Sorting a Hash

Problem

You need to work with the elements of a hash in a particular order.

Solution

Use keys to get a list of the keys, then sort them based on the ordering you want:

# %HASH is the hash to sort @keys = sort {
 criterion()
}
(keys %hash); foreach $key (@keys) {
 $value = $hash{$key};
 # do something with $key, $value }

Discussion

Even though you can't directly maintain a hash in a specific order (unless you use the Tie::IxHash module mentioned in ), you can access its entries in any order.

This technique offers many variations on the same basic mechanism: You extract the keys, reorder them using the sort function, and then process the entries in the new order. All the sorting tricks shown in Arrays, can be used here. Let's look at some applications.

The following code simply uses sort to order the keys alphabetically:

foreach $food (sort keys %food_color) {
 print "$food is $food_color{$food}.\n";
}

This sorts the keys by their associated values:

foreach $food (sort {
 $food_color{$a} cmp $food_color{$b}
}
keys %food_color) {
 print "$food is $food_color{$food}.\n";
}

This sorts by length of the values:

@foods = sort {
 length($food_color{$a}) <=> length($food_color{$b})
}
keys %food_color; foreach $food (@foods) {
 print "$food is $food_color{$food}.\n";
}

Sorting a Hash

Problem

Solution

Discussion

See Also