Ruby if...else, case, unless


Ruby offers conditional structures that are pretty common to modern languages. Here we will explain the more commonly used 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:

      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
      

This will produce following result:

      x is 1
      

 

Ruby if modifier:

Syntax:

      code if condition
      

Executes code if the conditional is true.

Example:

      condition = 1
      puts "true" if condition
      

This will produce following result:

      true
      

 

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:

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

This will produce following result:

      x is less than 2
      

 

Ruby unless modifier:

Syntax:

      code unless conditional
      

Executes code if conditional is false.

Example:

      condition =  1
      puts "1 -- Value is set\n" if condition
      puts "2 -- Value is set\n" unless condition
      
      condition = false
      puts "3 -- Value is set\n" unless condition
      

This will produce 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:

      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 following result:

      little child