Perl until Statement

Summary: in this tutorial, you’ll learn about the Perl until loop statement that executes a block of code as long as the test condition is false.

Introduction to Perl until statement

The while loop statement executes a block of code repeatedly as long as a condition is true.

If you replace the keyword while by the keyword until, the test is reversed i.e., it executes a block of code as long as the condition is false.

The until statement is the opposite of the while statement.

Both until and while statements evaluate the condition at the beginning of each iteration.

The following illustrates the syntax of the until statement:

until(expression){
   # code block
}Code language: Perl (perl)

Perl provides an optional block for the until statement: continue, as shown below:

until(condition){
  # until block
}
continue{
  # continue block
}Code language: Perl (perl)

The continue block is executed after each current iteration. The continue block is rarely used in practice.

Different from the for loop statement, the until loop statement does not have the special loop variable i.e., $_. To use a loop variable, you must declare it explicitly before entering the loop or in the test condition.

The following flowchart illustrates the Perl until statement:

Perl until statement examples

Perl until statement example

The following example demonstrates how to use the until statement:

#!/usr/bin/perl
use warnings;
use strict;

my $counter = 5;

until($counter == 0){
   print("$counter \n");
   $counter--;
}Code language: Perl (perl)

5
4
3
2
1

How it works:

• First, we initialized the $counter variable to 5.
• Next, the until statement checked if $counter is equal to zero before entering the loop. Because the result of the evaluation was false at the first iteration, the loop is entered.
• Then, we displayed the value of the $counter and decreased 1 from its current value.
• After that, the second iteration is reached so the expression $counter == 0 is evaluated again. The loop continued as long as the result of the expression is false.
• Finally, after 5 iterations, the $counter was equal to zero, which made the condition become true, so the loop terminated.

Perl until statement and array example

The following example demonstrates how to use the until statement with an array.

#!/usr/bin/perl
use warnings;
use strict;

my $counter = 0;
my @keywords = qw(until while do for loop);

until(!scalar @keywords) {
  $counter++;
  print shift(@keywords) . "\n";

}
print("$counter elements removed!\n");Code language: PHP (php)

until
while
do
for
loop
5 elements removed!Code language: JavaScript (javascript)

In each iteration, we used the shift() function to remove the element of the @keyword array and increased the $counter variable by 1. The loop is terminated when the @keyword array has no more elements.

In this tutorial, you learned how to use the Perl until statement to execute a code block repeatedly as long as a condition is false.