Programming Perl, 4th Edition

foreach Loops

This loop iterates over a list of values by setting the control variable (VAR) to each successive element of the list:

for my VAR (LIST) {
    ...
}

If “my VAR” is omitted, the global $_ is used. You can omit the my, but only when use strict is turned off, so don’t.

For historical reasons, the foreach keyword is a synonym for the for keyword, so you can use for and foreach interchangeably, whichever you think is more readable in a given situation. We tend to prefer for because we are lazy and because it is more readable, especially with the my. (Don’t worry—Perl can easily distinguish for (@ARGV) from for ($i=0; $i <=$#ARGV; $i++) because the latter contains semicolons.) Here are some examples:

$sum = 0;
for my $value (@array) { $sum += $value }

for my $count (10,9,8,7,6,5,4,3,2,1,"BOOM") {  # do a countdown
    say $count;
    sleep(1);
}

for (reverse "BOOM", 1 .. 10) {                # same thing
    say;
    sleep(1);
}

for my $field (split /:/, $data) {             # any LIST expression
    say "Field contains: '$field'";
}

for my $key (sort keys %hash) {
    say "$key => $hash{$key}";
}

That last one is the canonical way to print out the values of a hash in sorted order. See the keys and sort entries in Chapter 27 for more elaborate examples.

There is no way to tell where you are in the list. You may compare adjacent elements by remembering the previous one in a variable, but sometimes you just have to break down and write a three-part loop with subscripts. That’s why we have two different loops, after all.

If LIST consists ...