Simple hash search by value

Question

I have a simple hash, and would like to return the $key based on $value criteria. That is, for line 14, what code would I need to return the $key where the $value is "yellow"?

1  #!/usr/bin/perl
2
3  # This program creates a hash then
4  # prints out what is in the hash
5
6  %fruit = (
7   'apple' => ['red','green'],
8   'kiwi' => 'green',
9   'banana' => 'yellow',
10  );
11
12 print "The apple is @{$fruit{apple}}.\n";
13 print "The kiwi is $fruit{kiwi}.\n";
14 print "What is yellow? ";

Answer 1

grep is the right tool for this job:

my @all_matches = grep { $fruit{$_} eq 'yellow' } keys %fruit;
print("$_ ") foreach @matching_keys;

my ($any_match) = grep { $fruit{$_} eq 'yellow' } keys %fruit;

Answer 2

I'm not so sure that's easy to do efficiently with a one-way hash. The whole point of a hash is to convert the key into a value (or position of the value if you're looking under the covers). You can do an exhaustive search over all the values, collecting the keys as you go but that's not as efficient as a hash lookup.

In order to go the other way efficiently, you might want to consider a two-way hash, something like:

%fruit = (
    'apple' => ['red','green'],
    'kiwi' => 'green',
    'banana' => 'yellow',
);
%antifruit = (
    'red' => 'apple',
    'green' => ['apple','kiwi'],
    'yellow' => 'banana',
);
print "The apple is @{$fruit{'apple'}}.\n";
print "The kiwi is $fruit{'kiwi'}.\n";
print "A yellow thing is $antifruit{'yellow'}.\n";