1. Variables:

Kinds of variables
Ruby has three kinds of variables: 
variables and constants have no type. 

$ 	global variable
@ 	instance variable (object)
[a-z] or _ 	local variable
[A-Z] 	constant

Pseudo-variables: 
self: variable that refers to the currently executing object 
(equivalent of this in C++ or Java). its value is main.
nil: constant whose value is assigned to uninitialized variables. 

We have:

ruby> number = 23; print number, "\n"; defined? number
23
"local-variable(in-block)"
ruby> @number = 23; print @number, "\n"; defined? @number
23
"instance-variable"
ruby> $number =23; print $number, "\n"; defined? $number
23
"global-variable"
ruby> @@number = 23; print @@number, "\n"; defined? @@number
23
"class variable"



2. Examples:
Example 1:

ruby> self
   main
ruby> nil
   nil


Example 2:
 
ruby> i=8
8
ruby> i
ERR: undefined local variable or method `i' for #
ruby>
But:
ruby> @i=18
18
ruby> @i=18
18
ruby>

Example 3: Using case
The case statement to test a sequence 
of conditions is similar to switch in C and Java or PHP.

10..15 means the range between 10 and 15, inclusive. 
We will test here whether the value of the object @i 
falls within that range:

ruby> @i=18
18
ruby> case @i
ruby|      when 1, 10..15
ruby|        puts "1..15"
ruby|      when 16..20
ruby|        puts "16..20"
ruby| end
16..20
nil
ruby>



Example 4: Looping with "if", "while" and "for"
4.1.IF

ruby> @i = 10
10
ruby> puts "Less than a week." if @i<7
nil
ruby> puts "Week has seven days." if @i==7
nil
ruby> puts "A couple of weeks." if @i>7
A couple of weeks.
nil
ruby>


4.1.WHILE

ruby> @sum =0
0
ruby> while @sum <4
ruby| puts "@sum = " + @sum.to_s
ruby| @sum +=1
ruby| end
@sum = 0
@sum = 1
@sum = 2
@sum = 3
nil
ruby>



4.3.FOR

ruby> for number in (1..4)
ruby| puts number
ruby| end
1
2
3
4
1..4
ruby>



Example 5: The :: operator shows the module to consider
The :: operator tells the ruby interpreter which module

ruby> sqrt(36)
ERR: undefined method `sqrt' for #
ruby> Math.sqrt(36)
6.0
ruby> Math::sqrt(36)
6.0
ruby> include Math
Object
ruby> sqrt(36)
6.0
ruby> PI
3.14159265358979
ruby>