[lug] Perl - How to pass by Reference?

[Siegfried Heintze]

This is a great exercise that is not covered well in any of the tutorials I
read when I was trying to learn perl. I found this lesson very painful.

First: Why are you trying to pass by reference? 

There are easier alternatives like returning a list as the function value.
The list would contain the changed values.

You can pass a scalar by reference. Are you a C programmer? There are some
analogs if you are.

To pass a scalar by reference you insert the "\" in front of the function
argument in the main program. C programmers are accustomed to using a "&"
but that has different meanings in perl.

To receive a scalar by reference inside the function implementation, you use
an additional "$". So everywhere you have "$val" you would put "$$val". C
programmers would put an extra "*" in front but that also has a different
meaning in Perl.

Actually all arguments are passed by reference in perl. The reason your
program is not working is that you made a copy of the original when you
typed "my $val = pop @_". With the changes above, you will be making a copy
of the reference, and not the original. So an alternative would be to say
"$[0] = 'goodbye'; '. Nice transparent code, huh? 

Beware: things get more complex when passing arrays as function arguments.

Siegfried 

-----Original Message-----
From: lug-bounces at lug.boulder.co.us [mailto:lug-bounces at lug.boulder.co.us]
On Behalf Of Bill Thoen
Sent: Wednesday, March 01, 2006 3:37 PM
To: lug at lug.boulder.co.us
Subject: [lug] Perl - How to pass by Reference?


In this contrived example, I want to pass $test by reference so the
changeit() subroutine can change it, but I can't figure out how to do this
with 'strict' and 'warnings' turned on. Can anyone here tell me what I need
to do to fix this? (as you can tell, I'm trying to learn Perl)

#!/usr/bin/perl

use strict;
use warnings;

&main();

sub main {
  my $test='Hello';

  if (&changeit ($test)) {
    print "$test\n";
  }
  else {
    print "failed\n";
  }
}

sub changeit {
  my $val = pop @_;

  if ($val eq 'Hello') {
    $val = 'Goodbye';
    return 1;
  }
  else {
    return 0;
  }
}


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