Perl : Dereference between @_ and $_

Question

I have a question with the following Code:

#!/usr/bin/perl

use strict;
use warnings;

my %dmax=("dad" => "aaa","asd" => "bbb");
my %dmin=("dad" => "ccc","asd" => "ddd");

&foreach_schleife(\%dmax,\%dmin);

sub foreach_schleife {
        my $concat;
        my $i=0;

        foreach my $keys (sort keys %{$_[0]}) {
                while ($_[$i]) {
                        $concat.="$_[$i]{$keys} ";
                        print $_[$i]{$keys}."\n";
                        $i++;
                }
                $i=0;
                $concat="";
        }
}

The Output is:

   bbb
   ddd
   aaa
   ccc

I don't understand this. Normally you must dereference references on hashes,arrays etc. Why not here? Its enough to write :

$_[$i]{$keys}."\n";

and not something like that:

$$_[$i]{$keys}."\n";

Why? Has it something to do with the speciality of the variable @_/$_?

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Axeman · Answer 1 · 2011-12-12T14:01:49.300

3

My guess is that because an array (or a hash, for that matter) can only contain hash references your second act of indexing means that the reference is understood.

I think the developers need to document this a little better.

To see that it is not special to *_, you can try this before the loop:

my @a = @_;

And this during:

print $a[$i]{$keys}."\n";

I think the main thing is that if you only have a scalar reference as the base, then at least one -> is required. So

my ( $damxr, $dminr ) = @_;

would require

$dmaxr->{ $key };

edited Dec 12 '11 at 14:01

answered Dec 12 '11 at 13:45

Axeman

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+1. In my opinion that short-hand notation is only useful for golfing, since it doesn't make sense even to seasoned Perl developers. – flesk Dec 12 '11 at 14:21
@flesk, you're entitled to your opinion. I think having a lot of `->` is excessive noise, if all you want to do is index. – Axeman Dec 12 '11 at 16:42

score 3 · Answer 2 · answered Dec 12 '11 at 14:17

The reason why you don't have to dereference $_[0] and $_[1] is that $_[$i]{$keys} is a valid short-hand notation for $_[$i]->{$keys} when your reference is in an array or a hash.

$$_[$i]{$keys} won't work, because it will try to dereference the special variable $_ to a scalar. The correct syntax is %{$_[$i]}, but then you'll have to use %{$_[$i]}->{$keys}, which is more verbose.

score 0 · Accepted Answer · answered Dec 12 '11 at 13:46

0

@_ is the array of subroutine arguments, hence $_[$index] accesses the element at $index

Dereferencing is only good if you have references, but @_ isn't one.

answered Dec 12 '11 at 13:46

Ingo

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Ok @_ is no reference but its elements are.But i think the second reply to my posting give the probable right answer. – Hakan Kiyar Dec 12 '11 at 13:54

Perl : Dereference between @_ and $_

3 Answers3