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5.6. 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

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.



See Also

The documentation for the CPAN module Tie::IxHash; Recipe 13.15











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