Tie::SortHash - Perl module to keep hashes in a sorted order

  use Tie::SortHash;

  my %people = (
                'John Doe'  => 33,
                'Jane Doe'  => 29,
                'Jim Smith' => 15,
               );

  my $sortblock = q(
                    my $c = (split /\s+/, $a)[1];
                    my $d = (split /\s+/, $b)[1];

                           $c cmp $d
                              ||
                    $hash{$a} <=> $hash{$b}
                   );

  tie %people, 'Tie::SortHash', \%people, $sortblock;

  foreach my $name ( keys %people ) {
    print $name . " is " . $people{$name} . " years old.\n";
  }

  # This output will always be
  Jane Doe is 29 years old.
  John Doe is 33 years old.
  Jim Smith is 15 years old.

This module is a designed to be a light weight hash sorting mechanism. It is often frustrating to have a hash return elements in a random order, such as when using the keys(), values() and each() functions, or simply when iterating over them.

In order to tie() your hash to Tie::SortHash, you can use any of these methods:

  tie HASH, 'Tie::SortHash', HASHREF, SORTBLOCK;

  tie HASH, 'Tie::SortHash', HASHREF;

  tie HASH, 'Tie::SortHash';

It is important to remember that if you have elements in your HASH already, you must supply a reference to that hash in HASHREF.

For example:

  tie %people, 'Tie::SortHash', \%people;

If you don't, %people will be set to an empty hash. You probably don't want that.

Tie::SortHash implements all the methods that a tied hash class should. These are: TIEHASH, CLEAR, DELETE, DESTROY, EXISTS, FETCH, FIRSTKEY, NEXTKEY and STORE. With the exception of a few, these all work as they would on a normal hash. Those exceptions include:

FIRSTKEY

This will produce the first key according to the "sortblock".

NEXTKEY

This will produce each key according to the "sortblock", excluding the first which is hanled by FIRSTKEY.

It is a really bad idea to change the "sortblock" in the middle of an iteration, unless you actually want to. ( I'd be interested in why, though. )

After you have tied your hash, you can change the sort block at any time. Some examples include:

  (tied %people)->sortblock( q( $hash{$b} <=> $hash{$a} ) );

or:

  my $tied_ref = tie my %people, 'Tie::SortHash', \%people;

  $tied_ref->sortblock( q(
                          $hash{$a} <=> $hash{$b}
                                    ||
                                 $b cmp $a
                      )  );  

It is important to remember a few things about the sort block.

Always pass the sort block in a non-interpolated scalar

This allows you to have greater control over the sorting that you would like to do. With out it, you couldn't sort by value because your program would complain that %hash hasn't been declared. And $a and <$b> would need to be represented more like $Tie::SortHash::a.

%hash is generic within your sort block.

This is because the internal representation of the tie hash is most likley not representative of the hash you're tieing. And it allows the ability to manipulate and sort accoring to value.

In other words, within your "sortblock", %hash is the Tie::SortHashs' representation of your hash.

What happens when you have a syntax error in your "sortblock"?

The program dies, just like it would with any other syntax error. You will recieve a nice message ( $@ ) when this occurs. It will die when you try to assign to the "sortblock".

Casey Tweten, crt@kiski.net

Copyright (c) 2000 Casey Tweten. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

Version 1.00 Aug 19, 2000

perl(1), perltie, perlfaq4.