Perl Cookbook

Doing Something with Every Element in a List

Problem

You want to repeat a procedure for every element in a list.

Often you use an array to collect information you’re interested in; for instance, login names of users who have exceeded their disk quota. When you finish collecting the information, you want to process it by doing something with every element in the array. In the disk quota example, you might send each user a stern mail message.

Solution

Use a foreach loop:

foreach $item (LIST) {
    # do something with $item
}

Discussion

Let’s say we’ve used @bad_users to compile a list of users over their allotted disk quota. To call some complain() subroutine for each one we’d use:

foreach $user (@bad_users) {
        complain($user);
}

Rarely is this recipe so simply applied. Instead, we often use functions to generate the list:

foreach $var (sort keys %ENV) {
    print "$var=$ENV{$var}\n";
}

Here we’re using sort and keys to build a sorted list of environment variable names. In situations where the list will be used more than once, you’ll obviously keep it around by saving in an array. But for one-shot processing, it’s often tidier to process the list directly.

Not only can we add complexity to this formula by building up the list in the foreach, we can also add complexity by doing more work inside the code block. A common application of foreach is to gather information on every element of a list, and then decide (based on that information) whether to do something. For instance, returning to the disk quota ...