Ruby Loops - while, for, until, break, redo and retry

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

Ruby while Statement:

Syntax:

while conditional [do]
   code
end

Executes code while conditional is true. A while loop's conditional is separated from code by the reserved word do, a newline, backslash \, or a semicolon ;.

Example:

#!/usr/bin/ruby

$i = 0
$num = 5

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

This will produce the following result:

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

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

Example:

#!/usr/bin/ruby

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

This will produce the following result:

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:

until conditional [do]
   code
end

Executes code while conditional is false. An until statement's conditional is separated from code by the reserved word do, a newline, or a semicolon.

Example:

#!/usr/bin/ruby

$i = 0
$num = 5

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

This will produce the following result:

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

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

Example:

#!/usr/bin/ruby

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

This will produce the following result:

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

Executes code once for each element in expression.

Example:

#!/usr/bin/ruby

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

Here, we have defined the range 0..5. The statement for i in 0..5 will allow i to take values in the range from 0 to 5 (including 5). This will produce the following result:

Value of local variable is 0
Value of local variable is 1
Value of local variable is 2
Value of local variable is 3
Value of local variable is 4
Value of local variable is 5

A for...in loop is almost exactly equivalent to:

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

except that a for loop doesn't create a new scope for local variables. A for loop's expression is separated from code by the reserved word do, a newline, or a semicolon.

Example:

#!/usr/bin/ruby

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

This will produce the following result:

Value of local variable is 0
Value of local variable is 1
Value of local variable is 2
Value of local variable is 3
Value of local variable is 4
Value of local variable is 5

Ruby break Statement:

Syntax:

break

Terminates the most internal loop. Terminates a method with an associated block if called within the block (with the method returning nil).

Example:

#!/usr/bin/ruby

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

This will produce the following result:

Value of local variable is 0
Value of local variable is 1
Value of local variable is 2

Ruby next Statement:

Syntax:

next

Jumps to next iteration of the most internal loop. Terminates execution of a block if called within a block (with yield or call returning nil).

Example:

#!/usr/bin/ruby

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

This will produce the following result:

Value of local variable is 2
Value of local variable is 3
Value of local variable is 4
Value of local variable is 5

Ruby redo Statement:

Syntax:

redo

Restarts this iteration of the most internal loop, without checking loop condition. Restarts yield or call if called within a block.

Example:

#!/usr/bin/ruby

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

This will produce the following result and will go in an infinite loop:

Value of local variable is 0
Value of local variable is 0
............................

Ruby retry Statement:

Syntax:

retry

If retry appears in rescue clause of begin expression, restart from the beginning of the 1begin body.

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

If retry appears in the iterator, the block, or the body of the for expression, restarts the invocation of the iterator call. Arguments to the iterator is re-evaluated.

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

Example:

#!/usr/bin/ruby

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

This will produce the following result and will go in an infinite loop:

Value of local variable is 1
Value of local variable is 2
Value of local variable is 1
Value of local variable is 2
Value of local variable is 1
Value of local variable is 2
............................