Learning Perl Objects, References & Modules, from O'Reilly. | 5

Learning Perl Objects, References & Modules Chapter 3: Introduction to References

References to Hashes

Just as you can take a reference to an array, you can also take a reference to a hash. Once again, you use the backslash as the "take a reference to" operator:

my %gilligan_info = (
  name => 'Gilligan',
  hat => 'White',
  shirt => 'Red',
  position => 'First Mate',
);
my $hash_ref = \%gilligan_info;

You can dereference a hash reference to get back to the original data. The strategy is similar to dereferencing an array reference. Write the hash syntax as you would have without references, and then replace the name of the hash with a pair of curly braces surrounding the thing holding the reference. For example, to pick a particular value for a given key, use:

my $name = $ gilligan_info { 'name' };
my $name = $ { $hash_ref } { 'name' };

In this case, the curly braces have two different meanings. The first pair denotes the expression returning a reference, while the second pair delimits the expression for the hash key.

To perform an operation on the entire hash, you proceed similarly:

my @keys = keys % gilligan_info;
my @keys = keys % { $hash_ref };

As with array references, you can use shortcuts to replace the complex curly-braced forms under some circumstances. For example, if the only thing inside the curly braces is a simple scalar variable (as shown in these examples so far), you can drop the curly braces:

my $name = $$hash_ref{'name'};
my @keys = keys %$hash_ref;

Like an array reference, when referring to a specific hash element, you can use an arrow form:

my $name = $hash_ref->{'name'};

Because a hash reference fits wherever a scalar fits, you can create an array of hash references:

my %gilligan_info = (
  name => 'Gilligan',
  hat => 'White',
  shirt => 'Red',
  position => 'First Mate',
);
my %skipper_info = (
  name => 'Skipper',
  hat => 'Black',
  shirt => 'Blue',
  position => 'Captain',
);
my @crew = (\%gilligan_info, \%skipper_info);

Thus, $crew[0] is a hash reference to the information about Gilligan. You can get to Gilligan's name via any one of:

${ $crew[0] } { 'name' }
my $ref = $crew[0]; $$ref{'name'}
$crew[0]->{'name'}
$crew[0]{'name'}

On that last one, you can still drop the arrow between "subscripty kinds of things," even though one is an array bracket and one is a hash brace.

Let's print a crew roster:

my %gilligan_info = (
  name => 'Gilligan',
  hat => 'White',
  shirt => 'Red',
  position => 'First Mate',
);
my %skipper_info = (
  name => 'Skipper',
  hat => 'Black',
  shirt => 'Blue',
  position => 'Captain',
);
my @crew = (\%gilligan_info, \%skipper_info);
 
my $format = "%-15s %-7s %-7s %-15s\n";
printf $format, qw(Name Shirt Hat Position);
for my $crewmember (@crew) {
  printf $format,
    $crewmember->{'name'},
    $crewmember->{'shirt'},
    $crewmember->{'hat'},
    $crewmember->{'position'};
}

That last part looks very repetitive. You can shorten it with a hash slice. Again, if the original syntax is:

@ gilligan_info { qw(name position) }

the hash slice notation from a reference looks like:

@ { $hash_ref } { qw(name position) }

You can drop the first brace pair because the only thing within is a simple scalar value, yielding:

@ $hash_ref { qw(name position) }

Thus, you can replace that final loop with:

for my $crewmember (@crew) {
  printf $format, @$crewmember{qw(name shirt hat position)};
}

There is no shortcut form with an arrow (->) for array slices or hash slices, just as there is no shortcut for entire arrays or hashes.

A hash reference prints as a string that looks like HASH(0x1a2b3c), showing the hexadecimal memory address of the hash. That's not very useful to an end user and only barely more usable to the programmer, except as an indication of the lack of appropriate dereferencing.

Exercises

The answers for all exercises can be found in the Appendix.

Exercise 1 [5 min]

How many different things do these expressions refer to?

$ginger->[2][1]
${$ginger[2]}[1]
$ginger->[2]->[1]
${$ginger->[2]}[1]

Exercise 2 [30 min]

Using the final version of check_required_items, write a subroutine check_items_for_all that takes a hash reference as its only parameter, pointing at a hash whose keys are the people aboard the Minnow, and whose corresponding values are array references of the things they intend to bring on board.

For example, the hash reference might be constructed like so:

my @gilligan = ... gilligan items ...;
my @skipper = ... skipper items ...;
my @professor = ... professor items ...;
my %all = (
  "Gilligan" => \@gilligan,
  "Skipper" => \@skipper,
  "Professor" => \@professor,
);
check_items_for_all(\%all);

The newly constructed subroutine should call check_required_items for each person in the hash, updating their provisions list to include the required items.

1. There are more efficient ways to check list membership for large lists, but for a few items, this is probably the easiest way to do so with just a few lines of code.

2. Actually, assigning new scalars to elements of @_ after the shift modifies the corresponding variable being passed, but that still wouldn't let you extend the array with additional mandatory provisions.

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.

4. It's not that it hasn't been discussed repeatedly by the Perl developers; it's just that nobody has come up with a nice backward-compatible syntax with universal appeal.