Re^3: Sub hash param by reference

It's a whole lot easier and more subtle than you might expect. Perl actually passes parameters as aliases in @_ so we can:

use warnings;
use strict;

my %hash_rec = (id => "001");

doSub2($hash_rec{id2});

print "main: id2 = $hash_rec{id2}\n";


sub doSub2 {
    $_[0] = "Hello alias";
}
[download]

Prints:

main: id2 = Hello alias
[download]

We don't usually take advantage of that trick because without due care it's likely to lead to hard to maintain code.

Premature optimization is the root of all job security

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Re^4: Sub hash param by reference
by hankcoder (Scribe) on Jul 08, 2016 at 04:54 UTC

Hello GrandFather, thank you for your feedback and Wow, I never know param can do like that. I'm not quite understand what is $_[0] but I tested with it and it works well.

sub doSub2 {
   # use name var instead of $_[0] so can easier to recall it
   my ($ref_value) = \$_[0];
   $$ref_value = "Hello alias";
}
[download]

[reply]
[d/l]

Re^5: Sub hash param by reference

by Athanasius (Archbishop) on Jul 10, 2016 at 08:44 UTC

Hello hankcoder,

I'm not quite understand what is $_[0]

The special variable @_¹ is documented as follows:

Within a subroutine the array @_ contains the parameters passed to that subroutine. Inside a subroutine, @_ is the default array for the array operators pop and shift.

Now, in Perl² you access the individual elements of array @my_array as $my_array[0], $my_array[1], $my_array[2], .... (Note that the first element has index zero.) In the same way, the array @_ (which is named “ _ ”) has elements $_[0], $_[1], $_[2], .... So $_[0] is the first parameter passed to the subroutine.

¹You can also call it @ARG if you use English; — see perlvar#General-Variables.
²That is, Perl 5 and earlier. I believe Perl 6 uses a different syntax.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,