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Programming Perl, 3rd Edition
book

Programming Perl, 3rd Edition

by Larry Wall, Tom Christiansen, Jon Orwant
July 2000
Intermediate to advanced content levelIntermediate to advanced
1104 pages
35h 1m
English
O'Reilly Media, Inc.
Content preview from Programming Perl, 3rd Edition

Hashes of Hashes

A multidimensional hash is the most flexible of Perl's nested structures. It's like building up a record that itself contains other records. At each level, you index into the hash with a string (quoted when necessary). Remember, however, that the key/value pairs in the hash won't come out in any particular order; you can use the sort function to retrieve the pairs in whatever order you like.

Composition of a Hash of Hashes

You can create a hash of anonymous hashes as follows:

%HoH = (
    flintstones => {
        husband   => "fred",
        pal       => "barney",
    },
    jetsons => {
        husband   => "george",
        wife      => "jane",
        "his boy" => "elroy",  # Key quotes needed.
    },
    simpsons => {
        husband   => "homer",
        wife      => "marge",
        kid       => "bart",
    },
);

To add another anonymous hash to %HoH, you can simply say:

$HoH{ mash } = {
    captain  => "pierce",
    major    => "burns",
    corporal => "radar",
};

Generation of a Hash of Hashes

Here are some techniques for populating a hash of hashes. To read from a file with the following format:

flintstones: husband=fred pal=barney wife=wilma pet=dino

you could use either of the following two loops:

while ( <> ) {
    next unless s/^(.*?):\s*//;
    $who = $1;
    for $field ( split ) {
        ($key, $value) = split /=/, $field;
        $HoH{$who}{$key} = $value;
    }
}

while ( <> ) {
    next unless s/^(.*?):\s*//;
    $who = $1;
    $rec = {};
    $HoH{$who} = $rec;
    for $field ( split ) {
        ($key, $value) = split /=/, $field;
        $rec->{$key} = $value;
    }
}

If you have a subroutine get_family that returns a list of key/value pairs, you can ...

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Publisher Resources

ISBN: 0596000278Supplemental ContentErrata