Hello World in Ruby

puts "Hello, world!"

General Rules
Comments start with # and ends with newline
Newlines or semicolon can be used separate expressions
Everything in Ruby is an object. This includes constants such as 5
Since even operators are methods, they can be overridden
Elegant code can be written using blocks feature

Ruby Keywords

alias   and     BEGIN   begin   break   case    class   def     defined 
do      else    elsif   END     end     ensure  false   for     if 
in      module  next    nil     not     or      redo    rescue  retry 
return  self    super   then    true    undef   unless  until   when 
while   yield

Ruby Types
Basic types are numbers, strings, ranges, regexen, symbols, arrays, and hashes.

Symbols
:symbol
Unique and comparable values that can be substituted for string keys.

Ranges
1..10 - includes last value (10)
1…10 - excludes last value (10)

Arrays
var = Array.new
var = [10,20]
var = %w (string1,string2)

Hashes
var = Hash.new
var = {1=>2, 2=>3, 3=>4}

Variable Types
$global_variables
@@class_variables
@instance_variables
CONSTANT (caps)
local_variables

Dummy Variables
self - current method’s object
nil
true,false
__FILE__
__LINE__

Predefined Variables

$!         The exception message set by raise call. 
$@         Array of backtrace of the last exception thrown. 
$&         The string matched by the last successful pattern match. 
$`         The string to the left  of the last successful match. 
$'         The string to the right of the last successful match. 
$+         The last bracket matched by the last successful match. 
$1         The Nth group of the last successful match. May be > 1. 
$~         The information about the last match. 
$=         The flag for case insensitive, nil by default. 
$/         The input record separator, newline by default. 
$         The output record separator for the print and IO#write. 
$,         The output field separator for the print and Array#join. 
$;         The default separator for String#split. 
$.         The current input line number of the last file that was read. 
$<         The virtual concatenation file of the files from command line. 
$>         The default output for print, printf. $stdout by default. 
$_         The last input line of string by gets or readline. 
$0         Contains the name of the script being executed. 
$*         Command line arguments given for the script sans args. 
$$         The process number of the Ruby running this script. 
$?         The status of the last executed child process. 
$:         Load path for scripts and binary modules by load or require. 
$"         The array contains the module names loaded by require. 
$DEBUG     The status of the -d switch. 
$FILENAME  Current input file from $<. Same as $<.filename. 
$LOAD_PATH The alias to the $:. 
$stderr    The current standard error output. 
$stdin     The current standard input. 
$stdout    The current standard output. 
$VERBOSE   The verbose flag, which is set by the -v switch.

Operator Precedence (highest to lowest)
:: .
[]
**
-(unary) +(unary) ! ~
* / %
+ -
<< >>
&
| ^
> >= < <=
<=> == === != =~ !~
&&
||
.. …
=(+=, -=…)
not
and or

Ruby Control Structures

if boolean-expression [then]
code-block
elsif boolean-expression [then]
code-block
else
code-block
end

unless boolean-expression [then]
code-block
else
code-block
end

expression if boolean-expression

expression unless boolean-expression

case target-expr
when comparison [, comparison]… [then]
code-block
when comparison [, comparison]… [then]
code-block
[else
code-block]
end

loop do
code-block
end

while boolean-expression [do]
code-block
end

until boolean-expression [do]
code-block
end

begin
code-block
end while boolean-expression

begin
code-block
end until boolean-expression

for name[, name]… in expression [do]
code-block
end

expression.each do | name[, name]… |
code-block
end

expression while boolean-expression

expression until boolean-expression

Method Invocation and Definition
method_call
object.method_call
Class::method_call

def method_name
method_body
end

Blocks
method_invocation do … end
method_invocation {
…
}

Exception Handling (2 forms)

begin
  code_block
rescue
  error_handler
ensure
  must_execute_even_on_error
end
catch (:label_key) do ... end
throw :label_key jumps back to matching catch.

Classes

class Name < superclass
private
  class_body
public
  class_body
protected
  class_body
end

Accessor Shortcuts
attr_reader
attr_writer
attr