Perl Cookbook

Sorting an Array Numerically

Problem

You want to sort a list of numbers, but Perl’s sort (by default) sorts alphabetically in ASCII order.

Solution

Use Perl’s sort function and the <=> numerical comparison operator:

@sorted = sort { $a <=> $b } @unsorted;

Discussion

The sort function takes an optional code block, which lets you replace the default alphabetic comparison subroutine with your own. This comparison function is called each time sort has to compare two values. The values to compare are loaded into the special package variables $a and $b, which are automatically localized.

The comparison function should return a negative number if $a ought to appear before $b in the output list, 0 if they’re the same and their order doesn’t matter, or a positive number if $a ought to appear after $b. Perl has two operators that behave this way: <=> for sorting numbers in ascending numeric order, and cmp for sorting strings in ascending alphabetic order. By default, sort uses cmp-style comparisons.

Here’s code that sorts the list of PIDs in @pids, lets the user select one, then sends it a TERM signal followed by a KILL signal. We use a code block that compares $a to $b with <=> to sort numerically:

# @pids is an unsorted array of process IDs
foreach my $pid (sort { $a <=> $b } @pids) {
    print "$pid\n";
}
print "Select a process ID to kill:\n";
chomp ($pid = <>);
die "Exiting ... \n" unless $pid && $pid =~ /^\d+$/;
kill('TERM',$pid);
sleep 2;
kill('KILL',$pid);

If you use $a <=> $b or $a cmp ...