Explanation of scope

Scope

# VARIABLE SCOPE

=Begin

$example         A global variable
@example         An instance variable
example          A local variable
Example          A constant
@@example        A class variable

"nil" and "self" cannot be assigned values

=End

$global = 10
defined? $global

#LOCAL

1.9.3p194 :008 > def add(num1, num2)
1.9.3p194 :009?>   num1 + num2
1.9.3p194 :010?>   end
 => nil 
1.9.3p194 :011 > add(2,2)
 => 4 
1.9.3p194 :012 > num1
NameError: undefined local variable or method `num1' for main:Object
	from (irb#1):12


#GLOBAL

#Global variables can be accessed AND CHANGED from anywhere in the program.  Strongly discouraged.
1.9.3p194 :013 > $0
 => "irb"


gets
A Job   
 => "A Job\n" 
1.9.3p194 :002 > $_
 => "A Job\n" 
1.9.3p194 :003 > a = 4+4
 => 8 
1.9.3p194 :004 > $_
 => "A Job\n"


#CLASS
1.9.3p194 :001 > class Polygon
1.9.3p194 :002?>     @@sides = 10
1.9.3p194 :003?>     def self.sides
1.9.3p194 :004?>         @@sides
1.9.3p194 :005?>       end
1.9.3p194 :006?>   end
 => nil 
1.9.3p194 :007 > 
1.9.3p194 :008 >   puts Polygon.sides
10
 => nil 
1.9.3p194 :009 > class Triangle < Polygon
1.9.3p194 :010?>     @@sides = 3
1.9.3p194 :011?>   end
 => 3 
1.9.3p194 :012 > puts Triangle.sides
3
 => nil 
1.9.3p194 :013 > puts Polygon.sides
3
 => nil 

#INSTANCE

1.9.3p194 :014 > class Polygon
1.9.3p194 :015?>     @sides = 10
1.9.3p194 :016?>   end
 => 10 

1.9.3p194 :017 > puts Polygon.class_variables
@@sides
 => nil 
1.9.3p194 :018 > puts Polygon.instance_variables
@sides
 => nil

#CONSTANT