Re: Invisible Array Loop Counter?

From: Paul Lalli (ittyspam_at_yahoo.com)
Date: 06/02/04 

Date: Tue, 1 Jun 2004 18:47:08 -0400

On Tue, 1 Jun 2004, zzapper wrote:

> In fact none of the three methods actually shortened the array
>
> @t=@r=@q=qw(fred joe freddy sick);
>
> for (@q)
> {
> delete $q[$cnt] if /fred/;
> $cnt++;
> }
> print @q;
>
> for (0..$#r)
> {
> delete $r[$_] if $r[$_] =~ /fred/;
> }
> print @r;
>
> for (@t)
> {
> $_="" if /fred/;
> }
> print @t;
>
> print $#q;
> print $#r;
> print $#t;

Running your code, for all three methods, we see that the sizes of the
arrays are still four. However, printing out the values shows only the
'joe' and 'sick' elements. Why is this? Well, I modified your code to
print out the full contents of the arrays, using Data::Dumper.

use Data::Dumper;
@t=@r=@q=qw(fred joe freddy sick);
for (@q)
{
   delete $q[$cnt] if /fred/;
   $cnt++;
}
for (0..$#r)
{
   delete $r[$_] if $r[$_] =~ /fred/;
}
for (@t)
{
   $_="" if /fred/;
}

print Dumper(\@q)
print Dumper(\@r)
print Dumper(\@t);
__END__

And here's the output:

$VAR1 = [
          undef,
          'joe',
          ${\$VAR1->[0]},
          'sick'
        ];
$VAR1 = [
          undef,
          'joe',
          ${\$VAR1->[0]},
          'sick'
        ];
$VAR1 = [
          '',
          'joe',
          '',
          'sick'
        ];

What we see here is that in the first method, the first element you tried
to delete actually became undef, and the third element became a reference
back to that first element. (Right? Someone back me up here, or tell me
if that's wrong).
In the second method, a similar event happened.
In the third, you successfully modified the array so that the first and
third elements are equal to the empty string.

So in all cases, you modified the actual elements, without removing them
from the array. This is the behavior of the 'delete' function. (See
perldoc -f delete for more info).

Compare this to the method I suggested:
use strict;
use warnings;
use Data::Dumper;

my @q = qw(fred joe freddy sick);
@q = grep { !/fred/ } @q;
print Dumper (\@q);
__END__

$VAR1 = [
          'joe',
          'sick'
        ];

This method actually removes the elements from the array.

Paul Lalli

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