Return a Deep Hash Reference from Array of Keys

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

Greetings Fellow Monks, Monkettes & Monkees...

I was writing a program today and hit upon a Perl question I've never thought about before. Suppose I have some sort of hash structure (such as the one in my example below) and I would like to "reach in" and manipulate one of the nested hashes and that I know the necessary keys and in the correct order, but I only have them in an array form (i.e., qw/level1key level2key.../ etc.) What is the best method to return a reference from the hash using an array of keys?

I have constructed the following example which seems to work acceptably, but my question is: "Is the only and/or best approach to solve the problem?"

(Btw, the hash in the example is completely arbitrary and has no real-world significance)

Thanks

Jwhitten

#! /bin/perl

use Data::Dumper;

my $hash = {
        'wheels' => {
                'four' => {
                        'car' => {
                                'plain' => {
                                        'sporty' => 'honda',
                                        'retro' => 'volkswagon',
                                        },
                                'fancy' => {
                                        'red' => 'ferrari',
                                        'silver' => 'maserati',
                                        },
                                },
                        'truck' => {
                                'light' => {
                                        'chevy' => 'pickup',
                                        },
                                'utility' => {
                                        'us' => 'mail',
                                        'city' => 'trash',
                                        'muni' => 'power',
                                        },
                                },
                        },
                'one' => 'Unicycle',
                'two' => {
                        'pedals' => 'bicycle',
                        'motorized' => 'motorcycle',
                        },
                'three' => 'tricycle',
                },
        };


##---------------------------
## DEEP REFERENCE
##---------------------------

fetch($hash, 'wheels');
fetch($hash, 'wheels', 'one');
fetch($hash, 'wheels', 'four', 'truck', 'utility');
fetch($hash, 'wheels', 'four', 'truck', 'utility', 'blah'); # doesn't 
+exist

sub fetch {
        my ($hash, @keys) = @_;
        print "==> Fetching [" . join(', ', @keys) . ']' . "\n";
        my $results = deep_reference($hash, @keys);
        print Dumper($results), "\n\n";
        }

sub deep_reference {
        my ($hash, @keys) = @_;
        my $ref = undef;
        my $code = '$hash->' . join('->', map { "{$_}" } @keys);
        return eval $code;
        }
[download]

Produces:

==> Fetching [wheels]
$VAR1 = {
          'three' => 'tricycle',
          'one' => 'Unicycle',
          'two' => {
                     'pedals' => 'bicycle',
                     'motorized' => 'motorcycle'
                   },
          'four' => {
                      'car' => {
                                 'plain' => {
                                              'sporty' => 'honda',
                                              'retro' => 'volkswagon'
                                            },
                                 'fancy' => {
                                              'silver' => 'maserati',
                                              'red' => 'ferrari'
                                            }
                               },
                      'truck' => {
                                   'light' => {
                                                'chevy' => 'pickup'
                                              },
                                   'utility' => {
                                                  'city' => 'trash',
                                                  'muni' => 'power',
                                                  'us' => 'mail'
                                                }
                                 }
                    }
        };


==> Fetching [wheels, one]
$VAR1 = 'Unicycle';


==> Fetching [wheels, four, truck, utility]
$VAR1 = {
          'city' => 'trash',
          'muni' => 'power',
          'us' => 'mail'
        };


==> Fetching [wheels, four, truck, utility, blah]
$VAR1 = undef;
[download]

Comment on Return a Deep Hash Reference from Array of Keys Select or Download Code

Replies are listed 'Best First'.
Re: Return a Deep Hash Reference from Array of Keys by choroba (Cardinal) on Aug 14, 2013 at 21:49 UTC
See Data::Diver: `use Data::Diver 'Dive'; print Dive($hash, qw(wheels four truck utility muni));` [download] لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l]
Re^2: Return a Deep Hash Reference from Array of Keys by bratwiz (Sexton) on Aug 15, 2013 at 14:12 UTC
Thanks everybody for your responses! I didn't know about Data::Diver. I played around with it yesterday and its a very nice package and does exactly what I need. Thanks! John	[reply]
Re: Return a Deep Hash Reference from Array of Keys by tangent (Parson) on Aug 14, 2013 at 21:52 UTC
I doubt if this is the 'best' method but this is how I would do it: `sub fetch { my ($hash, @keys) = @_; print "==> Fetching [" . join(', ', @keys) . ']' . "\n"; for my $key (@keys) { return unless ref $hash eq 'HASH' && exists $hash->{$key}; $hash = $hash->{$key}; } print Dumper($hash), "\n\n"; }` [download]	[reply] [d/l]
Re: Return a Deep Hash Reference from Array of Keys by Laurent_R (Canon) on Aug 15, 2013 at 09:30 UTC
What about something like this: `DB<9> @list = qw /wheels four/; DB<10> print Dumper $hash->{shift @list}{shift @list}; $VAR1 = { 'car' => { 'plain' => { 'sporty' => 'honda', 'retro' => 'volkswagon' }, 'fancy' => { 'silver' => 'maserati', 'red' => 'ferrari' } }, 'truck' => { 'light' => { 'chevy' => 'pickup' }, 'utility' => { 'city' => 'trash', 'muni' => 'power', 'us' => 'mail' } } }; DB<11>` [download]	[reply] [d/l]
Re^2: Return a Deep Hash Reference from Array of Keys by bratwiz (Sexton) on Aug 16, 2013 at 20:38 UTC
Interesting idea, but it doesn't actually work. Here is the output: #! /bin/perl use Data::Dumper; my $hash = { 'wheels' => { 'four' => { 'car' => { 'plain' => { 'sporty' => 'honda', 'retro' => 'volkswagon', }, 'fancy' => { 'red' => 'ferrari', 'silver' => 'maserati', }, }, 'truck' => { 'light' => { 'chevy' => 'pickup', }, 'utility' => { 'us' => 'mail', 'city' => 'trash', 'muni' => 'power', 'corp' => { 'dom' => 'dominion p +ower', 'vap' => 'virginia p +ower', }, }, }, }, 'one' => 'Unicycle', 'two' => { 'pedals' => 'bicycle', 'motorized' => 'motorcycle', }, 'three' => 'tricycle', }; ##--------------------------- ## DEEP REFERENCE ##--------------------------- fetch2($hash, 'wheels'); fetch2($hash, 'wheels', 'four', 'truck', 'utility', 'blah'); fetch2($hash, 'wheels', 'four', 'truck', 'utility', 'corp', 'dom'); ## Using your suggested technique sub fetch2 { my ($hash, @keys) = @_; print "==> Fetching [" . join(', ', @keys) . ']' . "\n"; print Dumper $hash->{shift @keys}{shift @keys}; } [download] Produces: ==> Fetching [wheels] $VAR1 = undef; ==> Fetching [wheels, four, truck, utility, blah] $VAR1 = { 'car' => { 'plain' => { 'sporty' => 'honda', 'retro' => 'volkswagon' }, 'fancy' => { 'silver' => 'maserati', 'red' => 'ferrari' } }, 'truck' => { 'light' => { 'chevy' => 'pickup' }, 'utility' => { 'city' => 'trash', 'muni' => 'power', 'us' => 'mail', 'corp' => { 'dom' => 'dominion p +ower', 'vap' => 'virginia p +ower' } } } }; ==> Fetching [wheels, four, truck, utility, corp, dom] $VAR1 = { 'car' => { 'plain' => { 'sporty' => 'honda', 'retro' => 'volkswagon' }, 'fancy' => { 'silver' => 'maserati', 'red' => 'ferrari' } }, 'truck' => { 'light' => { 'chevy' => 'pickup' }, 'utility' => { 'city' => 'trash', 'muni' => 'power', 'us' => 'mail', 'corp' => { 'dom' => 'dominion p +ower', 'vap' => 'virginia p +ower' } } } }; [download] Whereas the expected result should be: ==> Fetching [wheels] $VAR1 = { 'three' => 'tricycle', 'one' => 'Unicycle', 'two' => { 'pedals' => 'bicycle', 'motorized' => 'motorcycle' }, 'four' => { 'car' => { 'plain' => { 'sporty' => 'honda', 'retro' => 'volkswagon' }, 'fancy' => { 'silver' => 'maserati', 'red' => 'ferrari' } }, 'truck' => { 'light' => { 'chevy' => 'pickup' }, 'utility' => { 'city' => 'trash', 'muni' => 'power', 'us' => 'mail', 'corp' => { 'dom' => + 'dominion power', 'vap' => + 'virginia power' } } } } }; ==> Fetching [wheels, four, truck, utility, blah] (nothing, because it doesn't exist) ==> Fetching [wheels, four, truck, utility, corp, dom] $VAR1 = 'dominion power'; [download]	[reply] [d/l] [select]
Re^3: Return a Deep Hash Reference from Array of Keys by Laurent_R (Canon) on Aug 16, 2013 at 21:13 UTC
The example under the debugger that I gave works only for a two-level hash dereferencing. It has to be adapted for other levels. I slightly modified the program you made based on my idea so that it takes into account the number of elements in the array of keys, as follows: #! /bin/perl use Data::Dumper; my $hash = { 'wheels' => { 'four' => { 'car' => { 'plain' => { 'sporty' => 'honda', 'retro' => 'volkswagon', }, 'fancy' => { 'red' => 'ferrari', 'silver' => 'maserati', }, }, 'truck' => { 'light' => { 'chevy' => 'pickup', }, 'utility' => { 'us' => 'mail', 'city' => 'trash', 'muni' => 'power', 'corp' => { 'dom' => 'dominion p +ower', 'vap' => 'virginia p +ower', }, }, }, }, 'one' => 'Unicycle', 'two' => { 'pedals' => 'bicycle', 'motorized' => 'motorcycle', }, 'three' => 'tricycle', } }; ##--------------------------- ## DEEP REFERENCE ##--------------------------- fetch2($hash, 'wheels'); fetch2($hash, 'wheels', 'four', 'truck'); fetch2($hash, 'wheels', 'four', 'truck', 'utility'); fetch2($hash, 'wheels', 'four', 'truck', 'utility', 'blah'); fetch2($hash, 'wheels', 'four', 'truck', 'utility', 'corp', 'dom'); ## Using your suggested technique sub fetch2 { my ($hash, @keys) = @_; print "==> Fetching [" . join(', ', @keys) . ']' . "\n"; my $command_string = 'print Dumper $hash->' . '{shift @keys}' +x scalar @keys; eval "$command_string"; } [download] And I get the following output, which seems correct to me: Read more... (4 kB)	[reply] [d/l] [select]
Re^4: Return a Deep Hash Reference from Array of Keys by bratwiz (Sexton) on Sep 13, 2013 at 18:20 UTC
Re^5: Return a Deep Hash Reference from Array of Keys by Laurent_R (Canon) on Sep 13, 2013 at 18:43 UTC