what annotations are and their purpose check this
// out: http://docs.oracle.com/javase/tutorial/java/annotations/
@Override
public void setGear(int gear) {
this.gear = 0;
}
}
// Object casting
// Since the PennyFarthing class is extending the Bicycle class, we can say
// a PennyFarthing is a Bicycle and write :
// Bicycle bicycle = new PennyFarthing();
// This is called object casting where an object is taken for another one. There
// are lots of details and deals with some more intermediate concepts here:
// https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html
// Interfaces
// Interface declaration syntax
// <access-level> interface <interface-name> extends <super-interfaces> {
// // Constants
// // Method declarations
// }
// Example - Food:
public interface Edible {
public void eat(); // Any class that implements this interface, must
// implement this method.
}
public interface Digestible {
public void digest();
// Since Java 8, interfaces can have default method.
public default void defaultMethod() {
System.out.println("Hi from default method ...");
}
}
// We can now create a class that implements both of these interfaces.
public class Fruit implements Edible, Digestible {
@Override
public void eat() {
// ...
}
@Override
public void digest() {
// ...
}
}
// In Java, you can extend only one class, but you can implement many
// interfaces. For example:
public class ExampleClass extends ExampleClassParent implements InterfaceOne,
InterfaceTwo {
@Override
public void InterfaceOneMethod() {
}
@Override
public void InterfaceTwoMethod() {
}
}
// Abstract Classes
// Abstract Class declaration syntax
// <access-level> abstract class <abstract-class-name> extends
// <super-abstract-classes> {
// // Constants and variables
// // Method declarations
// }
// Abstract Classes cannot be instantiated.
// Abstract classes may define abstract methods.
// Abstract methods have no body and are marked abstract
// Non-abstract child classes must @Override all abstract methods
// from their super-classes.
// Abstract classes can be useful when combining repetitive logic
// with customised behavior, but as Abstract classes require
// inheritance, they violate "Composition over inheritance"
// so consider other approaches using composition.
// https://en.wikipedia.org/wiki/Composition_over_inheritance
public abstract class Animal
{
private int age;
public abstract void makeSound();
// Method can have a body
public void eat()
{
System.out.println("I am an animal and I am Eating.");
// Note: We can access private variable here.
age = 30;
}
public void printAge()
{
System.out.println(age);
}
// Abstract classes can have main method.
public static void main(String[] args)
{
System.out.println("I am abstract");
}
}
class Dog extends Animal
{
// Note still have to override the abstract methods in the
// abstract class.
@Override
public void makeSound()
{
System.out.println("Bark");
// age = 30; ==> ERROR! age is private to Animal
}
// NOTE: You will get an error if you used the
// @Override annotation here, since java doesn't allow
// overriding of static methods.
// What is happening here is called METHOD HIDING.
// Check out this SO post: http://stackoverflow.com/questions/16313649/
public static void main(String[] args)
{
Dog pluto = new Dog();
pluto.makeSound();
pluto.eat();
pluto.printAge();
}
}
// Final Classes
// Final Class declaration syntax
// <access-level> final <final-class-name> {
// // Constants and variables
// // Method declarations
// }
// Final classes are classes that cannot be inherited from and are therefore a
// final child. In a way, final classes are the opposite of abstract classes
// because abstract classes must be extended, but final classes cannot be
// extended.
public final class SaberToothedCat extends Animal
{
// Note still have to override the abstract methods in the
// abstract class.
@Override
public void makeSound()
{
System.out.println("Roar");
}
}
// Final Methods
public abstract class Mammal()
{
// Final Method Syntax:
// <access modifier> final <return type> <function name>(<args>)
// Final methods, like, final classes cannot be overridden by a child
// class, and are therefore the final implementation of the method.
public final boolean isWarmBlooded()
{
return true;
}
}
// Java Records are a concise way to define immutable data carrier classes, automatically
// generating boilerplate code like constructors, equals(), hashCode()and toString().
// This automatically creates an immutable class Person with fields name and age.
public record Person(String name, int age) {}
Person p = new Person("Alice", 30);
// Enum Type
//
// An enum type is a special data type that enables for a variable to be a set
// of predefined constants. The variable must be equal to one of the values
// that have been predefined for it. Because they are constants, the names of
// an enum type's fields are in uppercase letters. In the Java programming
// language, you define an enum type by using the enum keyword. For example,
// you would specify a days-of-the-week enum type as:
public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}
// We can use our enum Day like that:
public class EnumTest {
// Variable Enum
Day day;
public EnumTest(Day day) {
this.day = day;
}
public void tellItLikeItIs() {
switch (day) {
case MONDAY:
System.out.println("Mondays are bad.");
break;
case FRIDAY:
System.out.println("Fridays are better.");
break;
case SATURDAY:
case SUNDAY:
System.out.println("Weekends are best.");
break;
default:
System.out.println("Midweek days are so-so.");
break;
}
}
public static void main(String[] args) {
EnumTest firstDay = new EnumTest(Day.MONDAY);
firstDay.tellItLikeItIs(); // => Mondays are bad.
EnumTest thirdDay = new EnumTest(Day.WEDNESDAY);
thirdDay.tellItLikeItIs(); // => Midweek days are so-so.
}
}
// Enum types are much more powerful than we show above.
// The enum body can include methods and other fields.
// You can see more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
// Getting Started with Lambda Expressions
//
// New to Java version 8 are lambda expressions. Lambdas are more commonly found
// in functional programming languages, which means they are methods which can
// be created without belonging to a class, passed around as if it were itself
// an object, and executed on demand.
//
// Final note, lambdas must implement a functional interface. A functional
// interface is one which has only a single abstract method declared. It can
// have any number of default methods. Lambda expressions can be used as an
// instance of that functional interface. Any interface meeting the requirements
// is treated as a functional interface. You can read more about interfaces
// above.
//
import java.util.Map;
import java.util.HashMap;
import java.util.function.*;
import java.security.SecureRandom;
public class Lambdas {
public static void main(String[] args) {
// Lambda declaration syntax:
// <zero or more parameters> -> <expression body or statement block>
// We will use this hashmap in our examples below.
Map<String, String> planets = new HashMap<>();
planets.put("Mercury", "87.969");
planets.put("Venus", "224.7");
planets.put("Earth", "365.2564");
planets.put("Mars", "687");
planets.put("Jupiter", "4,332.59");
planets.put("Saturn", "10,759");
planets.put("Uranus", "30,688.5");
planets.put("Neptune", "60,182");
// Lambda with zero parameters using the Supplier functional interface
// from java.util.function.Supplier. The actual lambda expression is
// what comes after numPlanets =.
Supplier<String> numPlanets = () -> Integer.toString(planets.size());
System.out.format("Number of Planets: %s\n\n", numPlanets.get());
// Lambda with one parameter and using the Consumer functional interface
// from java.util.function.Consumer. This is because planets is a Map,
// which implements both Collection and Iterable. The forEach used here,
// found in Iterable, applies the lambda expression to each member of
// the Collection. The default implementation of forEach behaves as if:
/*
for (T t : this)
action.accept(t);
*/
// The actual lambda expression is the parameter passed to forEach.
planets.keySet().forEach((p) -> System.out.format("%s\n", p));
// If you are only passing a single argument, then the above can also be
// written as (note absent parentheses around p):
planets.keySet().forEach(p -> System.out.format("%s\n", p));
// Tracing the above, we see that planets is a HashMap, keySet() returns
// a Set of its keys, forEach applies each element as the lambda
// expression of: (parameter p) -> System.out.format("%s\n", p). Each
// time, the element is said to be "consumed" and the statement(s)
// referred to in the lambda body is applied. Remember the lambda body
// is what comes after the ->.
// The above without use of lambdas would look more traditionally like:
for (String planet : planets.keySet()) {
System.out.format("%s\n", planet);
}
// This example differs from the above in that a different forEach
// implementation is used: the forEach found in the HashMap class
// implementing the Map interface. This forEach accepts a BiConsumer,
// which generically speaking is a fancy way of saying it handles
// the Set of each Key -> Value pairs. This default implementation
// behaves as if:
/*
for (Map.Entry<K, V> entry : map.entrySet())
action.accept(entry.getKey(), entry.getValue());
*/
// The actual lambda expression is the parameter passed to forEach.
String orbits = "%s orbits the Sun in %s Earth days.\n";
planets.forEach((K, V) -> System.out.format(orbits, K, V));
// The above without use of lambdas would look more traditionally like:
for (String planet : planets.keySet()) {
System.out.format(orbits, planet, planets.get(planet));
}
// Or, if following more closely the specification provided by the
// default implementation:
for (Map.Entry<String, String> planet : planets.entrySet()) {
System.out.format(orbits, planet.getKey(), planet.getValue());
}
// These examples cover only the very basic use of lambdas. It might not
// seem like much or even very useful, but remember that a lambda can be
// created as an object that can later be passed as parameters to other
// methods.
}
}
JavaScript:
// Single-line comments start with two slashes.
/* Multiline comments start with slash-star,
and end with star-slash */
// Statements can be terminated by ;
doStuff();
// ... but they don't have to be, as semicolons are automatically inserted
// wherever there's a newline, except in certain cases.
doStuff()
// Because those cases can cause unexpected results, we'll keep on using
// semicolons in this guide.
///////////////////////////////////
// 1. Numbers, Strings and Operators
// JavaScript has one number type (which is a 64-bit IEEE 754 double).
// Doubles have a 52-bit mantissa, which is enough to store integers
// up to about 9✕10¹⁵ precisely.
3; // = 3
1.5; // = 1.5
// Some basic arithmetic works as you'd expect.
1 + 1; // = 2
0.1 + 0.2; // = 0.30000000000000004
8 - 1; // = 7
10 * 2; // = 20
35 / 5; // = 7
// Including uneven division.
5 / 2; // = 2.5
// And modulo division.
10 % 2; // = 0
30 % 4; // = 2
18.5 % 7; // = 4.5
// Bitwise operations also work; when you perform a bitwise operation your float
// is converted to a signed int *up to* 32 bits.
1 << 2; // = 4
// Precedence is enforced with parentheses.
(1 + 3) * 2; // = 8
// There are three special not-a-real-number values:
Infinity; // result of e.g. 1/0
-Infinity; // result of e.g. -1/0
NaN; // result of e.g. 0/0, stands for 'Not a Number'
// There's also a boolean type.
true;
false;
// Strings are created with ' or ".
'abc';
"Hello, world";
// Negation uses the ! symbol
!true; // = false
!false; // = true
// Equality is ===
1 === 1; // = true
2 === 1; // = false
// Inequality is !==
1 !== 1; // = false
2 !== 1; // = true
// More comparisons
1 < 10; // = true
1 > 10; // = false
2 <= 2; // = true
2 >= 2; // = true
// Strings are concatenated with +
"Hello " + "world!"; // = "Hello world!"
// ... which works with more than just strings
"1, 2, " + 3; // = "1, 2, 3"
"Hello " + ["world", "!"]; // = "Hello world,!"
// ...which can result in some weird behaviour...
13 + !0; // 14
"13" + !0; // '13true'
// and are compared with < and >
"a" < "b"; // = true
// Type coercion is performed for comparisons with double equals...
"5" == 5; // = true
null == undefined; // = true
// ...unless you use ===
"5" === 5; // = false
null === undefined; // = false
// You can access characters in a string with `charAt`
"This is a string".charAt(0); // = 'T'
// ...or use `substring` to get larger pieces.
"Hello world".substring(0, 5); // = "Hello"
// `length` is a property, so don't use ().
"Hello".length; // = 5
// There's also `null` and `undefined`.
null; // used to indicate a deliberate non-value
undefined; // used to indicate a value is not currently present (although
// `undefined` is actually a value itself)
// false, null, undefined, NaN, 0 and "" are falsy; everything else is truthy.
// Note that 0 is falsy and "0" is truthy, even though 0 == "0".
///////////////////////////////////
// 2. Variables, Arrays and Objects
// Variables are declared with the `var` keyword. JavaScript is dynamically
// typed, so you don't need to specify type. Assignment uses a single `=`
// character.
var someVar = 5;
// If you leave the var keyword off, you won't get an error...
someOtherVar = 10;
// ...but your variable will be created in the global scope, not in the scope
// you defined it in.
// Variables declared without being assigned to are set to undefined.
var someThirdVar; // = undefined
// If you want to declare a couple of variables, then you could use a comma
// separator
var someFourthVar = 2, someFifthVar = 4;
// There's shorthand for performing math operations on variables:
someVar += 5; // equivalent to someVar = someVar + 5; someVar is 10 now
someVar *= 10; // now someVar is 100
// and an even-shorter-hand for adding or subtracting 1
someVar++; // now someVar is 101
someVar--; // back to 100
// Arrays are ordered lists of values, of any type.
var myArray = ["Hello", 45, true];
// Their members can be accessed using the square-brackets subscript syntax.
// Array indices start at zero.
myArray[1]; // = 45
// Arrays are mutable and of variable length.
myArray.push("World");
myArray.length; // = 4
// Add/Modify at specific index
myArray[3] = "Hello";
// Add and remove element from front or back end of an array
myArray.unshift(3); // Add as the first element
someVar = myArray.shift(); // Remove first element and return it
myArray.push(3); // Add as the last element
someVar = myArray.pop(); // Remove last element and return it
// Join all elements of an array with semicolon
var myArray0 = [32,false,"js",12,56,90];
myArray0.join(";"); // = "32;false;js;12;56;90"
// Get subarray of elements from index 1 (include) to 4 (exclude)
myArray0.slice(1,4); // = [false,"js",12]
// Remove 4 elements starting from index 2, and insert there strings
// "hi","wr" and "ld"; return removed subarray
myArray0.splice(2,4,"hi","wr","ld"); // = ["js",12,56,90]
// myArray0 === [32,false,"hi","wr","ld"]
// JavaScript's objects are equivalent to "dictionaries" or "maps" in other
// languages: an unordered collection of key-value pairs.
var myObj = {key1: "Hello", key2: "World"};
// Keys are strings, but quotes aren't required if they're a valid
// JavaScript identifier. Values can be any type.
var myObj = {myKey: "myValue", "my other key": 4};
// Object attributes can also be accessed using the subscript syntax,
myObj["my other key"]; // = 4
// ... or using the dot syntax, provided the key is a valid identifier.
myObj.myKey; // = "myValue"
// Objects are mutable; values can be changed and new keys added.
myObj.myThirdKey = true;
// If you try to access a value that's not yet set, you'll get undefined.
myObj.myFourthKey; // = undefined
///////////////////////////////////
// 3. Logic and Control Structures
// The `if` structure works as you'd expect.
var count = 1;
if (count == 3){
// evaluated if count is 3
} else if (count == 4){
// evaluated if count is 4
} else {
// evaluated if it's not either 3 or 4
}
// As does `while`.
while (true){
// An infinite loop!
}
// Do-while loops are like while loops, except they always run at least once.
var input;
do {
input = getInput();
} while (!isValid(input));
// The `for` loop is the same as C and Java:
// initialization; continue condition; iteration.
for (var i = 0; i < 5; i++){
// will run 5 times
}
// Breaking out of labeled loops is similar to Java
outer:
for (var i = 0; i < 10; i++) {
for (var j = 0; j < 10; j++) {
if (i == 5 && j ==5) {
break outer;
// breaks out of outer loop instead of only the inner one
}
}
}
// The for/in statement allows iteration over properties of an object.
var description = "";
var person = {fname:"Paul", lname:"Ken", age:18};
for (var x in person){
description += person[x] + " ";
} // description = 'Paul Ken 18 '
// The for/of statement allows iteration over iterable objects (including the built-in String,
// Array, e.g. the Array-like arguments or NodeList objects, TypedArray, Map and Set,
// and user-defined iterables).
var myPets = "";
var pets = ["cat", "dog", "hamster", "hedgehog"];
for (var pet of pets){
myPets += pet + " ";
} // myPets = 'cat dog hamster hedgehog '
// && is logical and, || is logical or
if (house.size == "big" && house.colour == "blue"){
house.contains = "bear";
}
if (colour == "red" || colour == "blue"){
// colour is either red or blue
}
// && and || "short circuit", which is useful for setting default values.
var name = otherName || "default";
// The `switch` statement checks for equality with `===`.
// Use 'break' after each case
// or the cases after the correct one will be executed too.
grade = 'B';
switch (grade) {
case 'A':
console.log("Great job");
break;
case 'B':
console.log("OK job");
break;
case 'C':
console.log("You can do better");
break;
default:
console.log("Oy vey");
break;
}
///////////////////////////////////
// 4. Functions, Scope and Closures
// JavaScript functions are declared with the `function` keyword.
function myFunction(thing){
return thing.toUpperCase();
}
myFunction("foo"); // = "FOO"
// Note that the value to be returned must start on the same line as the
// `return` keyword, otherwise you'll always return `undefined` due to
// automatic semicolon insertion. Watch out for this when using Allman style.
function myFunction(){
return // <- semicolon automatically inserted here
{thisIsAn: 'object literal'};
}
myFunction(); // = undefined
// JavaScript functions are first class objects, so they can be reassigned to
// different variable names and passed to other functions as arguments - for
// example, when supplying an event handler:
function myFunction(){
// this code will be called in 5 seconds' time
}
setTimeout(myFunction, 5000);
// Note: setTimeout isn't part of the JS language, but is provided by browsers
// and Node.js.
// Another function provided by browsers is setInterval
function myFunction(){
// this code will be called every 5 seconds
}
setInterval(myFunction, 5000);
// Function objects don't even have to be declared with a name - you can write
// an anonymous function definition directly into the arguments of another.
setTimeout(function(){
// this code will be called in 5 seconds' time
}, 5000);
// JavaScript has function scope; functions get their own scope but other blocks
// do not.
if (true){
var i = 5;
}
i; // = 5 - not undefined as you'd expect in a block-scoped language
// This has led to a common pattern of "immediately-executing anonymous
// functions", which prevent temporary variables from leaking into the global
// scope.
(function(){
var temporary = 5;
// We can access the global scope by assigning to the "global object", which
// in a web browser is always `window`. The global object may have a
// different name in non-browser environments such as Node.js.
window.permanent = 10;
})();
temporary; // raises ReferenceError
permanent; // = 10
// One of JavaScript's most powerful features is closures. If a function is
// defined inside another function, the inner function has access to all the
// outer function's variables, even after the outer function exits.
function sayHelloInFiveSeconds(name){
var prompt = "Hello, " + name + "!";
// Inner functions are put in the local scope by default, as if they were
// declared with `var`.
function inner(){
alert(prompt);
}
setTimeout(inner, 5000);
// setTimeout is asynchronous, so the sayHelloInFiveSeconds function will
// exit immediately, and setTimeout will call inner afterwards. However,
// because inner is "closed over" sayHelloInFiveSeconds, inner still has
// access to the `prompt` variable when it is finally called.
}
sayHelloInFiveSeconds("Adam"); // will open a popup with "Hello, Adam!" in 5s
///////////////////////////////////
// 5. More about Objects; Constructors and Prototypes
// Objects can contain functions.
var myObj = {
myFunc: function(){
return "Hello world!";
}
};
myObj.myFunc(); // = "Hello world!"
// When functions attached to an object are called, they can access the object
// they're attached to using the `this` keyword.
myObj = {
myString: "Hello world!",
myFunc: function(){
return this.myString;
}
};
myObj.myFunc(); // = "Hello world!"
// What `this` is set to has to do with how the function is called, not where
// it's defined. So, our function doesn't work if it isn't called in the
// context of the object.
var myFunc = myObj.myFunc;
myFunc(); // = undefined
// Inversely, a function can be assigned to the object and gain access to it
// through `this`, even if it wasn't attached when it was defined.
var myOtherFunc = function(){
return this.myString.toUpperCase();
};
myObj.myOtherFunc = myOtherFunc;
myObj.myOtherFunc(); // = "HELLO WORLD!"
// We can also specify a context for a function to execute in when we invoke it
// using `call` or `apply`.
var anotherFunc = function(s){
return this.myString + s;
};
anotherFunc.call(myObj, " And Hello Moon!"); // = "Hello World! And Hello Moon!"
// The `apply` function is nearly identical, but takes an array for an argument
// list.
anotherFunc.apply(myObj, [" And Hello Sun!"]); // = "Hello World! And Hello Sun!"
// This is useful when working with a function that accepts a sequence of
// arguments and you want to pass an array.
Math.min(42, 6, 27); // = 6
Math.min([42, 6, 27]); // = NaN (uh-oh!)
Math.min.apply(Math, [42, 6, 27]); // = 6
// But, `call` and `apply` are only temporary. When we want it to stick, we can
// use `bind`.
var boundFunc = anotherFunc.bind(myObj);
boundFunc(" And Hello Saturn!"); // = "Hello World! And Hello Saturn!"
// `bind` can also be used to partially apply (curry) a function.
var product = function(a, b){ return a * b; };
var doubler = product.bind(this, 2);
doubler(8); // = 16
// When you call a function with the `new` keyword, a new object is created, and
// made available to the function via the `this` keyword. Functions designed to be
// called like that are called constructors.
var MyConstructor = function(){
this.myNumber = 5;
};
myNewObj = new MyConstructor(); // = {myNumber: 5}
myNewObj.myNumber; // = 5
// Unlike most other popular object-oriented languages, JavaScript has no
// concept of 'instances' created from 'class' blueprints; instead, JavaScript
// combines instantiation and inheritance into a single concept: a 'prototype'.
// Every JavaScript object has a 'prototype'. When you go to access a property
// on an object that doesn't exist on the actual object, the interpreter will
// look at its prototype.
// Some JS implementations let you access an object's prototype on the magic
// property `__proto__`. While this is useful for explaining prototypes it's not
// part of the standard; we'll get to standard ways of using prototypes later.
var myObj = {
myString: "Hello world!"
};
var myPrototype = {
meaningOfLife: 42,
myFunc: function(){
return this.myString.toLowerCase();
}
};
myObj.__proto__ = myPrototype;
myObj.meaningOfLife; // = 42
// This works for functions, too.
myObj.myFunc(); // = "hello world!"
// Of course, if your property isn't on your prototype, the prototype's
// prototype is searched, and so on.
myPrototype.__proto__ = {
myBoolean: true
};
myObj.myBoolean; // = true
// There's no copying involved here; each object stores a reference to its
// prototype. This means we can alter the prototype and our changes will be
// reflected everywhere.
myPrototype.meaningOfLife = 43;
myObj.meaningOfLife; // = 43
// The for/in statement allows iteration over properties of an object,
// walking up the prototype chain until it sees a null prototype.
for (var x in myObj){
console.log(myObj[x]);
}
///prints:
// Hello world!
// 43
// [Function: myFunc]
// true
// To only consider properties attached to the object itself
// and not its prototypes, use the `hasOwnProperty()` check.
for (var x in myObj){
if (myObj.hasOwnProperty(x)){
console.log(myObj[x]);
}
}
///prints:
// Hello world!
// We mentioned that `__proto__` was non-standard, and there's no standard way to
// change the prototype of an existing object. However, there are two ways to
// create a new object with a given prototype.
// The first is Object.create, which is a recent addition to JS, and therefore
// not available in all implementations yet.
var myObj = Object.create(myPrototype);
myObj.meaningOfLife; // = 43
// The second way, which works anywhere, has to do with constructors.
// Constructors have a property called prototype. This is *not* the prototype of
// the constructor function itself; instead, it's the prototype that new objects
// are given when they're created with that constructor and the new keyword.
MyConstructor.prototype = {
myNumber: 5,
getMyNumber: function(){
return this.myNumber;
}
};
var myNewObj2 = new MyConstructor();
myNewObj2.getMyNumber(); // = 5
myNewObj2.myNumber = 6;
myNewObj2.getMyNumber(); // = 6
// Built-in types like strings and numbers also have constructors that create
// equivalent wrapper objects.
var myNumber = 12;
var myNumberObj = new Number(12);
myNumber == myNumberObj; // = true
// Except, they aren't exactly equivalent.
typeof myNumber; // = 'number'
typeof myNumberObj; // = 'object'
myNumber === myNumberObj; // = false
if (0){
// This code won't execute, because 0 is falsy.
}
if (new Number(0)){
// This code will execute, because wrapped numbers are objects, and objects
// are always truthy.
}
// However, the wrapper objects and the regular builtins share a prototype, so
// you can actually add functionality to a string, for instance.
String.prototype.firstCharacter = function(){
return this.charAt(0);
};
"abc".firstCharacter(); // = "a"
// This fact is often used in "polyfilling", which is implementing newer
// features of JavaScript in an older subset of JavaScript, so that they can be
// used in older environments such as outdated browsers.
// For instance, we mentioned that Object.create isn't yet available in all
// implementations, but we can still use it with this polyfill:
if (Object.create === undefined){ // don't overwrite it if it exists
Object.create = function(proto){
// make a temporary constructor with the right prototype
var Constructor = function(){};
Constructor.prototype = proto;
// then use it to create a new, appropriately-prototyped object
return new Constructor();
};
}
// ES6 Additions
// The "let" keyword allows you to define variables in a lexical scope,
// as opposed to a function scope like the var keyword does.
let name = "Billy";
// Variables defined with let can be reassigned new values.
name = "William";
// The "const" keyword allows you to define a variable in a lexical scope
// like with let, but you cannot reassign the value once one has been assigned.
const pi = 3.14;
pi = 4.13; // You cannot do this.
// There is a new syntax for functions in ES6 known as "lambda syntax".
// This allows functions to be defined in a lexical scope like with variables
// defined by const and let.
const isEven = (number) => {
return number % 2 === 0;
};
isEven(7); // false
// The "equivalent" of this function in the traditional syntax would look like this:
function isEven(number) {
return number % 2 === 0;
};
// I put the word "equivalent" in double quotes because a function defined
// using the lambda syntax cannot be called before the definition.
// The following is an example of invalid usage:
add(1, 8);
const add = (firstNumber, secondNumber) => {
return firstNumber + secondNumber;
};
Kotlin:
// Single-line comments start with //
/*
Multi-line comments look like this.
*/
// The "package" keyword works in the same way as in Java.
package com.learnxinyminutes.kotlin
/*
The entry point to a Kotlin program is a function named "main".
The function is passed an array containing any command-line arguments.
Since Kotlin 1.3 the "main" function can also be defined without
any parameters.
*/
fun main(args: Array<String>) {
/*
Declaring variables is done using either "var" or "val".
"val" declarations cannot be reassigned, whereas "vars" can.
*/
val fooVal = 10 // we cannot later reassign fooVal to something else
var fooVar = 10
fooVar = 20 // fooVar can be reassigned
/*
In most cases, Kotlin can determine what the type of a variable is,
so we don't have to explicitly specify it every time.
We can explicitly declare the type of a variable like so:
*/
val foo: Int = 7
/*
Strings can be represented in a similar way as in Java.
Escaping is done with a backslash.
*/
val fooString = "My String Is Here!"
val barString = "Printing on a new line?\nNo Problem!"
val bazString = "Do you want to add a tab?\tNo Problem!"
println(fooString)
println(barString)
println(bazString)
/*
A raw string is delimited by a triple quote (""").
Raw strings can contain newlines and any other characters.
*/
val fooRawString = """
fun helloWorld(val name : String) {
println("Hello, world!")
}
"""
println(fooRawString)
/*
Strings can contain template expressions.
A template expression starts with a dollar sign ($).
*/
val fooTemplateString = "$fooString has ${fooString.length} characters"
println(fooTemplateString) // => My String Is Here! has 18 characters
/*
For a variable to hold null it must be explicitly specified as nullable.
A variable can be specified as nullable by appending a ? to its type.
We can access a nullable variable by using the ?. operator.
We can use the ?: operator to specify an alternative value to use
if a variable is null.
*/
var fooNullable: String? = "abc"
println(fooNullable?.length) // => 3
println(fooNullable?.length ?: -1) // => 3
fooNullable = null
println(fooNullable?.length) // => null
println(fooNullable?.length ?: -1) // => -1
/*
Functions can be declared using the "fun" keyword.
Function arguments are specified in brackets after the function name.
Function arguments can optionally have a default value.
The function return type, if required, is specified after the arguments.
*/
fun hello(name: String = "world"): String {
return "Hello, $name!"
}
println(hello("foo")) // => Hello, foo!
println(hello(name = "bar")) // => Hello, bar!
println(hello()) // => Hello, world!
/*
A function parameter may be marked with the "vararg" keyword
to allow a variable number of arguments to be passed to the function.
*/
fun varargExample(vararg names: Int) {
println("Argument has ${names.size} elements")
}
varargExample() // => Argument has 0 elements
varargExample(1) // => Argument has 1 elements
varargExample(1, 2, 3) // => Argument has 3 elements
/*
When a function consists of a single expression then the curly brackets can
be omitted. The body is specified after the = symbol.
*/
fun odd(x: Int): Boolean = x % 2 == 1
println(odd(6)) // => false
println(odd(7)) // => true
// If the return type can be inferred then we don't need to specify it.
fun even(x: Int) = x % 2 == 0
println(even(6)) // => true
println(even(7)) // => false
/*
You can also use lambda functions, with the '->' operator seperating
the parameters from the function body.
*/
val fooLambda: (Int) -> Int = {n -> n + 1}
println(fooLambda(1)) // => 2
// Functions can take functions as arguments and return functions.
fun not(f: (Int) -> Boolean): (Int) -> Boolean {
return {n -> !f.invoke(n)}
}
// Named functions can be specified as arguments using the :: operator.
val notOdd = not(::odd)
val notEven = not(::even)
/*
Lambda expressions can be specified as arguments.
If it's the only argument parentheses can be omitted.
*/
val notZero = not {n -> n == 0}
/*
If a lambda has only one parameter
then its declaration can be omitted (along with the ->).
The name of the single parameter will be "it".
*/
val notPositive = not {it > 0}
for (i in 0..4) {
println("${notOdd(i)} ${notEven(i)} ${notZero(i)} ${notPositive(i)}")
}
// The "class" keyword is used to declare classes.
class ExampleClass(val x: Int) {
fun memberFunction(y: Int): Int {
return x + y
}
infix fun infixMemberFunction(y: Int): Int {
return x * y
}
}
/*
To create a new instance we call the constructor.
Note that Kotlin does not have a "new" keyword.
*/
val fooExampleClass = ExampleClass(7)
// Member functions can be called using dot notation.
println(fooExampleClass.memberFunction(4)) // => 11
/*
If a function has been marked with the "infix" keyword then it can be
called using infix notation.
*/
println(fooExampleClass infixMemberFunction 4) // => 28
/*
Data classes are a concise way to create classes that just hold data.
The "hashCode"/"equals" and "toString" methods are automatically generated.
*/
data class DataClassExample (val x: Int, val y: Int, val z: Int)
val fooData = DataClassExample(1, 2, 4)
println(fooData) // => DataClassExample(x=1, y=2, z=4)
// Data classes have a "copy" function.
val fooCopy = fooData.copy(y = 100)
println(fooCopy) // => DataClassExample(x=1, y=100, z=4)
// Objects can be destructured into multiple variables.
val (a, b, c) = fooCopy
println("$a $b $c") // => 1 100 4
// destructuring in "for" loop
for ((a, b, c) in listOf(fooData)) {
println("$a $b $c") // => 1 2 4
}
val mapData = mapOf("a" to 1, "b" to 2)
// Map.Entry is destructurable as well
for ((key, value) in mapData) {
println("$key -> $value")
}
// The "with" function is similar to the JavaScript "with" statement.
data class MutableDataClassExample (var x: Int, var y: Int, var z: Int)
val fooMutableData = MutableDataClassExample(7, 4, 9)
with (fooMutableData) {
x -= 2
y += 2
z--
}
println(fooMutableData) // => MutableDataClassExample(x=5, y=6, z=8)
/*
We can create a list using the "listOf" function.
The list will be immutable - elements cannot be added or removed.
*/
val fooList = listOf("a", "b", "c")
println(fooList.size) // => 3
println(fooList.first()) // => a
println(fooList.last()) // => c
// Elements of a list can be accessed by their index.
println(fooList[1]) // => b
// A mutable list can be created using the "mutableListOf" function.
val fooMutableList = mutableListOf("a", "b", "c")
fooMutableList.add("d")
println(fooMutableList.last()) // => d
println(fooMutableList.size) // => 4
// We can create a set using the "setOf" function.
val fooSet = setOf("a", "b", "c")
println(fooSet.contains("a")) // => true
println(fooSet.contains("z")) // => false
// We can create a map using the "mapOf" function.
val fooMap = mapOf("a" to 8, "b" to 7, "c" to 9)
// Map values can be accessed by their key.
println(fooMap["a"]) // => 8
/*
Sequences represent lazily-evaluated collections.
We can create a sequence using the "generateSequence" function.
*/
val fooSequence = generateSequence(1, { it + 1 })
val x = fooSequence.take(10).toList()
println(x) // => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
// An example of using a sequence to generate Fibonacci numbers:
fun fibonacciSequence(): Sequence<Long> {
var a = 0L
var b = 1L
fun next(): Long {
val result = a + b
a = b
b = result
return a
}
return generateSequence(::next)
}
val y = fibonacciSequence().take(10).toList()
println(y) // => [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
// Kotlin provides higher-order functions for working with collections.
val z = (1..9).map {it * 3}
.filter {it < 20}
.groupBy {it % 2 == 0}
.mapKeys {if (it.key) "even" else "odd"}
println(z) // => {odd=[3, 9, 15], even=[6, 12, 18]}
// A "for" loop can be used with anything that provides an iterator.
for (c in "hello") {
println(c)
}
// "while" loops work in the same way as other languages.
var ctr = 0
while (ctr < 5) {
println(ctr)
ctr++
}
do {
println(ctr)
ctr++
} while (ctr < 10)
/*
"if" can be used as an expression that returns a value.
For this reason the ternary ?: operator is not needed in Kotlin.
*/
val num = 5
val message = if (num % 2 == 0) "even" else "odd"
println("$num is $message") // => 5 is odd
// "when" can be used as an alternative to "if-else if" chains.
val i = 10
when {
i < 7 -> println("first block")
fooString.startsWith("hello") -> println("second block")
else -> println("else block")
}
// "when" can be used with an argument.
when (i) {
0, 21 -> println("0 or 21")
in 1..20 -> println("in the range 1 to 20")
else -> println("none of the above")
}
// "when" can be used as an expression that returns a value.
var result = when (i) {
0, 21 -> "0 or 21"
in 1..20 -> "in the range 1 to 20"
else -> "none of the above"
}
println(result)
/*
We can check if an object is of a particular type by using the "is" operator.
If an object passes a type check then it can be used as that type without
explicitly casting it.
*/
fun smartCastExample(x: Any) : Boolean {
if (x is Boolean) {
// x is automatically cast to Boolean
return x
} else if (x is Int) {
// x is automatically cast to Int
return x > 0
} else if (x is String) {
// x is automatically cast to String
return x.isNotEmpty()
} else {
return false
}
}
println(smartCastExample("Hello, world!")) // => true
println(smartCastExample("")) // => false
println(smartCastExample(5)) // => true
println(smartCastExample(0)) // => false
println(smartCastExample(true)) // => true
// Smartcast also works with when block
fun smartCastWhenExample(x: Any) = when (x) {
is Boolean -> x
is Int -> x > 0
is String -> x.isNotEmpty()
else -> false
}
/*
Extensions are a way to add new functionality to a class.
This is similar to C# extension methods.
*/
fun String.remove(c: Char): String {
return this.filter {it != c}
}
println("Hello, world!".remove('l')) // => Heo, word!
}
// Enum classes are similar to Java enum types.
enum class EnumExample {
A, B, C // Enum constants are separated with commas.
}
fun printEnum() = println(EnumExample.A) // => A
// Since each enum is an instance of the enum class, they can be initialized as:
enum class EnumExample(val value: Int) {
A(value = 1),
B(value = 2),
C(value = 3)
}
fun printProperty() = println(EnumExample.A.value) // => 1
// Every enum has properties to obtain its name and ordinal(position) in the enum class declaration:
fun printName() = println(EnumExample.A.name) // => A
fun printPosition() = println(EnumExample.A.ordinal) // => 0
/*
The "object" keyword can be used to create singleton objects.
We cannot instantiate it but we can refer to its unique instance by its name.
This is similar to Scala singleton objects.
*/
object ObjectExample {
fun hello(): String {
return "hello"
}
override fun toString(): String {
return "Hello, it's me, ${ObjectExample::class.simpleName}"
}
}
fun useSingletonObject() {
println(ObjectExample.hello()) // => hello
// In Kotlin, "Any" is the root of the class hierarchy, just like "Object" is in Java
val someRef: Any = ObjectExample
println(someRef) // => Hello, it's me, ObjectExample
}
/* The not-null assertion operator (!!) converts any value to a non-null type and
throws an exception if the value is null.
*/
var b: String? = "abc"
val l = b!!.length
data class Counter(var value: Int) {
// overload Counter += Int
operator fun plusAssign(increment: Int) {
this.value += increment
}
// overload Counter++ and ++Counter
operator fun inc() = Counter(value + 1)
// overload Counter + Counter
operator fun plus(other: Counter) = Counter(this.value + other.value)
// overload Counter * Counter
operator fun times(other: Counter) = Counter(this.value * other.value)
// overload Counter * Int
operator fun times(value: Int) = Counter(this.value * value)
// overload Counter in Counter
operator fun contains(other: Counter) = other.value == this.value
// overload Counter[Int] = Int
operator fun set(index: Int, value: Int) {
this.value = index + value
}
// overload Counter instance invocation
operator fun invoke() = println("The value of the counter is $value")
}
/* You can also overload operators through extension methods */
// overload -Counter
operator fun Counter.unaryMinus() = Counter(-this.value)
fun operatorOverloadingDemo() {
var counter1 = Counter(0)
var counter2 = Counter(5)
counter1 += 7
println(counter1) // => Counter(value=7)
println(counter1 + counter2) // => Counter(value=12)
println(counter1 * counter2) // => Counter(value=35)
println(counter2 * 2) // => Counter(value=10)
println(counter1 in Counter(5)) // => false
println(counter1 in Counter(7)) // => true
counter1[26] = 10
println(counter1) // => Counter(value=36)
counter1() // => The value of the counter is 36
println(-counter2) // => Counter(value=-5)
}
Lua:
-- Two dashes start a one-line comment.
--[[
Adding two ['s and ]'s makes it a
multi-line comment.
--]]
----------------------------------------------------
-- 1. Variables and flow control.
----------------------------------------------------
num = 42 -- Numbers can be integer or floating point.
s = 'walternate' -- Immutable strings like Python.
t = "double-quotes are also fine"
u = [[ Double brackets
start and end
multi-line strings.]]
t = nil -- Undefines t; Lua has garbage collection.
-- Blocks are denoted with keywords like do/end:
while num < 50 do
num = num + 1 -- No ++ or += type operators.
end
-- If clauses:
if num > 40 then
print('over 40')
elseif s ~= 'walternate' then -- ~= is not equals.
-- Equality check is == like Python; ok for strs.
io.write('not over 40\n') -- Defaults to stdout.
else
-- Variables are global by default.
thisIsGlobal = 5 -- Camel case is common.
-- How to make a variable local:
local line = io.read() -- Reads next stdin line.
-- String concatenation uses the .. operator:
print('Winter is coming, ' .. line)
end
-- Undefined variables return nil.
-- This is not an error:
foo = anUnknownVariable -- Now foo = nil.
aBoolValue = false
-- Only nil and false are falsy; 0 and '' are true!
if not aBoolValue then print('it was false') end
-- 'or' and 'and' are short-circuited.
-- This is similar to the a?b:c operator in C/js:
ans = aBoolValue and 'yes' or 'no' --> 'no'
karlSum = 0
for i = 1, 100 do -- The range includes both ends.
karlSum = karlSum + i
end
-- Use "100, 1, -1" as the range to count down:
fredSum = 0
for j = 100, 1, -1 do fredSum = fredSum + j end
-- In general, the range is begin, end[, step].
-- Another loop construct:
repeat
print('the way of the future')
num = num - 1
until num == 0
----------------------------------------------------
-- 2. Functions.
----------------------------------------------------
function fib(n)
if n < 2 then return 1 end
return fib(n - 2) + fib(n - 1)
end
-- Closures and anonymous functions are ok:
function adder(x)
-- The returned function is created when adder is
-- called, and remembers the value of x:
return function (y) return x + y end
end
a1 = adder(9)
a2 = adder(36)
print(a1(16)) --> 25
print(a2(64)) --> 100
-- Returns, func calls, and assignments all work
-- with lists that may be mismatched in length.
-- Unmatched receivers are nil;
-- unmatched senders are discarded.
x, y, z = 1, 2, 3, 4
-- Now x = 1, y = 2, z = 3, and 4 is thrown away.
function bar(a, b, c)
print(a, b, c)
return 4, 8, 15, 16, 23, 42
end
x, y = bar('zaphod') --> prints "zaphod nil nil"
-- Now x = 4, y = 8, values 15...42 are discarded.
-- Functions are first-class, may be local/global.
-- These are the same:
function f(x) return x * x end
f = function (x) return x * x end
-- And so are these:
local function g(x) return math.sin(x) end
local g; g = function (x) return math.sin(x) end
-- the 'local g' decl makes g-self-references ok.
-- Trig funcs work in radians, by the way.
-- Calls with one string param don't need parens:
print 'hello' -- Works fine.
----------------------------------------------------
-- 3. Tables.
----------------------------------------------------
-- Tables = Lua's only compound data structure;
-- they are associative arrays.
-- Similar to php arrays or js objects, they are
-- hash-lookup dicts that can also be used as lists.
-- Using tables as dictionaries / maps:
-- Dict literals have string keys by default:
t = {key1 = 'value1', key2 = false}
-- String keys can use js-like dot notation:
print(t.key1) -- Prints 'value1'.
t.newKey = {} -- Adds a new key/value pair.
t.key2 = nil -- Removes key2 from the table.
-- Literal notation for any (non-nil) value as key:
u = {['@!#'] = 'qbert', [{}] = 1729, [6.28] = 'tau'}
print(u[6.28]) -- prints "tau"
-- Key matching is basically by value for numbers
-- and strings, but by identity for tables.
a = u['@!#'] -- Now a = 'qbert'.
b = u[{}] -- We might expect 1729, but it's nil:
-- b = nil since the lookup fails. It fails
-- because the key we used is not the same object
-- as the one used to store the original value. So
-- strings & numbers are more portable keys.
-- A one-table-param function call needs no parens:
function h(x) print(x.key1) end
h{key1 = 'Sonmi~451'} -- Prints 'Sonmi~451'.
for key, val in pairs(u) do -- Table iteration.
print(key, val)
end
-- _G is a special table of all globals.
print(_G['_G'] == _G) -- Prints 'true'.
-- Using tables as lists / arrays:
-- List literals implicitly set up int keys:
v = {'value1', 'value2', 1.21, 'gigawatts'}
for i = 1, #v do -- #v is the size of v for lists.
print(v[i]) -- Indices start at 1 !! SO CRAZY!
end
-- A 'list' is not a real type. v is just a table
-- with consecutive integer keys, treated as a list.
----------------------------------------------------
-- 3.1 Metatables and metamethods.
----------------------------------------------------
-- A table can have a metatable that gives the table
-- operator-overloadish behavior. Later we'll see
-- how metatables support js-prototype behavior.
f1 = {a = 1, b = 2} -- Represents the fraction a/b.
f2 = {a = 2, b = 3}
-- This would fail:
-- s = f1 + f2
metafraction = {}
function metafraction.__add(f1, f2)
sum = {}
sum.b = f1.b * f2.b
sum.a = f1.a * f2.b + f2.a * f1.b
return sum
end
setmetatable(f1, metafraction)
setmetatable(f2, metafraction)
s = f1 + f2 -- call __add(f1, f2) on f1's metatable
-- f1, f2 have no key for their metatable, unlike
-- prototypes in js, so you must retrieve it as in
-- getmetatable(f1). The metatable is a normal table
-- with keys that Lua knows about, like __add.
-- But the next line fails since s has no metatable:
-- t = s + s
-- Class-like patterns given below would fix this.
-- An __index on a metatable overloads dot lookups:
defaultFavs = {animal = 'gru', food = 'donuts'}
myFavs = {food = 'pizza'}
setmetatable(myFavs, {__index = defaultFavs})
eatenBy = myFavs.animal -- works! thanks, metatable
-- Direct table lookups that fail will retry using
-- the metatable's __index value, and this recurses.
-- An __index value can also be a function(tbl, key)
-- for more customized lookups.
-- Values of __index,add, .. are called metamethods.
-- Full list. Here a is a table with the metamethod.
-- __add(a, b) for a + b
-- __sub(a, b) for a - b
-- __mul(a, b) for a * b
-- __div(a, b) for a / b
-- __mod(a, b) for a % b
-- __pow(a, b) for a ^ b
-- __unm(a) for -a
-- __concat(a, b) for a .. b
-- __len(a) for #a
-- __eq(a, b) for a == b
-- __lt(a, b) for a < b
-- __le(a, b) for a <= b
-- __index(a, b) <fn or a table> for a.b
-- __newindex(a, b, c) for a.b = c
-- __call(a, ...) for a(...)
----------------------------------------------------
-- 3.2 Class-like tables and inheritance.
----------------------------------------------------
-- Classes aren't built in; there are different ways
-- to make them using tables and metatables.
-- Explanation for this example is below it.
Dog = {} -- 1.
function Dog:new() -- 2.
newObj = {sound = 'woof'} -- 3.
self.__index = self -- 4.
return setmetatable(newObj, self) -- 5.
end
function Dog:makeSound() -- 6.
print('I say ' .. self.sound)
end
mrDog = Dog:new() -- 7.
mrDog:makeSound() -- 'I say woof' -- 8.
-- 1. Dog acts like a class; it's really a table.
-- 2. function tablename:fn(...) is the same as
-- function tablename.fn(self, ...)
-- The : just adds a first arg called self.
-- Read 7 & 8 below for how self gets its value.
-- 3. newObj will be an instance of class Dog.
-- 4. self = the class being instantiated. Often
-- self = Dog, but inheritance can change it.
-- newObj gets self's functions when we set both
-- newObj's metatable and self's __index to self.
-- 5. Reminder: setmetatable returns its first arg.
-- 6. The : works as in 2, but this time we expect
-- self to be an instance instead of a class.
-- 7. Same as Dog.new(Dog), so self = Dog in new().
-- 8. Same as mrDog.makeSound(mrDog); self = mrDog.
----------------------------------------------------
-- Inheritance example:
LoudDog = Dog:new() -- 1.
function LoudDog:makeSound()
s = self.sound .. ' ' -- 2.
print(s .. s .. s)
end
seymour = LoudDog:new() -- 3.
seymour:makeSound() -- 'woof woof woof' -- 4.
-- 1. LoudDog gets Dog's methods and variables.
-- 2. self has a 'sound' key from new(), see 3.
-- 3. Same as LoudDog.new(LoudDog), and converted to
-- Dog.new(LoudDog) as LoudDog has no 'new' key,
-- but does have __index = Dog on its metatable.
-- Result: seymour's metatable is LoudDog, and
-- LoudDog.__index = LoudDog. So seymour.key will
-- = seymour.key, LoudDog.key, Dog.key, whichever
-- table is the first with the given key.
-- 4. The 'makeSound' key is found in LoudDog; this
-- is the same as LoudDog.makeSound(seymour).
-- If needed, a subclass's new() is like the base's:
function LoudDog:new()
newObj = {}
-- set up newObj
self.__index = self
return setmetatable(newObj, self)
end
----------------------------------------------------
-- 4. Modules.
----------------------------------------------------
--[[ I'm commenting out this section so the rest of
-- this script remains runnable.
-- Suppose the file mod.lua looks like this:
local M = {}
local function sayMyName()
print('Hrunkner')
end
function M.sayHello()
print('Why hello there')
sayMyName()
end
return M
-- Another file can use mod.lua's functionality:
local mod = require('mod') -- Run the file mod.lua.
-- require is the standard way to include modules.
-- require acts like: (if not cached; see below)
local mod = (function ()
<contents of mod.lua>
end)()
-- It's like mod.lua is a function body, so that
-- locals inside mod.lua are invisible outside it.
-- This works because mod here = M in mod.lua:
mod.sayHello() -- Prints: Why hello there Hrunkner
-- This is wrong; sayMyName only exists in mod.lua:
mod.sayMyName() -- error
-- require's return values are cached so a file is
-- run at most once, even when require'd many times.
-- Suppose mod2.lua contains "print('Hi!')".
local a = require('mod2') -- Prints Hi!
local b = require('mod2') -- Doesn't print; a=b.
-- dofile is like require without caching:
dofile('mod2.lua') --> Hi!
dofile('mod2.lua') --> Hi! (runs it again)
-- loadfile loads a lua file but doesn't run it yet.
f = loadfile('mod2.lua') -- Call f() to run it.
-- load is loadfile for strings.
-- (loadstring is deprecated, use load instead)
g = load('print(343)') -- Returns a function.
g() -- Prints out 343; nothing printed before now.
--]]
Objective-C:
// Single-line comments start with //
/*
Multi-line comments look like this
*/
// XCode supports pragma mark directive that improve jump bar readability
#pragma mark Navigation Functions // New tag on jump bar named 'Navigation Functions'
#pragma mark - Navigation Functions // Same tag, now with a separator
// Imports the Foundation headers with #import
// Use <> to import global files (in general frameworks)
// Use "" to import local files (from project)
#import <Foundation/Foundation.h>
#import "MyClass.h"
// If you enable modules for iOS >= 7.0 or OS X >= 10.9 projects in
// Xcode 5 you can import frameworks like that:
@import Foundation;
// Your program's entry point is a function called
// main with an integer return type
int main (int argc, const char * argv[])
{
// Create an autorelease pool to manage the memory into the program
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
// If using automatic reference counting (ARC), use @autoreleasepool instead:
@autoreleasepool {
// Use NSLog to print lines to the console
NSLog(@"Hello World!"); // Print the string "Hello World!"
///////////////////////////////////////
// Types & Variables
///////////////////////////////////////
// Primitive declarations
int myPrimitive1 = 1;
long myPrimitive2 = 234554664565;
// Object declarations
// Put the * in front of the variable names for strongly-typed object declarations
MyClass *myObject1 = nil; // Strong typing
id myObject2 = nil; // Weak typing
// %@ is an object
// 'description' is a convention to display the value of the Objects
NSLog(@"%@ and %@", myObject1, [myObject2 description]); // prints => "(null) and (null)"
// String
NSString *worldString = @"World";
NSLog(@"Hello %@!", worldString); // prints => "Hello World!"
// NSMutableString is a mutable version of the NSString object
NSMutableString *mutableString = [NSMutableString stringWithString:@"Hello"];
[mutableString appendString:@" World!"];
NSLog(@"%@", mutableString); // prints => "Hello World!"
// Character literals
NSNumber *theLetterZNumber = @'Z';
char theLetterZ = [theLetterZNumber charValue]; // or 'Z'
NSLog(@"%c", theLetterZ);
// Integral literals
NSNumber *fortyTwoNumber = @42;
int fortyTwo = [fortyTwoNumber intValue]; // or 42
NSLog(@"%i", fortyTwo);
NSNumber *fortyTwoUnsignedNumber = @42U;
unsigned int fortyTwoUnsigned = [fortyTwoUnsignedNumber unsignedIntValue]; // or 42
NSLog(@"%u", fortyTwoUnsigned);
NSNumber *fortyTwoShortNumber = [NSNumber numberWithShort:42];
short fortyTwoShort = [fortyTwoShortNumber shortValue]; // or 42
NSLog(@"%hi", fortyTwoShort);
NSNumber *fortyOneShortNumber = [NSNumber numberWithShort:41];
unsigned short fortyOneUnsigned = [fortyOneShortNumber unsignedShortValue]; // or 41
NSLog(@"%u", fortyOneUnsigned);
NSNumber *fortyTwoLongNumber = @42L;
long fortyTwoLong = [fortyTwoLongNumber longValue]; // or 42
NSLog(@"%li", fortyTwoLong);
NSNumber *fiftyThreeLongNumber = @53L;
unsigned long fiftyThreeUnsigned = [fiftyThreeLongNumber unsignedLongValue]; // or 53
NSLog(@"%lu", fiftyThreeUnsigned);
// Floating point literals
NSNumber *piFloatNumber = @3.141592654F;
float piFloat = [piFloatNumber floatValue]; // or 3.141592654f
NSLog(@"%f", piFloat); // prints => 3.141592654
NSLog(@"%5.2f", piFloat); // prints => " 3.14"
NSNumber *piDoubleNumber = @3.1415926535;
double piDouble = [piDoubleNumber doubleValue]; // or 3.1415926535
NSLog(@"%f", piDouble);
NSLog(@"%4.2f", piDouble); // prints => "3.14"
// NSDecimalNumber is a fixed-point class that's more precise than float or double
NSDecimalNumber *oneDecNum = [NSDecimalNumber decimalNumberWithString:@"10.99"];
NSDecimalNumber *twoDecNum = [NSDecimalNumber decimalNumberWithString:@"5.002"];
// NSDecimalNumber isn't able to use standard +, -, *, / operators so it provides its own:
[oneDecNum decimalNumberByAdding:twoDecNum];
[oneDecNum decimalNumberBySubtracting:twoDecNum];
[oneDecNum decimalNumberByMultiplyingBy:twoDecNum];
[oneDecNum decimalNumberByDividingBy:twoDecNum];
NSLog(@"%@", oneDecNum); // prints => 10.99 as NSDecimalNumber is immutable
// BOOL literals
NSNumber *yesNumber = @YES;
NSNumber *noNumber = @NO;
// or
BOOL yesBool = YES;
BOOL noBool = NO;
NSLog(@"%i", yesBool); // prints => 1
// Array object
// May contain different data types, but must be an Objective-C object
NSArray *anArray = @[@1, @2, @3, @4];
NSNumber *thirdNumber = anArray[2];
NSLog(@"Third number = %@", thirdNumber); // prints => "Third number = 3"
// Since Xcode 7, NSArray objects can be typed (Generics)
NSArray<NSString *> *stringArray = @[@"hello", @"world"];
// NSMutableArray is a mutable version of NSArray, allowing you to change
// the items in the array and to extend or shrink the array object.
// Convenient, but not as efficient as NSArray.
NSMutableArray *mutableArray = [NSMutableArray arrayWithCapacity:2];
[mutableArray addObject:@"Hello"];
[mutableArray addObject:@"World"];
[mutableArray removeObjectAtIndex:0];
NSLog(@"%@", [mutableArray objectAtIndex:0]); // prints => "World"
// Dictionary object
NSDictionary *aDictionary = @{ @"key1" : @"value1", @"key2" : @"value2" };
NSObject *valueObject = aDictionary[@"A Key"];
NSLog(@"Object = %@", valueObject); // prints => "Object = (null)"
// Since Xcode 7, NSDictionary objects can be typed (Generics)
NSDictionary<NSString *, NSNumber *> *numberDictionary = @{@"a": @1, @"b": @2};
// NSMutableDictionary also available as a mutable dictionary object
NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionaryWithCapacity:2];
[mutableDictionary setObject:@"value1" forKey:@"key1"];
[mutableDictionary setObject:@"value2" forKey:@"key2"];
[mutableDictionary removeObjectForKey:@"key1"];
// Change types from Mutable To Immutable
//In general [object mutableCopy] will make the object mutable whereas [object copy] will make the object immutable
NSMutableDictionary *aMutableDictionary = [aDictionary mutableCopy];
NSDictionary *mutableDictionaryChanged = [mutableDictionary copy];
// Set object
NSSet *set = [NSSet setWithObjects:@"Hello", @"Hello", @"World", nil];
NSLog(@"%@", set); // prints => {(Hello, World)} (may be in different order)
// Since Xcode 7, NSSet objects can be typed (Generics)
NSSet<NSString *> *stringSet = [NSSet setWithObjects:@"hello", @"world", nil];
// NSMutableSet also available as a mutable set object
NSMutableSet *mutableSet = [NSMutableSet setWithCapacity:2];
[mutableSet addObject:@"Hello"];
[mutableSet addObject:@"Hello"];
NSLog(@"%@", mutableSet); // prints => {(Hello)}
///////////////////////////////////////
// Operators
///////////////////////////////////////
// The operators works like in the C language
// For example:
2 + 5; // => 7
4.2f + 5.1f; // => 9.3f
3 == 2; // => 0 (NO)
3 != 2; // => 1 (YES)
1 && 1; // => 1 (Logical and)
0 || 1; // => 1 (Logical or)
~0x0F; // => 0xF0 (bitwise negation)
0x0F & 0xF0; // => 0x00 (bitwise AND)
0x01 << 1; // => 0x02 (bitwise left shift (by 1))
///////////////////////////////////////
// Control Structures
///////////////////////////////////////
// If-Else statement
if (NO)
{
NSLog(@"I am never run");
} else if (0)
{
NSLog(@"I am also never run");
} else
{
NSLog(@"I print");
}
// Switch statement
switch (2)
{
case 0:
{
NSLog(@"I am never run");
} break;
case 1:
{
NSLog(@"I am also never run");
} break;
default:
{
NSLog(@"I print");
} break;
}
// While loops statements
int ii = 0;
while (ii < 4)
{
NSLog(@"%d,", ii++); // ii++ increments ii in-place, after using its value
} // prints => "0,"
// "1,"
// "2,"
// "3,"
// For loops statements
int jj;
for (jj=0; jj < 4; jj++)
{
NSLog(@"%d,", jj);
} // prints => "0,"
// "1,"
// "2,"
// "3,"
// Foreach statements
NSArray *values = @[@0, @1, @2, @3];
for (NSNumber *value in values)
{
NSLog(@"%@,", value);
} // prints => "0,"
// "1,"
// "2,"
// "3,"
// Object for loop statement. Can be used with any Objective-C object type
for (id item in values) {
NSLog(@"%@,", item);
} // prints => "0,"
// "1,"
// "2,"
// "3,"
// Try-Catch-Finally statements
@try
{
// Your statements here
@throw [NSException exceptionWithName:@"FileNotFoundException"
reason:@"File Not Found on System" userInfo:nil];
} @catch (NSException * e) // use: @catch (id exceptionName) to catch all objects.
{
NSLog(@"Exception: %@", e);
} @finally
{
NSLog(@"Finally. Time to clean up.");
} // prints => "Exception: File Not Found on System"
// "Finally. Time to clean up."
// NSError objects are useful for function arguments to populate on user mistakes.
NSError *error = [NSError errorWithDomain:@"Invalid email." code:4 userInfo:nil];
///////////////////////////////////////
// Objects
///////////////////////////////////////
// Create an object instance by allocating memory and initializing it
// An object is not fully functional until both steps have been completed
MyClass *myObject = [[MyClass alloc] init];
// The Objective-C model of object-oriented programming is based on message
// passing to object instances
// In Objective-C one does not simply call a method; one sends a message
[myObject instanceMethodWithParameter:@"Steve Jobs"];
// Clean up the memory you used into your program
[pool drain];
// End of @autoreleasepool
}
// End the program
return 0;
}
///////////////////////////////////////
// Classes And Functions
///////////////////////////////////////
// Declare your class in a header file (MyClass.h):
// Class declaration syntax:
// @interface ClassName : ParentClassName <ImplementedProtocols>
// {
// type name; <= variable declarations;
// }
// @property type name; <= property declarations
// -/+ (type) Method declarations; <= Method declarations
// @end
@interface MyClass : NSObject <MyProtocol> // NSObject is Objective-C's base object class.
{
// Instance variable declarations (can exist in either interface or implementation file)
int count; // Protected access by default.
@private id data; // Private access (More convenient to declare in implementation file)
NSString *name;
}
// Convenient notation for public access variables to auto generate a setter method
// By default, setter method name is 'set' followed by @property variable name
@property int propInt; // Setter method name = 'setPropInt'
@property (copy) id copyId; // (copy) => Copy the object during assignment
// (readonly) => Cannot set value outside @interface
@property (readonly) NSString *roString; // Use @synthesize in @implementation to create accessor
// You can customize the getter and setter names instead of using default 'set' name:
@property (getter=lengthGet, setter=lengthSet:) int length;
// Methods
+/- (return type)methodSignature:(Parameter Type *)parameterName;
// + for class methods:
+ (NSString *)classMethod;
+ (MyClass *)myClassFromHeight:(NSNumber *)defaultHeight;
// - for instance methods:
- (NSString *)instanceMethodWithParameter:(NSString *)string;
- (NSNumber *)methodAParameterAsString:(NSString*)string andAParameterAsNumber:(NSNumber *)number;
// Constructor methods with arguments:
- (id)initWithDistance:(int)defaultDistance;
// Objective-C method names are very descriptive. Always name methods according to their arguments
@end // States the end of the interface
// To access public variables from the implementation file, @property generates a setter method
// automatically. Method name is 'set' followed by @property variable name:
MyClass *myClass = [[MyClass alloc] init]; // create MyClass object instance
[myClass setCount:10];
NSLog(@"%d", [myClass count]); // prints => 10
// Or using the custom getter and setter method defined in @interface:
[myClass lengthSet:32];
NSLog(@"%i", [myClass lengthGet]); // prints => 32
// For convenience, you may use dot notation to set and access object instance variables:
myClass.count = 45;
NSLog(@"%i", myClass.count); // prints => 45
// Call class methods:
NSString *classMethodString = [MyClass classMethod];
MyClass *classFromName = [MyClass myClassFromName:@"Hello"];
// Call instance methods:
MyClass *myClass = [[MyClass alloc] init]; // Create MyClass object instance
NSString *stringFromInstanceMethod = [myClass instanceMethodWithParameter:@"Hello"];
// Selectors
// Way to dynamically represent methods. Used to call methods of a class, pass methods
// through functions to tell other classes they should call it, and to save methods
// as a variable
// SEL is the data type. @selector() returns a selector from method name provided
// methodAParameterAsString:andAParameterAsNumber: is method name for method in MyClass
SEL selectorVar = @selector(methodAParameterAsString:andAParameterAsNumber:);
if ([myClass respondsToSelector:selectorVar]) { // Checks if class contains method
// Must put all method arguments into one object to send to performSelector function
NSArray *arguments = [NSArray arrayWithObjects:@"Hello", @4, nil];
[myClass performSelector:selectorVar withObject:arguments]; // Calls the method
} else {
// NSStringFromSelector() returns a NSString of the method name of a given selector
NSLog(@"MyClass does not have method: %@", NSStringFromSelector(selectedVar));
}
// Implement the methods in an implementation (MyClass.m) file:
@implementation MyClass {
long distance; // Private access instance variable
NSNumber *height;
}
// To access a public variable from the interface file, use '_' followed by variable name:
_count = 5; // References "int count" from MyClass interface
// Access variables defined in implementation file:
distance = 18; // References "long distance" from MyClass implementation
// To use @property variable in implementation, use @synthesize to create accessor variable:
@synthesize roString = _roString; // _roString available now in @implementation
// Called before calling any class methods or instantiating any objects
+ (void)initialize
{
if (self == [MyClass class]) {
distance = 0;
}
}
// Counterpart to initialize method. Called when an object's reference count is zero
- (void)dealloc
{
[height release]; // If not using ARC, make sure to release class variable objects
[super dealloc]; // and call parent class dealloc
}
// Constructors are a way of creating instances of a class
// This is a default constructor which is called when the object is initialized.
- (id)init
{
if ((self = [super init])) // 'super' used to access methods from parent class
{
self.count = 1; // 'self' used for object to call itself
}
return self;
}
// Can create constructors that contain arguments:
- (id)initWithDistance:(int)defaultDistance
{
distance = defaultDistance;
return self;
}
+ (NSString *)classMethod
{
return @"Some string";
}
+ (MyClass *)myClassFromHeight:(NSNumber *)defaultHeight
{
height = defaultHeight;
return [[self alloc] init];
}
- (NSString *)instanceMethodWithParameter:(NSString *)string
{
return @"New string";
}
- (NSNumber *)methodAParameterAsString:(NSString*)string andAParameterAsNumber:(NSNumber *)number
{
return @42;
}
// Objective-C does not have private method declarations, but you can simulate them.
// To simulate a private method, create the method in the @implementation but not in the @interface.
- (NSNumber *)secretPrivateMethod {
return @72;
}
[self secretPrivateMethod]; // Calls private method
// Methods declared into MyProtocol
- (void)myProtocolMethod
{
// statements
}
@end // States the end of the implementation
///////////////////////////////////////
// Categories
///////////////////////////////////////
// A category is a group of methods designed to extend a class. They allow you to add new methods
// to an existing class for organizational purposes. This is not to be mistaken with subclasses.
// Subclasses are meant to CHANGE functionality of an object while categories instead ADD
// functionality to an object.
// Categories allow you to:
// -- Add methods to an existing class for organizational purposes.
// -- Allow you to extend Objective-C object classes (ex: NSString) to add your own methods.
// -- Add ability to create protected and private methods to classes.
// NOTE: Do not override methods of the base class in a category even though you have the ability
// to. Overriding methods may cause compiler errors later between different categories and it
// ruins the purpose of categories to only ADD functionality. Subclass instead to override methods.
// Here is a simple Car base class.
@interface Car : NSObject
@property NSString *make;
@property NSString *color;
- (void)turnOn;
- (void)accelerate;
@end
// And the simple Car base class implementation:
#import "Car.h"
@implementation Car
@synthesize make = _make;
@synthesize color = _color;
- (void)turnOn {
NSLog(@"Car is on.");
}
- (void)accelerate {
NSLog(@"Accelerating.");
}
@end
// Now, if we wanted to create a Truck object, we would instead create a subclass of Car as it would
// be changing the functionality of the Car to behave like a truck. But lets say we want to just add
// functionality to this existing Car. A good example would be to clean the car. So we would create
// a category to add these cleaning methods:
// @interface filename: Car+Clean.h (BaseClassName+CategoryName.h)
#import "Car.h" // Make sure to import base class to extend.
@interface Car (Clean) // The category name is inside () following the name of the base class.
- (void)washWindows; // Names of the new methods we are adding to our Car object.
- (void)wax;
@end
// @implementation filename: Car+Clean.m (BaseClassName+CategoryName.m)
#import "Car+Clean.h" // Import the Clean category's @interface file.
@implementation Car (Clean)
- (void)washWindows {
NSLog(@"Windows washed.");
}
- (void)wax {
NSLog(@"Waxed.");
}
@end
// Any Car object instance has the ability to use a category. All they need to do is import it:
#import "Car+Clean.h" // Import as many different categories as you want to use.
#import "Car.h" // Also need to import base class to use it's original functionality.
int main (int argc, const char * argv[]) {
@autoreleasepool {
Car *mustang = [[Car alloc] init];
mustang.color = @"Red";
mustang.make = @"Ford";
[mustang turnOn]; // Use methods from base Car class.
[mustang washWindows]; // Use methods from Car's Clean category.
}
return 0;
}
// Objective-C does not have protected method declarations but you can simulate them.
// Create a category containing all of the protected methods, then import it ONLY into the
// @implementation file of a class belonging to the Car class:
@interface Car (Protected) // Naming category 'Protected' to remember methods are protected.
- (void)lockCar; // Methods listed here may only be created by Car objects.
@end
//To use protected methods, import the category, then implement the methods:
#import "Car+Protected.h" // Remember, import in the @implementation file only.
@implementation Car
- (void)lockCar {
NSLog(@"Car locked."); // Instances of Car can't use lockCar because it's not in the @interface.
}
@end
///////////////////////////////////////
// Extensions
///////////////////////////////////////
// Extensions allow you to override public access property attributes and methods of an @interface.
// @interface filename: Shape.h
@interface Shape : NSObject // Base Shape class extension overrides below.
@property (readonly) NSNumber *numOfSides;
- (int)getNumOfSides;
@end
// You can override numOfSides variable or getNumOfSides method to edit them with an extension:
// @implementation filename: Shape.m
#import "Shape.h"
// Extensions live in the same file as the class @implementation.
@interface Shape () // () after base class name declares an extension.
@property (copy) NSNumber *numOfSides; // Make numOfSides copy instead of readonly.
-(NSNumber)getNumOfSides; // Make getNumOfSides return a NSNumber instead of an int.
-(void)privateMethod; // You can also create new private methods inside of extensions.
@end
// The main @implementation:
@implementation Shape
@synthesize numOfSides = _numOfSides;
-(NSNumber)getNumOfSides { // All statements inside of extension must be in the @implementation.
return _numOfSides;
}
-(void)privateMethod {
NSLog(@"Private method created by extension. Shape instances cannot call me.");
}
@end
// Starting in Xcode 7.0, you can create Generic classes,
// allowing you to provide greater type safety and clarity
// without writing excessive boilerplate.
@interface Result<__covariant A> : NSObject
- (void)handleSuccess:(void(^)(A))success
failure:(void(^)(NSError *))failure;
@property (nonatomic) A object;
@end
// we can now declare instances of this class like
Result<NSNumber *> *result;
Result<NSArray *> *result;
// Each of these cases would be equivalent to rewriting Result's interface
// and substituting the appropriate type for A
@interface Result : NSObject
- (void)handleSuccess:(void(^)(NSArray *))success
failure:(void(^)(NSError *))failure;
@property (nonatomic) NSArray * object;
@end
@interface Result : NSObject
- (void)handleSuccess:(void(^)(NSNumber *))success
failure:(void(^)(NSError *))failure;
@property (nonatomic) NSNumber * object;
@end
// It should be obvious, however, that writing one
// Class to solve a problem is always preferable to writing two
// Note that Clang will not accept generic types in @implementations,
// so your @implemnation of Result would have to look like this:
@implementation Result
- (void)handleSuccess:(void (^)(id))success
failure:(void (^)(NSError *))failure {
// Do something
}
@end
///////////////////////////////////////
// Protocols
///////////////////////////////////////
// A protocol declares methods that can be implemented by any class.
// Protocols are not classes themselves. They simply define an interface
// that other objects are responsible for implementing.
// @protocol filename: "CarUtilities.h"
@protocol CarUtilities <NSObject> // <NSObject> => Name of another protocol this protocol includes.
@property BOOL engineOn; // Adopting class must @synthesize all defined @properties and
- (void)turnOnEngine; // all defined methods.
@end
// Below is an example class implementing the protocol.
#import "CarUtilities.h" // Import the @protocol file.
@interface Car : NSObject <CarUtilities> // Name of protocol goes inside <>
// You don't need the @property or method names here for CarUtilities. Only @implementation does.
- (void)turnOnEngineWithUtilities:(id <CarUtilities>)car; // You can use protocols as data too.
@end
// The @implementation needs to implement the @properties and methods for the protocol.
@implementation Car : NSObject <CarUtilities>
@synthesize engineOn = _engineOn; // Create a @synthesize statement for the engineOn @property.
- (void)turnOnEngine { // Implement turnOnEngine however you would like. Protocols do not define
_engineOn = YES; // how you implement a method, it just requires that you do implement it.
}
// You may use a protocol as data as you know what methods and variables it has implemented.
- (void)turnOnEngineWithCarUtilities:(id <CarUtilities>)objectOfSomeKind {
[objectOfSomeKind engineOn]; // You have access to object variables
[objectOfSomeKind turnOnEngine]; // and the methods inside.
[objectOfSomeKind engineOn]; // May or may not be YES. Class implements it however it wants.
}
@end
// Instances of Car now have access to the protocol.
Car *carInstance = [[Car alloc] init];
[carInstance setEngineOn:NO];
[carInstance turnOnEngine];
if ([carInstance engineOn]) {
NSLog(@"Car engine is on."); // prints => "Car engine is on."
}
// Make sure to check if an object of type 'id' implements a protocol before calling protocol methods:
if ([myClass conformsToProtocol:@protocol(CarUtilities)]) {
NSLog(@"This does not run as the MyClass class does not implement the CarUtilities protocol.");
} else if ([carInstance conformsToProtocol:@protocol(CarUtilities)]) {
NSLog(@"This does run as the Car class implements the CarUtilities protocol.");
}
// Categories may implement protocols as well: @interface Car (CarCategory) <CarUtilities>
// You may implement many protocols: @interface Car : NSObject <CarUtilities, CarCleaning>
// NOTE: If two or more protocols rely on each other, make sure to forward-declare them:
#import "Brother.h"
@protocol Brother; // Forward-declare statement. Without it, compiler will throw error.
@protocol Sister <NSObject>
- (void)beNiceToBrother:(id <Brother>)brother;
@end
// See the problem is that Sister relies on Brother, and Brother relies on Sister.
#import "Sister.h"
@protocol Sister; // These lines stop the recursion, resolving the issue.
@protocol Brother <NSObject>
- (void)beNiceToSister:(id <Sister>)sister;
@end
///////////////////////////////////////
// Blocks
///////////////////////////////////////
// Blocks are statements of code, just like a function, that are able to be used as data.
// Below is a simple block with an integer argument that returns the argument plus 4.
^(int n) {
return n + 4;
}
int (^addUp)(int n); // Declare a variable to store a block.
void (^noParameterBlockVar)(void); // Example variable declaration of block with no arguments.
// Blocks have access to variables in the same scope. But the variables are readonly and the
// value passed to the block is the value of the variable when the block is created.
int outsideVar = 17; // If we edit outsideVar after declaring addUp, outsideVar is STILL 17.
__block long mutableVar = 3; // __block makes variables writable to blocks, unlike outsideVar.
addUp = ^(int n) { // Remove (int n) to have a block that doesn't take in any parameters.
NSLog(@"You may have as many lines in a block as you would like.");
NSSet *blockSet; // Also, you can declare local variables.
mutableVar = 32; // Assigning new value to __block variable.
return n + outsideVar; // Return statements are optional.
}
int addUp = addUp(10 + 16); // Calls block code with arguments.
// Blocks are often used as arguments to functions to be called later, or for callbacks.
@implementation BlockExample : NSObject
- (void)runBlock:(void (^)(NSString))block {
NSLog(@"Block argument returns nothing and takes in a NSString object.");
block(@"Argument given to block to execute."); // Calling block.
}
@end
///////////////////////////////////////
// Memory Management
///////////////////////////////////////
/*
For each object used in an application, memory must be allocated for that object. When the application
is done using that object, memory must be deallocated to ensure application efficiency.
Objective-C does not use garbage collection and instead uses reference counting. As long as
there is at least one reference to an object (also called "owning" an object), then the object
will be available to use (known as "ownership").
When an instance owns an object, its reference counter is increments by one. When the
object is released, the reference counter decrements by one. When reference count is zero,
the object is removed from memory.
With all object interactions, follow the pattern of:
(1) create the object, (2) use the object, (3) then free the object from memory.
*/
MyClass *classVar = [MyClass alloc]; // 'alloc' sets classVar's reference count to one. Returns pointer to object
[classVar release]; // Decrements classVar's reference count
// 'retain' claims ownership of existing object instance and increments reference count. Returns pointer to object
MyClass *newVar = [classVar retain]; // If classVar is released, object is still in memory because newVar is owner
[classVar autorelease]; // Removes ownership of object at end of @autoreleasepool block. Returns pointer to object
// @property can use 'retain' and 'assign' as well for small convenient definitions
@property (retain) MyClass *instance; // Release old value and retain a new one (strong reference)
@property (assign) NSSet *set; // Pointer to new value without retaining/releasing old (weak reference)
// Automatic Reference Counting (ARC)
// Because memory management can be a pain, Xcode 4.2 and iOS 4 introduced Automatic Reference Counting (ARC).
// ARC is a compiler feature that inserts retain, release, and autorelease automatically for you, so when using ARC,
// you must not use retain, release, or autorelease
MyClass *arcMyClass = [[MyClass alloc] init];
// ... code using arcMyClass
// Without ARC, you will need to call: [arcMyClass release] after you're done using arcMyClass. But with ARC,
// there is no need. It will insert this release statement for you
// As for the 'assign' and 'retain' @property attributes, with ARC you use 'weak' and 'strong'
@property (weak) MyClass *weakVar; // 'weak' does not take ownership of object. If original instance's reference count
// is set to zero, weakVar will automatically receive value of nil to avoid application crashing
@property (strong) MyClass *strongVar; // 'strong' takes ownership of object. Ensures object will stay in memory to use
// For regular variables (not @property declared variables), use the following:
__strong NSString *strongString; // Default. Variable is retained in memory until it leaves it's scope
__weak NSSet *weakSet; // Weak reference to existing object. When existing object is released, weakSet is set to nil
__unsafe_unretained NSArray *unsafeArray; // Like __weak, but unsafeArray not set to nil when existing object is released
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