5.7 Selecting and Altering Complex Data

You can use these operators on more complex data. Taking the provisions list from Chapter 4:

my %provisions = (
  "The Skipper" =>
    [qw(blue_shirt hat jacket preserver sunscreen)],
  "The Professor" =>
    [qw(sunscreen water_bottle slide_rule batteries radio)],
  "Gilligan" =>
    [qw(red_shirt hat lucky_socks water_bottle)],
);

In this case, $provisions{"The Professor"} gives an array reference of the provisions brought by the Professor, and $provisions{"Gilligan"}[-1] gives the last item Gilligan thought to bring.

Run a few queries against this data. Who brought fewer than five items?

my @packed_light = grep @{ $provisions{$_} } < 5, keys %provisions;

In this case, $_ is the name of a person. Take that name, look up the array reference of the provisions for that person, dereference that in a scalar context to get the count of provisions, and then compare it to 5. And wouldn't you know it; the only name is Gilligan.

Here's a trickier one. Who brought a water bottle?

my @all_wet = grep {
  my @items = @{ $provisions{$_} };
  grep $_ eq "water_bottle", @items;
} keys %provisions;

Starting with the list of names again (keys %provisions), pull up all the packed items first, and then use that list in an inner grep to count the number of those items that equal water_bottle. If the count is 0, there's no bottle, so the result is false for the outer grep. If the count is nonzero, you have a bottle, so the result is true for the outer grep. Now you see that the Skipper will be a bit thirsty later, without any relief.

You can also perform transformations. For example, turn this hash into a list of array references, with each array containing two items. The first is the original person's name; the second is a reference to an array of the provisions for that person:

my @remapped_list = map {
  [ $_ => $provisions{$_} ];
} keys %provisions;

The keys of %provisions are names of the people. For each name, construct a two-element list of the name and the corresponding provisions array reference. This list is inside an anonymous array constructor, so you get back a reference to a newly created array for each person. Three names in; three references out.^[7]

^[7] If you had left the inner brackets off, you'd end up with six items out. That's not very useful, unless you're creating a different hash from them.

Or, let's go a different way. Turn the input hash into a series of references to arrays. Each array will have a person's name and one of the items they brought:

my @person_item_pairs = map {
  my $person = $_;
  my @items = @{ $provisions{$person} };
  map [$person => $_], @items;
} keys %provisions;

Yes, a map within a map. The outer map selects one person at a time. The name is saved into $person, and then the item list is extracted from the hash. The inner map walks over this item list, executing the expression to construct an anonymous array reference for each item. The anonymous array contains the person's name and the provision item.

You had to use $person here to hold the outer $_ temporarily. Otherwise, you can't refer to both temporary values for the outer map and the inner map.