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5.7. Retrieving from a Hash in Insertion Order

5.7.1. Problem

The keys
and each functions traverse the hash elements in a
strange order, and you want them in the order in which you inserted
them.

5.7.2. Solution

Use the
Tie::IxHash module.

use Tie::IxHash;
tie %HASH, "Tie::IxHash";
# manipulate %HASH
@keys = keys %HASH;         # @keys is in insertion order

5.7.3. Discussion

Tie::IxHash makes keys, each,
and values return the hash elements in the order
they were added. This often removes the need to preprocess the hash
keys with a complex sort comparison or maintain a
distinct array containing the keys in the order they were inserted
into the hash.

Tie::IxHash also provides an
object-oriented interface to splice,
push, pop,
shift, unshift,
keys, values, and
delete, among others.

Here''s an example, showing both keys and
each:

# initialize
use Tie::IxHash;
tie %food_color, "Tie::IxHash";
$food_color{"Banana"} = "Yellow";
$food_color{"Apple"}  = "Green";
$food_color{"Lemon"}  = "Yellow";
print "In insertion order, the foods are:\n";
foreach $food (keys %food_color) {
print "  $food\n";
}
print "Still in insertion order, the foods'' colors are:\n";
while (( $food, $color ) = each %food_color ) {
print "$food is colored $color.\n";
}
In insertion order, the foods are:
  Banana
  Apple
  Lemon
Still in insertion order, the foods'' colors are:
Banana is colored Yellow.
Apple is colored Green.
Lemon is colored Yellow.

5.7.4. See Also

The documentation for the CPAN module Tie::IxHash; Recipe 13.5