[Perl-beginners] Order in which keys are stored in a hash

In the perlvar documentation<http://perldoc.perl.org/perldata.html#List-value-constructors>,
there's a little blurb that says *Note that just because a hash is
initialized in that order doesn't mean that it comes out in that order. See
sort <http://perldoc.perl.org/functions/sort.html> for examples of how to
arrange for an output ordering.* It's easy to miss if you didn't know to
look for it.

My limited understanding is that Perl handles the order internally. Code
should never rely on a specific order.

Hash lookups are faster if the keys do not maintain any order. If you
add more elements, you will find that the order of the keys will change.
One thing Perl does guarantee is that the order of keys and values
will be the same provided you do not add or remove anything from the
hash. This allows things like this: inserting one hash in another.

#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dumper;

# Make Data::Dumper pretty
$Data::Dumper::Sortkeys = 1;
$Data::Dumper::Indent = 1;

# Set maximum depth for Data::Dumper, zero means unlimited
local $Data::Dumper::Maxdepth = 0;

# TBD

my %src_hash = (
a => 1,
b => 2,
c => 3,
);

my %dst_hash = (
x => -1,
y => -2,
z => -3,
);

print 'BEFORE: ', Dumper \%src_hash, \%dst_hash;

@dst_hash{keys %src_hash} = values %src_hash;

print 'AFTER: ', Dumper \%src_hash, \%dst_hash;

__END__

Oops! The reply mail went only to Shawn. I am sorry about that.
I assumed that the Reply-To header would be set to the list address :(
Should have been more careful. I was just asking if starting off with
the Llama book is a good start or should I be starting off with some
other book( and of course the Internet). Thanks

Hi Balachandran,
The order is determined by perl 5's internal hashing implementation and cannot
be relied on. If you want a particular order use an array, sort the keys first
or use a CPAN module. For more information see the resources on:

http://perl-begin.org/topics/hashes/

What you can rely on is for the order of the keys in the hash to remain the
same as long as it is not modified and this is the order in keys(), values()
and each().
Also see:

http://www.nntp.perl.org/group/perl.beginners/2011/03/msg116305.html

Regards,

Shlomi Fish