Misprinting combinations / iterator

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

Hi all! I'm a new Perl programmer and I'm trying to output combinations of numbers (1 to 10) to a file, but all I can get as output is a list of "ARRAY(0x4eeeec)" (for instance), both in the screen and written on the file. I've searched a little all over the place (Perl Monks included) but I can't find a similar situation / problem. Since I'm a new Perl user, I think my code is printing not an actual value but an address - I guess I'm misusing the iterator, but I can't find out why and how to fix it. Any help will be greatly appreciated. Thanks, João Fernandes

#!/usr/bin/perl
use strict;
use warnings;

use Algorithm::Combinatorics qw(combinations);

my @data = qw(1 2 3 4 5 6 7 8 9 10);
my $k = 4;

open(my $output, ">", "combinations.txt") or die "Can't open combinati
+ons.txt: $!";

my $iter = combinations(\@data, $k);

while (my $combination = $iter->next)
{
    print "$combination\n";
    print $output "$combination\n";
}

close $output or die "$output: $!";
[download]

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Re: Misprinting combinations / iterator by tangent (Parson) on Mar 27, 2012 at 15:36 UTC
The $combination variable returned by the iterator is an array reference, so you need to de-reference it in order to print. The following will print each combination on a line, with each element separated by a tab character: `print $output join("\t",@{$combination}) . "\n";` [download] The @{ } construct is what de-references the array reference	[reply] [d/l]
Re^2: Misprinting combinations / iterator by blacknail (Initiate) on Mar 27, 2012 at 16:01 UTC
Thanks for the explanation, tangent :]	[reply]
Re: Misprinting combinations / iterator by moritz (Cardinal) on Mar 27, 2012 at 15:36 UTC
If you see something like `ARRAY(0x4eeeec)`, it means you have an array reference. They are thoroughly explained in perllol and perlreftut. The fix is to dereferencing them before printing, for example `print "@$combination\n";` Perl 6 - second systems done right	[reply] [d/l] [select]
Re^2: Misprinting combinations / iterator by blacknail (Initiate) on Mar 27, 2012 at 15:59 UTC
Thanks, moritz. I had already looked at perlol, but haven't understand it fully.	[reply]
Re: Misprinting combinations / iterator by Marshall (Canon) on Mar 27, 2012 at 15:54 UTC
I haven't used this particular module, but from looking at the documentation.. $combination is a reference to an array (an arrayref) - that needs to be deferenced. `print "@$combination\n";` looks like what you need.	[reply] [d/l]
Re^2: Misprinting combinations / iterator by blacknail (Initiate) on Mar 27, 2012 at 16:03 UTC
You're right, Marshall! Thank you all for your kind and quick answers! :]	[reply]
Re^3: Misprinting combinations / iterator by Marshall (Canon) on Mar 27, 2012 at 20:18 UTC
The examples in the module's docs weren't completely explanatory - I can see how this was confusing! There is a "core" module (meaning that is pre-installed and exists on all Perl installations), Data::Dumper. It can print a representation of any Perl data structure - WOW! You need to give it a reference to the structure. This is fantastic debugging tool. There is no equivalent in 'C' or 'Java' of which I am aware. `use Data::Dumper; # in your loop put... print Dumper \$combination;` [download]	[reply] [d/l]