Variables

From RubySpec

1 Local Variables
2 Global Variables
3 Instance Variables
4 Class Instance Variables
5 Class Variables

Local Variables

Local variables start with a lower case letter[a-z] or an underscore. Variables are lexically scoped, but only within blocks/closures. All local variables in a non-block scope are at the same level, as in the following example:

a = 1
if true
  a = 2 # reassigns same a
  b = 3 # assigns b for the first time
end
puts b # ok, b is still part of the same scope

Local variables often must be "declared" before they are used in a block. This is generally accomplished by assigning them to some value or to nil

a = nil # "declare" a
b, c = nil # "declare" b and c
1.times {
  a = 1 # assigns same a
  d = 2 # d gets scoped only within this block, since it did not exist previously
}
puts a # => 1
puts d # error...no d variable in this scope

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Global Variables

Global variables start with a $ and can then be followed by [a-zA-Z0-9] or an underscore. There are other global 'variables', such as $!, $@, $$, $`, $&, $^, $*, $", $', $_,, $, $; and $:. See the Ruby-Doc page on English.rb for a complete list with descriptions.

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Instance Variables

Instance variables start with @ and can then be followed by [a-zA-Z0-9] or an underscore. Must be initialized inside an instance method, otherwise, if initialized outside any method, will become Class Instance Variable.

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Class Instance Variables

Class instance variables start with @ and can then be followed by [a-zA-Z0-9] or an underscore. Variables initialized outside any method (but inside the class definition). These variables can be accessed only from class methods and instance methods of this class.

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