What does it mean when you try to print an array or hash using Perl and you get, Array(0xd3888)?

Question

What does it mean when you try to print an array or hash and you see the following; Array(0xd3888) or HASH(0xd3978)?

EXAMPLE

CODE

my @data =          


        

Answer 1

It's hard to tell whether you meant it or not, but the reason why you're getting array references is because you're not printing what you think you are.

You started out right when iterating over the 'rows' of @data with:

foreach my $line (@data) { ... }

However, the next line is a no-go. It seems that you're confusing text strings with an array structure. Yes, each row contains strings, but Perl treats @data as an array, not a string.

split is used to convert strings to arrays. It doesn't operate on arrays! Same goes for chomp (with an irrelevant exception).

What you'll want to do is replace the contents of the foreach loop with the following:

foreach my $line (@data) {

    print $line->[0].", ".$line->[1].", ".$line->[2]."\n";
}

You'll notice the -> notation, which is there for a reason. $line refers to an array. It is not an array itself. The -> arrows deference the array, allowing you access to individual elements of the array referenced by $line.

If you're not comfortable with the idea of deferencing with arrows (and most beginners usually aren't), you can create a temporary array as shown below and use that instead.

foreach my $line (@data) {

    my @random = @{ $line };
    print $random[0].", ".$random[1].", ".$random[2]."\n";
}

OUTPUT

1_TEST, 1_T, 1_TESTER
2_TEST, 2_T, 2_TESTER
3_TEST, 3_T, 3_TESTER
4_TEST, 4_T, 4_TESTER
5_TEST, 5_T, 5_TESTER
6_TEST, 6_T, ^_TESTER

A one-liner might go something like print "@$_\n" for @data; (which is a bit OTT), but if you want to just print the array to see what it looks like (say, for debugging purposes), I'd recommend using the Data::Dump module, which pretty-prints arrays and hashes for you without you having to worry about it too much.

Just put use Data::Dump 'dump'; at beginning of your script, and then dump @data;. As simple as that!

Answer 2

You're printing a reference to the hash or array, rather than the contents OF that.

In the particular code you're describing I seem to recall that Perl automagically makes the foreach looping index variable (my $line in your code) into an "alias" (a sort of reference I guess) of the value at each stage through the loop.

So $line is a reference to @data[x] ... which is, at each iteration, some array. To get at one of the element of @data[0] you'd need the $ sigil (because the elements of the array at @data[0] are scalars). However $line[0] is a reference to some package/global variable that doesn't exist (use warnings; use strict; will tell you that, BTW).

[Edited after Ether pointed out my ignorance] @data is a list of anonymous array references; each of which contains a list of scalars. Thus you have to use the sort of explicit de-referencing I describe below:

What you need is something more like:

print ${$line}[0], ${$line}[1], ${$line}[2], "\n";

... notice that the ${xxx}[0] is ensuring that the xxx is derefenced, then indexing is performed on the result of the dereference, which is then extracted as a scalar.

I testing this as well:

print $$line[0], $$line[1], $$line[2], "\n";

... and it seems to work. (However, I think that the first form is more clear, even if it's more verbose).

Personally I chalk this up to yet another gotchya in Perl.

[Further editorializing] I still count this as a "gotchya." Stuff like this, and the fact that most of the responses to this question have been technically correct while utterly failing to show any effort to actually help the original poster, has once again reminded me why I shifted to Python so many years ago. The code I posted works, of course, and probably accomplishes what the OP was attempting. My explanation was wholly wrong. I saw the word "alias" in the `perlsyn` man page and remembered that there were some funky semantics somewhere there; so I totally missed the part that [...] is creating an anonymous reference. Unless you drink from the Perl Kool-Aid in deep drafts then even the simplest code cannot be explained.

Answer 3

It means you do not have an array; you have a reference to an array.

Note that an array is specified with round brackets - as a list; when you use the square bracket notation, you are creating a reference to an array.

foreach my $line (@data)
{
    my @array = @$line;
    print "$array[0] - $array[1] - $array[2]\n";
}

---

Illustrating the difference:

my @data = (
['1_TEST','1_T','1_TESTER'],
['2_TEST','2_T','2_TESTER'],
['3_TEST','3_T','3_TESTER'],
['4_TEST','4_T','4_TESTER'],
['5_TEST','5_T','5_TESTER'],
['6_TEST','6_T','^_TESTER']
);

# Original print loop
foreach my $line (@data)
{
    chomp($line);
    @random = split(/\|/,$line);
    print "".$random[0]."".$random[1]."".$random[2]."","\n";
}

# Revised print loop
foreach my $line (@data)
{
    my @array = @$line;
    print "$array[0] - $array[1] - $array[2]\n";
}

Output

ARRAY(0x62c0f8)
ARRAY(0x649db8)
ARRAY(0x649980)
ARRAY(0x649e48)
ARRAY(0x649ec0)
ARRAY(0x649f38)
1_TEST - 1_T - 1_TESTER
2_TEST - 2_T - 2_TESTER
3_TEST - 3_T - 3_TESTER
4_TEST - 4_T - 4_TESTER
5_TEST - 5_T - 5_TESTER
6_TEST - 6_T - ^_TESTER