Extracting values from nested hashrefs

loris has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

If I have

my $animals = {
               gnu => {
                       humps => 0,
                      },
               dromedary => {
                             humps => 1,
                            },
               camel => {
                         humps => 2,
                        },
              };
[download]

is there any simple way of extracting an array of the values for humps without explicitly traversing the top-level hashrefs?

Thanks,

loris

Comment on Extracting values from nested hashrefs Select or Download Code

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Re: Extracting values from nested hashrefs by choroba (Cardinal) on Sep 12, 2012 at 09:13 UTC
I am not sure whether map plus double values counts as implicit traversing: `say for map values %$_, values %$animals;` [download] Update: simplified. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l]
Re^2: Extracting values from nested hashrefs by loris (Hermit) on Sep 12, 2012 at 09:35 UTC
Ah, sorry, my example was too simple. It should have been like this: `my $animals = { gnu => { humps => 0, mascot_for => 'emacs', }, dromedary => { humps => 1, mascot_for => 'perl', }, camel => { humps => 2, mascot_for => 'perl', }, };` [download] BTW, implicit traversing is OK; I just wanted to avoid some sort of loop with , say `while`. Thanks, loris	[reply] [d/l] [select]
Re^3: Extracting values from nested hashrefs by choroba (Cardinal) on Sep 12, 2012 at 09:45 UTC
You can use `map $_->{humps}, values %$animals;` [download] If your structure becomes even more complicated (e.g. different level of nesting), you should use recursion (or CPAN). لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l]
Re^3: Extracting values from nested hashrefs by Athanasius (Archbishop) on Sep 12, 2012 at 12:46 UTC
Here is an approach that should work no matter how complicated your data structure becomes (provided the data you want remains in the form `humps => d`): `#! perl use strict; use warnings; use Data::Dumper; my $animals = { gnu => { humps => 0, mascot_for => 'emacs', }, dromedary => { humps => 1, mascot_for => 'perl', }, camel => { humps => 2, mascot_for => 'perl', }, }; my $flat = Dumper($animals); print "$1\n" while $flat =~ / 'humps' \s+ => \s+ (\d+) /gx;` [download] Output: `2 0 1` [download] TMTOWTDI. Athanasius <°(((>< contra mundum	[reply] [d/l] [select]
Re: Extracting values from nested hashrefs by kcott (Archbishop) on Sep 12, 2012 at 09:25 UTC
G'day loris, Hash keys have no inherent order. If you want to retain the order you've shown, you'll need to specify it explicitly. Here's two solutions in the one script. Pick whichever is most appropriate for you. `#!/usr/bin/env perl use 5.010; use strict; use warnings; my $animals = { gnu => { humps => 0, }, dromedary => { humps => 1, }, camel => { humps => 2, }, }; my @humps; push @humps, $_->{humps} for map { $animals->{$_} } keys %$animals; say "Unsorted humps: @humps"; my @sorted_animals = qw{gnu dromedary camel}; my @sorted_humps; push @sorted_humps, $_->{humps} for map { $animals->{$_} } @sorted_ani +mals; say "Sorted humps: @sorted_humps";` [download] Outputs: `$ pm_nested_hash_extract.pl Unsorted humps: 2 0 1 Sorted humps: 0 1 2` [download] -- Ken	[reply] [d/l] [select]
Re^2: Extracting values from nested hashrefs by loris (Hermit) on Sep 12, 2012 at 09:51 UTC
The ordering happens to be irrelevant, so the unsorted version is fine. I see from your solution that the distinction I was trying to make between explicit and implicit was a bit foolish. What I meant was that I wanted to do things in a nifty perl-like way, as you have done, rather than in a dull, C-like way. Thanks, loris	[reply]
Re^3: Extracting values from nested hashrefs by Marshall (Canon) on Sep 12, 2012 at 11:51 UTC
Why be more cryptic that you need to be? Doing so will not be more efficient. `#!/usr/bin/perl -w use strict; my $animals = { gnu => { humps => 0, mascot_for => 'emacs', }, dromedary => { humps => 1, mascot_for => 'perl', }, camel => { humps => 2, mascot_for => 'perl', }, }; foreach my $critter (keys %$animals) { print "$critter "; #camel gnu dromedary } print "\n"; foreach my $critter (keys %$animals) #sort if you want to { print "$animals->{$critter}{humps} "; #2 0 1 } print "\n";` [download]	[reply] [d/l]
Re: Extracting values from nested hashrefs by trizen (Hermit) on Sep 12, 2012 at 13:39 UTC
Using recursion: `use 5.010; my $animals = { gnu => { humps => 0, mascot_for => 'emacs', }, dromedary => { humps => 1, zzz => { humps => 'nested', aaa => { humps => 'really nested', xxx => 0, }, }, mascot_for => 'perl', }, }; sub print_humps { my $hash_ref = shift; foreach my $key (keys %{$hash_ref}) { if (ref $hash_ref->{$key} eq "HASH") { print_humps($hash_ref->{$key}); } elsif ($key eq 'humps') { say $hash_ref->{$key}; } } } print_humps($animals); __END__ 0 1 nested really nested` [download]	[reply] [d/l]
Re: Extracting values from nested hashrefs by tobyink (Canon) on Sep 12, 2012 at 14:38 UTC
Here are four ways of doing it... use strict; use warnings; use Test::More; use B::Deparse; my $animals; my $deparse = B::Deparse->new; $deparse->ambient_pragmas(strict => 'all', warnings => 'all'); # Function to check that each solution works! sub this_works (&) { $animals = { gnu => { humps => 0, }, dromedary => { humps => 1, }, camel => { humps => 2, }, }; my @results = $_[0]->(); my $code = $deparse->coderef2text($_[0]); if (is_deeply [sort @results], [qw/ 0 1 2 /] ) { diag "this works $code"; } else { diag "FAILED! $code"; } } this_works { map($_->{humps}, values %$animals) }; this_works { map { $_->{humps} } values %$animals }; this_works { map { $animals->{$_}{humps} } keys %$animals }; this_works { require JSON::Path; JSON::Path->new('$..humps')->values($animals); }; done_testing(); [download] `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l]
Re: Extracting values from nested hashrefs by jdporter (Paladin) on Sep 12, 2012 at 13:58 UTC
I think you'll be interested in Data::Diver.	[reply]

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