Arrays of Hashes (Programming Perl)

9.3. Arrays of Hashes

An array of hashes is useful when you have a bunch of records that you'd like to access sequentially, and each record itself contains key/value pairs. Arrays of hashes are used less frequently than the other structures in this chapter.

9.3.1. Composition of an Array of Hashes

You can create an array of anonymous hashes as follows:

@AoH = (
    {
       husband  => "barney",
       wife     => "betty",
       son      => "bamm bamm",
    },
    {
       husband => "george",
       wife    => "jane",
       son     => "elroy",
    },

    {
       husband => "homer",
       wife    => "marge",
       son     => "bart",
    },
  );

To add another hash to the array, you can simply say:

push @AoH, { husband => "fred", wife => "wilma", daughter => "pebbles" };

9.3.2. Generation of an Array of Hashes

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

husband=fred friend=barney

you could use either of the following two loops:

while ( <> ) {
    $rec = {};
    for $field ( split ) {
        ($key, $value) = split /=/, $field;
        $rec->{$key} = $value;
    }
    push @AoH, $rec;
}

while ( <> ) {
    push @AoH, { split /[\s=]+/ };
}

If you have a subroutine get_next_pair that returns key/value pairs, you can use it to stuff @AoH with either of these two loops:

while ( @fields = get_next_pair() ) {
    push @AoH, { @fields };
}

while (<>) {
    push @AoH, { get_next_pair($_) };
}

You can append new members to an existing hash like so:

$AoH[0]{pet} = "dino";
$AoH[2]{pet} = "santa's little helper";

9.3.3. Access and Printing of an Array of Hashes

You can set a key/value pair of a particular hash as follows:

$AoH[0]{husband} = "fred";

To capitalize the husband of the second array, apply a substitution:

$AoH[1]{husband} =~ s/(\w)/\u$1/;

You can print all of the data as follows:

for $href ( @AoH ) {
    print "{ ";
    for $role ( keys %$href ) {
         print "$role=$href->{$role} ";
    }
    print "}\n";
}

and with indices:

for $i ( 0 .. $#AoH ) {
    print "$i is { ";
    for $role ( keys %{ $AoH[$i] } ) {
         print "$role=$AoH[$i]{$role} ";
    }
    print "}\n";
}

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