comment on

G'day johnfl68,

"Arrays, hashes, array of hashes, hash of arrays, etc., they have always been confusing for me. ... What am I missing?"

While you may be having some difficulty with certain data structures, I suspect your underlying problem is in the areas of data types, their references and the commands to use to manage them. This is particularly borne out by comments about getting HASH(0xd6a9690) instead of a string and, in a separate reply, "Can't use string ... as a HASH ref".

If you haven't already read perlintro, I suggest you do. If you have read it, I'd recommend revising the "Perl variable types" section. Don't be put off by terms such as "introductory" and "beginner"; this particular piece of documentation is peppered with links to more detailed information and advanced topics: it's an excellent stepping-stone when researching some specific area and you're not sure where to start. The section I pointed out has multiple links: for your immediate needs, perhaps start with perlreftut and perldata.

I put together the following script, which uses a data structure similar to yours, but which has a whole range of data types and references. It shows how each can be examined: is it defined? is it a reference? what sort of reference? does it have content? how to dereference? The first half of the examples are likely to be things you're dealing with generally; the second half are somewhat more advanced (perhaps come back to them at a later date).

#!/usr/bin/env perl -l

use strict;
use warnings;

use IO::Handle;

my $complex_data_hash_ref = {
    not_used_1 => '',
    example => {
        undefined           => undef,
        empty_string        => '',
        non_empty_string    => 'string',
        empty_array_ref     => [],
        non_empty_array_ref => [ 1, 2, 3 ],
        empty_hash_ref      => {},
        non_empty_hash_ref  => { x => 24, y => 25, z => 26 },
        scalar_ref          => \42,
        glob_ref            => \*STDOUT,
        code_ref            => sub { print "\n\t\tHello, world!\n" },
        ref_ref             => \\99,
        blessed_ref         => IO::Handle::->new(),
        ref_ref_ref         => \\\999,
        regex               => qr{match},
        vstring             => v65,
        vstring_ref         => \v65,
    },
    not_used_2 => '',
};

my @example_keys_to_parse = qw{
    undefined empty_string non_empty_string empty_array_ref 
    non_empty_array_ref empty_hash_ref non_empty_hash_ref
    glob_ref code_ref ref_ref blessed_ref ref_ref_ref
    regex vstring vstring_ref
};

for my $example_key (@example_keys_to_parse) {
    my $example_key_value
        = $complex_data_hash_ref->{example}{$example_key};
    examine_data($example_key, $example_key_value);
}

sub examine_data {
    my ($in_key, $in_value) = @_;

    print "Examining '$in_key':";

    if (ref $in_value) {
        print "\tThe value of '$in_key' is a reference.";
        print "\tThe value of '$in_key' is '$in_value'.";

        my $ref = ref $in_value;

        print "\tThe ref() value of '$in_key' is '$ref'.";

        if ($ref eq 'SCALAR') {
            my $deref = ${$in_value};
            print "\tIts dereferenced value is '$deref'.";
        }
        elsif ($ref eq 'ARRAY') {
            my @deref = @{$in_value};

            if (@deref) {
                print "\tIts dereferenced value has these elements: ",
                        "'@deref'.";
            }
            else {
                print "\tIts dereferenced value has no elements.";
            }
        }
        elsif ($ref eq 'HASH') {
            my %deref = %{$in_value};
            my @deref_keys = keys %deref;

            if (@deref_keys) {
                print "\tIts dereferenced value has these keys: ",
                        "'@deref_keys'.";

                my @deref_values = values %deref;

                print  "\tIts dereferenced value has these values: ",
                        "'@deref_values'.";
            }
            else {
                print "\tIts dereferenced value has no key-value pairs
+.";
            }
        }
        elsif ($ref eq 'GLOB') {
            print $in_value "\tA globref can be a filehandle opened fo
+r writing.";
        }
        elsif ($ref eq 'CODE') {
            $in_value->();
        }
        elsif ($ref eq 'REF') {
            examine_data('dereferenced__' . $in_key, ${$in_value});
        }
        else {
            print "\tTODO: Further work on this type ",
                    'is left as an exercise for the reader.';
        }
    }
    else {
        print "\tThe value of '$in_key' is not a reference.";

        if (defined $in_value) {
            print "\tThe value of '$in_key' is defined.";

            if (length $in_value) {
                print "\tThe value of '$in_key' is '$in_value'.";
            }
            else {
                print "\tThe value of '$in_key' is zero-length.";
            }
        }
        else {
            print "\tThe value of '$in_key' is not defined.";
        }
    }
}
[download]

Notes:

IO::Handle was only used to generate a value for "blessed_ref". It has no significance beyond the fact that its new() method returns a blessed reference.
While I've used ref() throughout; do read its documentation, particularly with respect to the reftype() function of Scalar::Util often being a better choice.
I've shown dereferencing as ${$ref}, @{$ref}, and so on. In simple situations like that, the braces are not necessary (i.e. $$ref, @$ref, etc. are fine); however, for more complex code, I recommend you use them (e.g. @{$obj->get_aref($h{x})}). If you're using a recent Perl version, consider Postfix Dereference Syntax (I personally think this makes the code easier to read, e.g. $obj->get_aref($h{x})->@*).
I've kept the syntax as simple as possible for didactic reasons. In a number of cases, I probably wouldn't normally be so verbose; you needn't be either.
You can find documentation for the functions I've used via Perl functions A-Z.

The output is rather lengthy. It's in the spoiler, below. You may find it's a useful exercise to determine what you think the output of each example is before looking.

— Ken

In reply to Re: If hash not defined help by kcott
in thread If hash not defined help by johnfl68

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