Array shuffle code produces 'Modification of non-creatable array value attempted' error

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

It's been a while since I've played in Perl but I remember the below shuffle code working. However, on the script I'm working on today says there is an error 'Modification of non-creatable array value attempted, subscript -1). The specific line it is complaining about is @$array$i,$j = @$array$j,$i; Any suggestions on how to fix this?

sub shuffle
{
    my $array = shift;
    my $i = @$array;
    while ( --$i )
    {
        my $j = int rand( $i+1 );
        @$array[$i,$j] = @$array[$j,$i];
    }
}
[download]

Comment on Array shuffle code produces 'Modification of non-creatable array value attempted' error Download Code

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Re: Array shuffle code produces 'Modification of non-creatable array value attempted' error by toolic (Bishop) on May 29, 2015 at 18:27 UTC
Works for me when I pass it a reference to a simple array on Perl version 5.12.2: `use warnings; use strict; use Data::Dumper; my $arr = [1..5]; shuffle($arr); print Dumper($arr); sub shuffle { my $array = shift; my $i = @$array; while ( --$i ) { my $j = int rand( $i+1 ); @$array[$i,$j] = @$array[$j,$i]; } } __END__ $VAR1 = [ 3, 5, 4, 1, 2 ];` [download] What version of Perl? How do you call the sub? See also List::Util::shuffle	[reply] [d/l]
Re: Array shuffle code produces 'Modification of non-creatable array value attempted' error by Anonymous Monk on May 29, 2015 at 19:42 UTC
The probable cause is passing an empty array to shuffle: `shuffle([]);` Suggested fix: `my $i = @$array \|\| return;`	[reply] [d/l] [select]
Re: Array shuffle code produces 'Modification of non-creatable array value attempted' error by stevieb (Canon) on May 29, 2015 at 18:48 UTC
toolic's code works on Perl 5.18.2 as well.	[reply]
Re: Array shuffle code produces 'Modification of non-creatable array value attempted' error by ikegami (Patriarch) on Jun 01, 2015 at 16:45 UTC
The error occurs when you assign to a negative index so large that it indicates a negative element. `$ perl -e' @a = qw( abc def ); $a[-1] = "ghi"; $a[-2] = "jkl"; $a[-3] = "mno"; ' Modification of non-creatable array value attempted, subscript -3 at - +e line 4.` [download] In your code, that will happen when an empty array is passed to `shuffle`. Fix by changing `while ( --$i )` [download] to `while ( --$i > 0 )` [download]	[reply] [d/l] [select]