Creating a hash of hashes of arrays

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

Hi Monks. I am a very new to hashes in Perl. In the following code, I am creating a hash of hashes of arrays.

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my @array1 = (
  "hello",
  "world"
);

my @array2 = (
  "here"
);

my @array3 = (
  "there"
);

my %inner_hash = (
  innerkey1 => \@array1,
  innerkey2 => \@array2,
  innerkey3 => \@array3
);

my %hash_of_hash_of_arrays = (
  key => \%inner_hash
);

print Dumper(\%hash_of_hash_of_arrays);

exit(0);
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This gives the output:

$VAR1 = {
          'key' => {
                     'innerkey2' => [
                                      'here'
                                    ],
                     'innerkey3' => [
                                      'there'
                                    ],
                     'innerkey1' => [
                                      'hello',
                                      'world'
                                    ]
                   }
        };
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Is there any way I can get this same output by defining only one variable? Something like:

my %hash_of_hash_of_arrays = {
           'key' => {
                      'innerkey2' => [
                                       'here'
                                     ],
                      'innerkey3' => [
                                       'there'
                                     ],
                      'innerkey1' => [
                                       'hello',
                                       'world'
                                     ]
                    }
         };
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...but that gives an error about "Reference found where even-sized list expected", and the output looks like:

$VAR1 = {
          'HASH(0x57e1d78)' => undef
        };
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which obviously is not what I want at all. Please help.

Comment on Creating a hash of hashes of arrays Select or Download Code

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Re: Creating a hash of hashes of arrays
by toolic (Bishop) on Dec 15, 2014 at 16:57 UTC

my %hof = (
          'key' => {
                     'innerkey2' => [
                                      'here'
                                    ],
                     'innerkey3' => [
                                      'there'
                                    ],
                     'innerkey1' => [
                                      'hello',
                                      'world'
                                    ]
                   }
        );
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The curlies are used to construct a reference to a hash.

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Re: Creating a hash of hashes of arrays
by Athanasius (Cardinal) on Dec 15, 2014 at 17:13 UTC

Hello estreb, and welcome to the Monastery!

As toolic says, to construct a hash you need to change the braces to parentheses:

my %hash = ( ... );
...
print Dumper(\%hash);
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Alternatively, keep the braces (which construct an anonymous hash and return a reference to it) and assign the result to a scalar variable (because a reference is always a scalar value, regardless of the type of thing it refers to):

my $hash_ref = { ... };
...
print Dumper($hash_ref);
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Either will give you the output you were expecting. See perlreftut, then perldsc.

Hope that helps,

Update: Fixed typo. Thanks to AnomalousMonk and Anonymous Monk.

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^2: Creating a hash of hashes of arrays

by Anonymous Monk on Dec 15, 2014 at 18:05 UTC

my %hash_ref = { ... };

my $hash_ref = { ... };

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Re: Creating a hash of hashes of arrays
by AnomalousMonk (Archbishop) on Dec 15, 2014 at 18:36 UTC

It's worth noting that Data::Dumper (core) and Data::Dump (my favorite) and many other, similar modules are designed to produce text that is compilable code. So compiling and running the exact OPed Data::Dumper output (with strict disabled!):

$VAR1 = {
           'key' => {
                      'innerkey2' => [
                                       'here'
                                     ],
                      'innerkey3' => [
                                       'there'
                                     ],
                      'innerkey1' => [
                                       'hello',
                                       'world'
                                     ]
                    }
         };

print Dumper $VAR1;
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strict

our

use warnings;
use strict;

use Data::Dump qw(dd);

my $something_else = {
    ...,
    };
dd $something_else;
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Update: ~~For some reason, I can't get either [mod://Data::Dump] or [cpan://Data::Dump] to link to the module, but I can see it on CPAN. You'll just have to fend for yourselves.~~ Later: [mod://Data::Dump] works now. And there was great rejoicing...

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Re: Creating a hash of hashes of arrays
by estreb (Novice) on Dec 15, 2014 at 19:14 UTC

Thanks, all. I have what I need.

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