Learning Perl, 5th Edition

Transforming Items from a List with map

Another common task is transforming items from a list. For example, suppose we have a list of numbers that should be formatted as “money numbers” for output, as with the subroutine &big_money (from Chapter 14). But we don’t want to modify the original data; we need a modified copy of the list just for output. Here’s one way to do that:

my @data = (4.75, 1.5, 2, 1234, 6.9456, 12345678.9, 29.95);
my @formatted_data;

foreach (@data) {
  push @formatted_data, &big_money($_);
}

That looks similar in form to the example code used at the beginning of the section on grep, doesn’t it? So it may not surprise you that the replacement code resembles the first grep example:

my @data = (4.75, 1.5, 2, 1234, 6.9456, 12345678.9, 29.95);

my @formatted_data = map { &big_money($_) } @data;

The map operator looks much like grep because it has the same kind of arguments: a block that uses $_, and a list of items to process. And it operates in a similar way, evaluating the block once for each item in the list, with $_ aliased to a different original list element each time. But the last expression of the block is used differently; instead of giving a Boolean value, the final value actually becomes part of the resulting list.^[*] Any grep or map statement could be rewritten as a foreach loop pushing items onto a temporary array. But the shorter way is typically more efficient and more convenient. Since the result of map or grep is a list, it can be passed directly to another ...