Ruby if Statement

Ruby provides several common conditional structures. Here, we will explain all the conditional statements and available modifiers in Ruby.

Ruby if...else Statement

Syntax

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

The if expression is used for conditional execution. The values false and nil are false, and everything else is true. Note that Ruby uses elsif, not else if or elif.

If the conditional is true, the code is executed. If the conditional is not true, the code specified in the else clause is executed.

Usually, we omit the reserved word then. If you want to write the entire if expression in one line, you must separate the condition and the code block with then, as shown below:

if a == 4 then a = 7 end

Example

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

x = 1
if x > 2
   puts "x is greater than 2"
elsif x <= 2 and x != 0
   puts "x is 1"
else
   puts "Cannot determine the value of x"
end

Output:

x is 1

Ruby if Modifier

Syntax

code if condition

The if modifier means that the code on the left is executed if the condition on the right is true.

Example

#!/usr/bin/ruby

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

Output:

debug

Ruby unless Statement

Syntax

unless conditional [then]
   code
[else
   code ]
end

The unless expression is the opposite of the if expression. If the conditional is false, the code is executed. If the conditional is true, the code specified in the else clause is executed.

Example

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

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

Output:

x is less than or equal to 2

Ruby unless Modifier

Syntax

code unless conditional

The code is executed if the conditional is false.

Example

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

$var = 1
print "1 -- This line is output\n" if $var
print "2 -- This line is not output\n" unless $var

$var = false
print "3 -- This line is output\n" unless $var

Output:

1 -- This line is output
3 -- This line is output

Ruby case Statement

Syntax

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

The case statement first matches an expression and then branches based on the match result.

It uses the === operator to compare the expression specified in the when clause. If it matches, the code in the when block is executed.

Usually, we omit the reserved word then. If you want to write the entire when expression in one line, you must separate the condition and the code block with then, as shown below:

when a == 4 then a = 7 end

Thus:

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

Is essentially similar to:

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

Example

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

$age = 5
case $age
when 0..2
    puts "Baby"
when 3..6
    puts "Child"
when 7..12
    puts "Child"
when 13..18
    puts "Teenager"
else
    puts "Other age group"
end

Output:

Child

When the "expression" part of the case statement is omitted, it evaluates the first when condition that is true.

foo = false
bar = true
quu = false

case
when foo then puts 'foo is true'
when bar then puts 'bar is true'
when quu then puts 'quu is true'
end
# Outputs "bar is true"