until loop - Beginning Perl Tutorial, Control Structures

Examples of Perl's until loop

As we've discussed, Perl's until loop is used to loop through a designated block of code until a specific condition is evaluated as true. Let's look at an example of Perl's until loop in action and break down exactly how it works, step by step.

$count = 10;
until ($count == 0) {
print "$count ";
$count--;
}
print "Blastoff.\n";

Running this simple Perl script produces the following output:

10 9 8 7 6 5 4 3 2 1 Blastoff.

First we set the string $count to a value of 10.

$count = 10;

Next comes the start of the until loop, and the expression in the parenthesis is evaluated:

until ($count == 0)

If the until expression is evaluated as false, the code inside the block is executed and the expression is re-evaluated. When it finally evaluates as true, the block is skipped and the rest of the Perl script is executed.

1. $count is set to a value of 10.
2. Is $count equal to 0? If not, continue, otherwise exit the until loop.
3. Execute the code block inside the until loop.
4. Return to step 2.

The end result is that $count starts at 10 and comes down by 1 every time the loop is executed. When we print the value of $count, we can see that the loop is executed until $count has a value equal to 0, at which point the loop stops and the word 'Blastoff' is printed.

1. An until loop is a Perl control structure.
2. It is used to step through a block of code until a specific condition is true.

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