How do I access values in a Perl Hash of Array of Arrays?

Question

I have built a hash containing arrays of arrays, let's call it %hash_multidim, such that the output from Data::Dumper looks like so:

      'Key1' => [
                   [
                     '-3.81',
                     '-1.91',
                     '-1.86',
                     '-1.70'
                   ],
                   [
                     '1.35',
                     '1.04',
                     '-1.01',
                     '-2.69'
                   ]
                 ],
      'Key2' => [
                    [
                      '-1.63'
                    ],
                    [
                      '-1.17'
                    ]
                  ],

Now, I would like to access and perform manipulations on the bottom-most level in this structure. For example, for 'Key1' I want to find the mean of the in row 1 (aka the mean of the array at [0]). Using List::Util qw(sum), I have written a subroutine called mean:

sub mean {
    return sum(@_)/scalar(@_);
}

However, if using the subroutine, eg:

my $test = mean($hash_multidim{Key1}[0]);
print $test;

I do not get what I expect. In fact, I get:

43678288

Where did I go wrong? If I try to evaluate the result of

$hash_multidim{Key1}[0]

everything looks kosher. E.g.,

@test2 = $hash_multidim{Key1}[0];
print Dumper(\@test2);

produces this output:

$VAR1 = [
          [
            '-3.81',
            '-1.91',
            '-1.86',
            '-1.70'
          ]
        ];

mob · Accepted Answer · 2013-08-19 20:44:10Z

4

$hash_multidim{Key1}[0] is an array reference, but mean expects a list. So you just need to "dereference" it. The syntax is a little tricky, but it is

my $test = mean( @{$hash_multidim{Key1}[0]} );

answered Aug 19, 2013 at 20:44

mob

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2 Comments

MCor Over a year ago

Thanks! My intuition was that it was a problem of dereferencing, but I messed up the syntax when attempting to fix it. I tend to panic when dereferencing part of a multidimensional data structure. Any tips (other than experience) or heuristics for knowing how/when to dereference? Actually, I'm fairly sure I already know when, but the how (for complex situations) often leaves me befuddled even after reading through perldocs.

mob Over a year ago

If your expression starts with $, then you always have a scalar (which could be a reference to an array or hash). If it starts with @ or %, then it's an array or hash.

ikegami · Accepted Answer · 2013-08-19 20:47:18Z

3

$hash_multidim{Key1}[0] is a scalar whose value is a reference to an array. You're passing that single scalar to mean when you mean to pass the values of the elements of the referenced array.

my $test = mean( @{ $hash_multidim{Key1}[0] } );

Note: The division operator cannot divide by a list, only by a number. As such, it imposes a scalar context on its operands. Your use of scalar is superfluous. You could simply use

sub mean { sum(@_)/@_ }

answered Aug 19, 2013 at 20:47

ikegami

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1 Comment

MCor Over a year ago

Thanks for the tip re: division by lists! I thought that I could write it your way, but I don't always program in Perl and I like to be explicit for myself (so I don't panic when I go back to my code later).

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