Filtering a hash of hash in perl

Question

I have a hash of hash that I need to filter. I did find how to do a lookup but it did not answer my question.

Say I have a hash of hash like that :

my %HoH = (
    flintstones => {
        husband   => "fred",
        pal       => "barney",
        town      => "springfield"
    },
    jetsons => {
        husband   => "george",
        wife      => "jane",
        "his boy" => "elroy",
    },
    simpsons => {
        husband   => "homer",
        wife      => "marge",
        kid       => "bart",
        town      => "springfield",
    },
);

I lets say I want all the townfolks from springfield. I want the same hash of hash in output without the outsiders.

my %HoH = (
    flintstones => {
        husband   => "fred",
        pal       => "barney",
        town      => "springfield"
    },
    simpsons => {
        husband   => "homer",
        wife      => "marge",
        kid       => "bart",
        town      => "springfield",
    },
);

It seems silly but can't figure out how to filter the struture. The goal would be to iterate on all the people of springfield after filtering.

I of course did some research and the closest thing I came by is hash slices. But they seem scary.

Answer 1

You first need to find the keys of the elements you want to remove:

grep { $HoH{$_}{town} eq 'springfield' } keys(%HoH)

Then you delete them:

delete $HoH{$_} for grep { $HoH{$_}{town} eq 'springfield' } keys(%HoH);

Or using a hash slice:

delete @HoH{ grep { $HoH{$_}{town} eq 'springfield' } keys(%HoH) };

Answer 2

If you need to grep or map through hashes, then you can consider to use grepp or mapp from List::Pairwise. Advantage is that there's no need to mention the original hash variable anymore in the grep/map code block, making it more "functional". So your problem could be solved like this:

use List::Pairwise qw(grepp);
%HoH = grepp { $b->{town} eq 'springfield' } %HoH; # $a is the current key, $b is the current value