Question: How do I sort a Perl hash by the hash key?

Answer: Sorting the output of a Perl hash by the hash key is fairly straightforward. It involves two Perl functions, keys and sort, along with the good old foreach statement.

This is easiest to demonstrate by example. Suppose we have a class of five students. Their names are kim, al, rocky, chrisy, and jane.

Creating a Perl hash

Suppose these students just took a test, and we stored their grades in a hash named grades (note that hashes were called associative arrays prior to the release of Perl 5).

The hash definition might look like this:

%grades = (
	kim       => 96,
	al        => 63,
	rocky     => 87,
	chrisy    => 96,
	jane      => 79,
);

If you're familiar with a Perl hash, you know that the student names are the hash keys, and the test scores are the hash values.

Printing a Perl hash

If you want to print out the student names and scores, sorted in alphabetical order by student name, you can use this following Perl code snippet:

print "\n\tGRADES SORTED BY STUDENT NAME:\n";
foreach $key (sort (keys(%grades))) {
   print "\t\t$key \t\t$grades{$key}\n";
}

The Perl keys function returns a normal array of the keys of the hash. Our sample hash is named grades, so the keys function returns the names of the students. One attribute of the keys function, however, is that the keys are returned in what appears to be a random order. Therefore, you need to use sort to sort the keys in alphabetical order to get the desired printout.

A complete Perl hash sort by key program

Here's a complete Perl hash sort example program that prints the contents of the grades hash, sorted by student name:

#!/usr/bin/perl -w
#
#  printHashByKey.pl  -  Created by DevDaily Interactive, Inc.
#

%grades = (
	kim       => 96,
	al        => 63,
	rocky     => 87,
	chrisy    => 96,
	jane      => 79,
);

print "\n\tGRADES SORTED BY STUDENT NAME:\n";
foreach $key (sort (keys(%grades))) {
   print "\t\t$key \t\t$grades{$key}\n";
}

The output of this program looks like this:

GRADES SORTED BY STUDENT NAME:
	al 		63
	chrisy 		96
	jane 		79
	kim 		96
	rocky 		87

Perl hash sorting - final thoughts

• We have a companion Perl FAQ named "How to sort a hash by the hash value" which demonstrates how to sort by the hash value, which is a little more tricky.
• We also have many devdaily.com Perl FAQs, and that list keeps growing every year.

Related Perl hash tutorials

I hope you found this short Perl hash tutorial helpful. We have many more Perl hash tutorials on this site, including the following:

Getting started Perl hash tutorials:

• Perl hash introduction/tutorial
• Perl foreach and while: how to loop over the elements in a Perl hash
• Perl hash add - How to add an element to a Perl hash
• How to print each element in a Perl hash

More advanced Perl hash tutorials:

• How to see if a Perl hash contains a key
• How to delete an element from a Perl hash by key

The hash in Perl - hash sorting tutorials:

• Perl hash sorting - Sort a Perl hash by the hash key
• Perl hash sorting - Sort a Perl hash by the hash value