(Perl) Dereferencing arrays for subroutines

Poetics · 05-22-2006, 10:23 AM

I'm trying to pass an array into a subroutine, but though the routine is small enough to be copied every time, proper coding suggets I use references. The difficulty resides in the fact that for some reason, my dereferencing isn't acting as I expect.

(simplified) example code follows:

Code:

@ref = 1..5;
@rray = &check(\@ref, "Apple");

sub check {
   $temp = shift;
   @temp = @{$temp};
   $apple = shift;
}

If I add in a print $temp, I get the expected memory reference for the array. However if I add in a print @temp I get a return of udef, which is certainly not what I expected.

Is my understanding of dereferencing just faulty or is something else going on here? Thanks in advance.

destuxor · 05-22-2006, 01:53 PM

I think your problem is in your usage of the "shift" function. Ordinarily, if I wanted to copy the array sent to a function I would just copy it like this:

Code:

sub check {
   @temp = @_;
   $apple = pop(@temp);
}

The below code,

Code:

   $temp = shift;
   @temp = @{$temp};

puts $_[0] into $temp, then attempts to create an array with a single element $temp in position zero - I think.
Does that answer your question?

aluser · 05-22-2006, 02:18 PM

Code:

15:16 aluser@alf:~/test/perl$ cat arrpass.pl
#! /usr/bin/perl

@ref = 1..5;
@rray = &check(\@ref, "Apple");

sub check {
        $temp = shift;
        @temp = @{$temp};
        $apple = shift;

        print "\$temp: $temp\n";
        print "\@temp: @temp\n";
}
15:16 aluser@alf:~/test/perl$ perl arrpass.pl
$temp: ARRAY(0x8152678)
@temp: 1 2 3 4 5

Works fine for me, I think your problem is somewhere else : )

destruxor: "@temp = @{$temp}" says to take $temp as an array reference, dereference it, and copy the result to @temp. $temp and @temp are two separate variables.

aluser · 05-22-2006, 02:19 PM

btw, variable names like "temp" will get you into big trouble eventually if you don't localize them with "my". Use "use strict" always, and especially when debugging things : )

Poetics · 05-22-2006, 03:36 PM

Quote:

Originally Posted by aluser

btw, variable names like "temp" will get you into big trouble eventually if you don't localize them with "my". Use "use strict" always, and especially when debugging things : )

Don't worry, that's far from the code in my program

destuxor, I don't think that's right -- $_[0] contains the first element of @_, which is the reference to the array.

The actual relevant code is posted below, taken from my project (slight immeterial edits)... I'm really wondering why this isn't working

Code:

if (@ns) { @txtns = &roundup(\@check, "NS"); }

sub roundup {
     $array = shift @_;
     @array = @{$array};
     $string = shift @_;
     foreach (@array) {
        @splits = split("--");
        push @c, "[Misc information here using the value of $i]";
        $i++;
     }
     $i = 0;
     @c;
}

I've put print statements (or 'push'd an array that was printed later) just after the first shift and then after the @array assignment, but I run into @array being empty for some reason, and as such, it never runs the foreach loop so @c is never added to.

chrism01 · 05-23-2006, 02:53 AM

I always do it the other way around:

Code:

calledsub($scalr, $scalar ... , @array)
sub calledsub
{
  $t1 = shift;
  $t2 = shift;
  .
  .
  .
  @array = @_;
}

BTW, using '&' to call a sub is old/deprecated unless passing a ref-to-sub; just use the sub name.

aluser · 05-23-2006, 03:51 PM

Code:

16:50 aluser@alf:~/test/perl$ cat arrpass.pl
#! /usr/bin/perl
@ns = (1, 2);
@check = 1..4;
if (@ns) { @txtns = &roundup(\@check, "NS"); }
print "@txtns\n";
sub roundup {
     $array = shift @_;
     @array = @{$array};
     $string = shift @_;
     foreach (@array) {
        @splits = split("--");
        push @c, "[Misc information here using the value of $i]";
        $i++;
     }
     $i = 0;
     @c;
}
16:51 aluser@alf:~/test/perl$ perl arrpass.pl
[Misc information here using the value of ] [Misc information here using the value of 1] [Misc information here using the value of 2] [Misc information here using the value of 3]

Once again, I think your problem is elsewhere...

Poetics · 05-23-2006, 05:27 PM

Thanks for the help

I'll keep poking at it. For some reason nothing in the [misc information] block is getting to @txtns.