Ruby if...else, case, unless

Ruby offers conditional structures that are pretty common to modern languages. Here, we will explain all the conditional statements and modifiers available in Ruby.

Ruby if...else Statement:

Syntax:

if conditional [then]
   code...
[elsif conditional [then]
   code...]...
[else
   code...]
end

if expressions are used for conditional execution. The values false and nil are false, and everything else are true. Notice Ruby uses elsif, not else if nor elif.
Executes code if the conditional is true. If the conditional is not true, code specified in the else clause is executed.
An if expression's conditional is separated from code by the reserved word then, a newline, or a semicolon.

Example:

#!/usr/bin/ruby

x=1
if x > 2
   puts "x is greater than 2"
elsif x <= 2 and x!=0
   puts "x is 1"
else
   puts "I can't guess the number"
end

x is 1

Ruby if modifier:

Syntax:

code if condition

Executes code if the conditional is true.

Example:

#!/usr/bin/ruby

$debug=1
print "debug\n" if $debug

This will produce the following result:

debug

Ruby unless Statement:

Syntax:

unless conditional [then]
   code
[else
   code ]
end

Executes code if conditional is false. If the conditional is true, code specified in the else clause is executed.

Example:

#!/usr/bin/ruby

x=1
unless x>2
   puts "x is less than 2"
 else
  puts "x is greater than 2"
end

This will produce the following result:

x is less than 2

Ruby unless modifier:

Syntax:

code unless conditional

Executes code if conditional is false.

Example:

#!/usr/bin/ruby

$var =  1
print "1 -- Value is set\n" if $var
print "2 -- Value is set\n" unless $var

$var = false
print "3 -- Value is set\n" unless $var

This will produce the following result:

1 -- Value is set
3 -- Value is set

Ruby case Statement

Syntax:

case expression
[when expression [, expression ...] [then]
   code ]...
[else
   code ]
end

Compares the expression specified by case and that specified by when using the === operator and executes the code of the when clause that matches.
The expression specified by the when clause is evaluated as the left operand. If no when clauses match, case executes the code of the else clause.
A when statement's expression is separated from code by the reserved word then, a newline, or a semicolon.
Thus:

case expr0
when expr1, expr2
   stmt1
when expr3, expr4
   stmt2
else
   stmt3
end

is basically similar to the following:

_tmp = expr0
if expr1 === _tmp || expr2 === _tmp
   stmt1
elsif expr3 === _tmp || expr4 === _tmp
   stmt2
else
   stmt3
end

Example:

#!/usr/bin/ruby

$age =  5
case $age
when 0 .. 2
    puts "baby"
when 3 .. 6
    puts "little child"
when 7 .. 12
    puts "child"
when 13 .. 18
    puts "youth"
else
    puts "adult"
end

This will produce the following result:

little child