Ruby if...else, case, unless

Ruby offers contional 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, codespecified 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

This will produce following result:

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 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 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 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 following result:

little child