Ruby Loops

Loops in Ruby are used to execute the same block of code a specified number of times. This section will detail all the loop statements supported by Ruby.

Ruby while Statement

Syntax

while conditional [do]
   code
end

Or

while conditional [:]
   code
end

The code is executed while the conditional is true.

The do or : can be omitted. However, if you want to write the while statement in one line, you must use do or : to separate the conditional from the code block.

Example

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

$i = 0
$num = 5

while $i < $num  do
   puts("Inside the loop i = #$i" )
   $i +=1
end

Output:

Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4

Ruby while Modifier

Syntax

code while condition

Or

begin
  code
end while conditional

The code is executed while the conditional is true.

If the while modifier follows a begin statement with no rescue or ensure clauses, the code is executed once before the conditional is evaluated.

Example

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

$i = 0
$num = 5
begin
   puts("Inside the loop i = #$i" )
   $i +=1
end while $i < $num

Output:

Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4

Ruby until Statement

Syntax

until conditional [do]
   code
end

The code is executed while the conditional is false.

The do can be omitted. However, if you want to write the until statement in one line, you must use do to separate the conditional from the code block.

Example

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

$i = 0
$num = 5

until $i > $num  do
   puts("Inside the loop i = #$i" )
   $i +=1;
end

Output:

Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4
Inside the loop i = 5

Ruby until Modifier

Syntax

code until conditional

Or

begin
   code
end until conditional

The code is executed while the conditional is false.

If the until modifier follows a begin statement with no rescue or ensure clauses, the code is executed once before the conditional is evaluated.

Example

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

$i = 0
$num = 5
begin
   puts("Inside the loop i = #$i" )
   $i +=1;
end until $i > $num

Output:

Inside the loop i = 0
Inside the loop i = 1
Inside the loop i = 2
Inside the loop i = 3
Inside the loop i = 4
Inside the loop i = 5

Ruby for Statement

Syntax

for variable [, variable ...] in expression [do]
   code
end

The expression is evaluated to get an object, and for each element in the expression, the code is executed once.

Example

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

for i in 0..5
   puts "The value of local variable is #{i}"
end

Here, we have defined a range 0..5. The statement for i in 0..5 allows i to take values from 0 to 5 inclusive.

Output:

The value of local variable is 0
The value of local variable is 1
The value of local variable is 2
The value of local variable is 3
The value of local variable is 4
The value of local variable is 5

The for...in loop is almost completely equivalent to:

(expression).each do |variable[, variable...]| code end

However, the for loop does not create a new scope for local variables.

The do can be omitted. However, if you want to write the for statement in one line, you must use do to separate the conditional from the code block.

Example

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

(0..5).each do |i|
   puts "The value of local variable is #{i}"
end

Output:

The value of local variable is 0
The value of local variable is 1
The value of local variable is 2
The value of local variable is 3
The value of local variable is 4
The value of local variable is 5

Ruby break Statement

Syntax

break

Terminates the innermost loop. If called within a block, it terminates the associated method (the method returns nil).

Example

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

for i in 0..5
   if i > 2 then
      break
   end
   puts "The value of local variable is #{i}"
end

Output:

The value of local variable is 0
The value of local variable is 1
The value of local variable is 2

Ruby next Statement

Syntax

next

Jumps to the next iteration of the loop. If called within a block, it terminates the block's execution (yield expression returns nil).

Example

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

for i in 0..5
   if i < 2 then
      next
   end
   puts "The value of local variable is #{i}"
end

Output:

The value of local variable is 2
The value of local variable is 3
The value of local variable is 4
The value of local variable is 5

Ruby redo Statement

Syntax

redo

Restarts the current iteration of the innermost loop, without checking the loop condition. If called within a block, it restarts the yield or call.

Example

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

for i in 0..5
   if i < 2 then
      puts "The value of local variable is #{i}"
      redo
   end
end

This will produce the following result and enter an infinite loop:

The value of local variable is 0
The value of local variable is 0
............................

Ruby retry Statement

Note: Versions 1.9 and later do not support retry in loops.

Syntax

retry

If retry appears in the rescue clause of a begin expression, it restarts from the beginning of the begin body.

begin
   do_something   # exception raised
rescue
   # handle error
   retry          # restart from beginning
end

If retry appears in an iterator, block, or the body of a for expression, it restarts the invocation of the iterator call. The arguments to the iterator are re-evaluated.

for i in 1..5
   retry if some_condition # restart from i == 1
end

Example

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

for i in 1..5
   retry if  i > 2
   puts "The value of local variable is #{i}"
end

This will produce the following result and enter an infinite loop:

The value of local variable is 1
The value of local variable is 2
The value of local variable is 1
The value of local variable is 2
The value of local variable is 1
The value of local variable is 2
............................