Retrieving from a Hash in Insertion Order

Problem

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

Solution

Use the Tie::IxHash module.

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

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.