Recipe 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:
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