ref array being modified unexpectively

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

Hello Monks,

I have a problem with ref array which i have spent sometimes debugging it. I fixed it but still don't understand how this can happen. Below is a shorten version of the code:

my $ra = [ 'A', 'B', 'C' ];

print STDERR '1. ra = ' . Dumper( $ra );

test_ra( $ra );

print STDERR '2. ra = ' . Dumper( $ra );

exit;


sub test_ra
{
    my $data = shift;
    foreach my $item ( @{ $data } )
    {
        $item = 'Z';
    }
}
[download]

After running the code:

1. ra = [ 'A','B','C' ]
2. ra = [ 'Z','Z','Z' ]
[download]

Apparently assigning some other value to $item modifies my ref array. This is something i'm not expecting. Off course when i copy the content of the array to another one (my @dup_array = @{$data}) and use it, the problem will disappeared.

Maybe i've overlooked something, but hopefully you can shine some light on the darkness.

TIA

PS: i'm using v5.8.8

Comment on ref array being modified unexpectively Select or Download Code

Replies are listed 'Best First'.
Re: ref array being modified unexpectively by davido (Cardinal) on Jul 29, 2011 at 08:52 UTC
Yes, there are two things you're overlooking: In perlsyn you'll see that within a `foreach` loop, the loop iterator variable (`$item`) becomes an alias to each element in the array being looped over. So if you modify $item, you modify the corresponding element within the loop's array. Also, since you're passing a reference to `test_ra()`, the subroutine is not acting upon a copy of the array referred to by $ra, but rather, the same exact array. In other words, if you were to `print scalar $ra` and also `print scalar $data`, you would see that they're the same reference. Dave	[reply] [d/l] [select]
Re: ref array being modified unexpectively by moritz (Cardinal) on Jul 29, 2011 at 08:52 UTC
The behavior you observe is to be expected, if one is sufficiently familiar with Perl 5. In particular none of the steps cause a copy of the array (hey, you're passing a reference after all!). Perhaps the most surprising point is that `foreach my $item` alises `$item` to the current list/array element. But again that's "works as designed". Perl 6 - second systems done right	[reply] [d/l] [select]