Passing arrays to a subroutine that prints each array separately

Question

I know that this is probably a simple fix, but I have not been able to find the answer through google and searching through the questions here.

My goal is to pass multiple arrays to a subroutine that simply iterates through each array separately and prints each array with something before and after it.

What I have:

@A1 = (1, 2, 3);
@A2 = (4, 5, 6);

printdata(@A1, @A2) ;

sub printdata {
   foreach(@_) {
      print "$_" ; 
      print "@@@"
      }
   }

What I am attempting to get is:

123@@@456@@@

Instead its treating both arrays as one and iterating through each variable in the array, Putting the separator after every variable vice the entire array.

1@@@2@@@3@@@etc.....

I am not sure how to get the subroutine to treat the arrays as separate rather than as one.

Any Help would be greatly appreciated!

What you've discovered is the List context in Perl. Lists are what get passed to methods (and almost any time you do use a paren with commas inside). They automatically flatten anything inside of them - arrays or hashes. This can be really convenient when passing arguments to functions (eg, you can build up all the arguments in an array, then pass it as a list to the function) or collating data from multiple sources (such as combining multiple hashes or arrays), but this idea of 'context' is new to most programmers and so can trip people up. :) — Robert P
– Robert P, Commented Jun 29, 2011 at 16:20

TLP · Accepted Answer · 2011-06-29 12:21:55Z

6

You need to pass the arrays as references:

@A1 = (1, 2, 3);
@A2 = (4, 5, 6);

printdata(\@A1, \@A2) ;

sub printdata {
   foreach(@_) {
      print @$_ ; 
      print "@@@"
      }
}

The sub call expands the arrays into a list of scalars, which is then passed to the sub within the @_ variable. E.g.:

printdata(@A1, @A2);

is equal to:

printdata(1,2,3,4,5,6);

answered Jun 29, 2011 at 12:21

TLP

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2 Comments

Kevin Over a year ago

Perfect thanks, I knew I was missing something simple. I don't exactly understand references, but I will be researching it now.

mac Over a year ago

Its not simple, by adding "\" he introduced a new concept of perl programming which is very different from your concept

Dave Cross · Accepted Answer · 2011-06-29 12:29:22Z

5

See the section on "Pass by Reference" in perldoc perlsub.

answered Jun 29, 2011 at 12:29

Dave Cross

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1 Comment

TLP Over a year ago

But what about the fishers? ;)

Axeman · Accepted Answer · 2011-06-29 12:45:56Z

2

use strict;
use warnings;
use English qw<$LIST_SEPARATOR>;

my @A1 = (1, 2, 3);
my @A2 = (4, 5, 6);
{   local $LIST_SEPARATOR = '';
    my @a = map { "@$_" } \@A1, \@A2;
    $LIST_SEPARATOR = '@@@';
    print "@a\n";
}

You also could have used join (po-tay-to, po-tah-to).

my @a = map { join( '', @$_ ) } \@A1, \@A2;
print join( '@@@', @a ), "\n";

answered Jun 29, 2011 at 12:45

Axeman

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3 Comments

TLP Over a year ago

I like the use of the somewhat obscure continue: for (@_) { print @$_ } continue { print "@@@" }

Axeman Over a year ago

@TLP, Cool!. I have never used continue, it has always looked like an ugly tack-on--probably a bias coming from other languages. I might need to reconsider.

TLP Over a year ago

I've never used it either. ;) I just thought this was a fun opportunity to try it out. Easy mnemonic: It does the same as the third element in a c-style for loop, for (init; inc; *continue*)

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