Intermediate Perl

Chapter 4. Introduction to References

References are the basis for complex data structures, object-oriented programming (OOP), and fancy subroutine magic. They’re the magic that was added between Perl version 4 and version 5 to make it all possible.

A Perl scalar variable holds a single value. An array holds an ordered list of one or more scalars. A hash holds a collection of scalars as values, keyed by other scalars. Although a scalar can be an arbitrary string, which allows complex data to be encoded into an array or hash, none of the three data types are well suited to complex data interrelationships. This is a job for the reference. Let’s look at the importance of references by starting with an example.

Performing the Same Task on Many Arrays

Before the Minnow can leave on an excursion (for example, a three-hour tour), we should check every passenger and crew member to ensure they have all the required trip items in their possession. Let’s say that, for maritime safety, every person on-board the Minnow needs to have a life preserver, some sunscreen, a water bottle, and a rain jacket. We can write a bit of code to check for the Skipper’s supplies:

my @required = qw(preserver sunscreen water_bottle jacket);
my @skipper  = qw(blue_shirt hat jacket preserver sunscreen);

for my $item (@required) {
  unless (grep $item eq $_, @skipper) { # not found in list?
    print "skipper is missing $item.\n";
  }
}

The grep in a scalar context returns the number of times the expression $item eq $_ returns ...