Re: Find common values in a hash and print the respective keys

[Anonymous Monk]

The invert() function from the Var::Pairs module can also do what you want:

my %your_hash = (
   aa => 1,
   bb => 2,
   cc => 3,
   dd => 4,
   ee => 2,
   ff => 5,
   gg => 3,
   hh => 2,
);

use Var::Pairs;

my %keys_for_each_value = invert(%your_hash);

use Data::Dumper 'Dumper';
warn Dumper \%keys_for_each_value;

[Jim]

Here's a version optimized for clarity and verisimilitude.

#!perl
#
# farm.pl

use strict;
use warnings;

# Lookup table of farm animals by home
my %home_of = (
    Cat     => 'House',
    Cow     => 'Field',
    Dog     => 'Yard',
    Goat    => 'Barn',
    Hamster => 'House',
    Hen     => 'Coop',
    Horse   => 'Barn',
    Sheep   => 'Barn',
    Pig     => 'Sty',
    Rooster => 'Yard',
);

my %animals_by;

# Add each animal to a list of farm animals by home...
while (my ($animal, $home) = each %home_of) {
    push @{ $animals_by{$home} }, $animal;
}

# Print ordered lists of animals by an ordered list of homes...
for my $home (sort keys %animals_by) {
    my $list_of_animals = join ', ', sort @{ $animals_by{$home} };

    print "$home\t$list_of_animals\n";
}

__END__
Barn    Goat, Horse, Sheep
Coop    Hen
Field   Cow
House   Cat, Hamster
Sty     Pig
Yard    Dog, Rooster