references changing a hash value

tcf03 has asked for the wisdom of the Perl Monks concerning the following question:

Can someone help me to understand the behavior of the following code? ~~Im unable to find this documented.~~

#!/usr/bin/perl -w

use strict;

my %animals=( 'cow'   => "bessie",
              'horse' => "ed",
              'duck'  => "goose",
              'dog'   => "rexx"
            );

print "my animals\n";
print_hash();

my $ranimals=\%animals;
$$ranimals{'duck'} = "franklin";

print "\nmy referenced animals\n";
print_hash();

sub print_hash
{
    for ( sort keys(%animals) )
    {
        print "$_ = $animals{$_}\n";
    }
}
[download]

when I change $$ranimals{'duck'} to shouldnt $animals{'duck'} still be "goose"? Or am I just greatly misunderstanding what this is doing?

Update
after typing this out I think Im seeing the error of my ways. Im going to leave the question stand though. My original intention was to review a lot of the code ive written over the years and bring my code more into line with my understanding of perl now. Ive recently read a few books such as "Effective Perl Programming" and "Advanced Perl Programming" and am trying to bring my skills to a higher level. I think I need to have another go-round with perlref.

Ted

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Re: references changing a hash value by davorg (Chancellor) on Apr 26, 2005 at 15:21 UTC
Try changing the line to `my $ranimals = { %animals };` to see the behaviour that you expected. In this case, you are creating a _copy_ of the hash and getting a reference to the new copy. In your original code you are taking a reference to the original (and only!) hash so any changes you make through the reference effect the original array. -- <http://www.dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply] [d/l]
Re: references changing a hash value by tlm (Prior) on Apr 26, 2005 at 13:53 UTC
No. When you take a reference to a hash (or anything else) with `\` you are not making a copy. Dereferencing the reference accesses the original. Read more... (991 Bytes) the lowliest monk	[reply] [d/l]
Re: references changing a hash value by splinky (Hermit) on Apr 26, 2005 at 14:01 UTC
$ranimals is a reference to %animals, so they both point at the same thing. So, when you change $$ranimals{'duck'}, you're also changing $animals{'duck'}.	[reply]