3.3 Dereferencing the Array Reference

If you look at @skipper, you'll see that it consists of two parts: the @ symbol and the name of the array. Similarly, the syntax $skipper[1] consists of the name of the array in the middle and some syntax around the outside to get at the second element of the array (index value 1 is the second element because you start counting index values at 0).

Here's the trick: any reference to an array can be placed in curly braces and written in place of the name of an array, ending up with a method to access the original array. That is, wherever you write skipper to name the array, you use the reference inside curly braces: { $items }. For example, both of these lines refer to the entire array:

@  skipper
@{ $items }

whereas both of these refer to the second item of the array:^[3]

^[3] Note that whitespace was added in these two displays to make the similar parts line up. This whitespace is legal in a program, even though most programs won't use it.

$  skipper [1]
${ $items }[1]

By using the reference form, you've decoupled the code and the method of array access from the actual array. Let's see how that changes the rest of this subroutine:

sub check_required_items {
  my $who = shift;
  my $items = shift;
  my @required = qw(preserver sunscreen water_bottle jacket);
  for my $item (@required) {
    unless (grep $item eq $_, @{$items}) { # not found in list?
      print "$who is missing $item.\n";
    }
  }
}

All you did was replace @_ (the copy of the provisions list) with @{$items}, a dereferencing of the reference to the original provisions array. Now you can call the subroutine a few times as before:

my @skipper = qw(blue_shirt hat jacket preserver sunscreen);
check_required_items("The Skipper", \@skipper);
my @professor = qw(sunscreen water_bottle slide_rule batteries radio);
check_required_items("Professor", \@professor);
my @gilligan = qw(red_shirt hat lucky_socks water_bottle);
check_required_items("Gilligan", \@gilligan);

In each case, $items points to a different array, so the same code applies to different arrays each time it is invoked. This is one of the most important uses of references: decoupling the code from the data structure on which it operates so the code can be reused more readily.

Passing the array by reference fixes the first of the two problems mentioned earlier. Now, instead of copying the entire provision list into the @_ array, you get a single element of a reference to that provisions array.

Could you have eliminated the two shifts at the beginning of the subroutine? Sure, at the expense of clarity:

sub check_required_items {
  my @required = qw(preserver sunscreen water_bottle jacket);
  for my $item (@required) {
    unless (grep $item eq $_, @{$_[1]}) { # not found in list?
      print "$_[0] is missing $item.\n";
    }
  }
}

You still have two elements in @_. The first element is the passenger or crew member name and is used in the error message. The second element is a reference to the correct provisions array, used in the grep expression.