gnubbs has asked for the wisdom of the Perl Monks concerning the following question:

A simple question. It has been a long time since I programmed in perl, and for some reason I can't seem to sort this problem out. References in perl....

#!/usr/local/bin/perl -w

use strict;

sub get_conf(\%);
my %cur_conf;

$cur_conf{test} = "test";

get_conf(\%cur_conf);
print "after: " . $cur_conf{test};

sub get_conf(\%) {

   my $href = @_;
   $href->{test} = "another test";
}

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To my tired newbie eyes, this looks like it should work but does not. Thanks for any help.

gnubbs

Replies are listed 'Best First'.
Re: Hashes and Functions
by jasonk (Parson) on Apr 10, 2003 at 18:00 UTC 
    my $href = @_;

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    Evaluating an array as a scalar returns the number of elements in the array, so at this point $href contains '1'. You want either my($href) = @_; or my $href = shift;, or something similar that will maintain the array context you are looking for.

    We're not surrounded, we're in a target-rich environment!
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      Thanks! This was something that took me a while to get right when I was first learning perl, and now I see why. In the camel book the line is:

      my ($cref, $dref) = @_;

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      For some reason, I had associated the () with the my... Now it makes perfect sense. Thanks.

      gnubbs
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        Actually, in that case the parens are associated with my. You can think of my as a function just like any other function and the parens group and specify its arguments. The reason you can assign to it is because my returns a list of its newly scoped arguments, which are then assigned to like any other list assignment.

        kelan

        Perl6 Grammar Student

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        IMHO, "shift" is the better way to get arguments off of @_. If you start using "shift", you'll never get confused about scalar vs. list context, etc.

        So,

             my ($cref, $dref) = @_;

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        becomes

             my $cref = shift;
     my $dref = shift;

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        Very clean, very easy. No confusion :-)

        HTH.

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Re: Hashes and Functions
by bart (Canon) on Apr 11, 2003 at 00:56 UTC
    You have two problems. One has been addressed in other replies, on your way to pass the parameter to the local, lexical variable.

    But a second problem is that prototypes change the way you have to call your sub. With a prototype \%, perl wants you to pass in a hash, and it will pass on a reference to it to the sub. So the call should look like: 

    get_conf(%cur_conf);

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    With that additional change, your code runs as expected, on my system (perl 5.6.1).

    Similar, with a prototype \@, you must pass in an array, not an array reference: 

    sub mypush (\@@) {
    my $aref = shift;
    push @$aref, @_;
}

@a = (1);
mypush @a, 2, 3, 4;
print "@a\n";

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    Resulting in: 1 2 3 4
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