This seems to be a bug in Data::Dumper. With the perl debugger (before the grep, thanks tye):

main::(-:1):    my @matrix = ();
  DB<1> n
main::(-:2):    $matrix[2]->[3] = "What's goin' on?";
  DB<1> n
main::(-:3):    grep {/Huh?/}@$_ for @matrix;
  DB<1> print @matrix
ARRAY(0x8103f2c)
  DB<2> x @matrix
0  undef
1  undef
2  ARRAY(0x8103f2c)
   0  empty slot
   1  empty slot
   2  empty slot
   3  'What\'s goin\' on?'
  DB<3>
[download]

With Data::Denter:

Before grep:
@
    ?
    ?
    @
        ?
        ?
        ?
        What's goin on?

After grep:
@
    @
    @
    @
        ?
        ?
        ?
        What's goin on?
[download]

... indicating undef (?) before the grep, as normal, and empty array refs (@) after the grep, as normal due to autovivification upon array dereference. Only Data::Dumper seems to get it wrong, or at least what seems to be wrong.

Yes, but try this one:


use Data::Denter;

my @matrix = ();
$matrix[2]->[3]= "What's goin on?";

print "Before grep:\n", Denter(\@matrix);

#print grep{ /Huh?/ }@$_ for @matrix;
print map { grep{ /Huh?/ }@$_ } @matrix;
print "\nAfter grep:\n", Denter(\@matrix);
[download]



Where is the bug then in Data::Dumper or in Data::Denter or in map, grep, for or autovivification?

$anarion=\$anarion;

s==q^QBY_^=,$_^=$[x7,print

It's not unreasonable that it is a Data::Dumper flaw. Recently I was using Data::Dumper on some not so complex structures and some of the values it output were array-refs (where no array-ref ought to live) i.e. it output the scalar form of the array-ref, not even an evalable form.

--
perl -pe "s/\b;([st])/'\1/mg"

Quick clarification: Note that the first set of output is from before grep gets run. It is the grep that autovivifies those empty array references.

        - tye (but my friends call me "Tye")

1) Perl has a special value for undef, called SV_UNDEF. Rather than create a separate 'undef' for each uninitialized value, Perl stores a pointer to this special undef value. Thus, the output from Data::Dumper is, in a way, correct; all of the uninitialized array elements are pointing to the same value, SV_UNDEF.

2) Auto-vivification. grep { 1 } @$_ for @matrix; treats each element of @matrix as an array reference, so the uninitialized elements are autovivified.

2.5) and why for can iterate over it and map chokes?

Take a look at my code above

$anarion=\$anarion;

s==q^QBY_^=,$_^=$[x7,print

dump_matrix();

sub dump_matrix {
   for my $i (0..2) {
       print "row: $i [ ";
       for my $j (0..3) {
           print "$matrix[$i][$j], ";
       }
       print "]\n";
   }
}
[download]

--Jim

Update: Ah. Thanks tachyon, I would've researched it but I've been competing for computer time today. Five year olds always win :(

Makes me wonder how Data::Dumper is traversing the structure

Data::Dumper is an XS module so it does it in C

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

Actually its both.

When I get some time i'll check to whether the pure perl version does the same thing.

Yves / DeMerphq
--
When to use Prototypes?

Also, IMO when using Dumper one should _always_ set Purity to be true

use Data::Dumper;
$Data::Dumper::Purity=1;
[download]

Yves / DeMerphq
--
When to use Prototypes?

grep { /Huh?/ } @matrix;
[download]

grep { /Huh?/ } @$_ for @matrix;
[download]

@matrix = ( ["\n\nNo "], ["I ", "did "], ["not mean that, ", "get it n
+ow?"] );

grep{print}@matrix;

grep{print}@$_ for @matrix;
[download]

perlman:perlref

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

I would report this as a bug to p5p, and also make sure to actually initialize all rows in the array of arrays before proceeding to use it as such.