Ace128 has asked for the wisdom of the Perl Monks concerning the following question:

Hey,

Ok, this is something that is supposed to be easy, but I cant make it work! I have the following: 
my %a;
my $a_ref = \%a;

&resetCounters();
&add($a_ref);

print $a{'modified'} . "\n";
print $a{'removed'};

sub resetCounters {

    $a{'added'} = 0;
    $a{'removed'} = 7;
    $a{'modified'}++;

    print $a{'modified'} . "\n";
}

sub add {

    my $a_ref2 = shift;
    my %temp = %{$a_ref2};
    $temp{'modified'}++;

    foreach (keys %temp) {
        print $_ . "  " . $temp{$_} . "\n";
    }
}

[download]
And this prints:
1
removed 7
added 0
modified 2
1
7

But I want it to be:
1
removed 7
added 0
modified 2
2 <-- Notice!
7

What am I missing?

Thanks,
Ace

Replies are listed 'Best First'.
Re: References on hashes!
by graff (Chancellor) on Jul 08, 2005 at 03:55 UTC
    If you really want the "add" sub to modify the contents of the original "%a" hash, you'd do it like this: 
    sub add {

    my $a_ref2 = shift;
    $$a_ref2{modified}++;  # (update: I forgot to include this step)

    foreach (keys %$a_ref2) {
        print $_ . "  " . $$a_ref2{$_} . "\n";
    }
}

    [download]
    (note the double sigils on the hash ref: %$a_ref2 refers to the entire hash, and  $$a_ref2{$_} refers to a single hash element; another way to do the latter is $a_ref2->{$_})
[reply]
[d/l]
[select]
      Em... what is going on here? The a_ref2's should be same hash, now shouldn't they? 
      use threads;
use threads::shared;

my %a;
my $a_ref = \%a;

&resetCounters();
threads->create('add', $a_ref);
threads->create('printCounters', $a_ref);
#threads->create('loop', $a_ref)->join;
#&printCounters();
&loop();

sub loop {
    while (1) {
        sleep 1;
    }
}

#&printCounters();

#print $a{'modified'} . "\n";

sub printCounters {

    my $a_ref2 = shift;
    print "print REF2: $a_ref2\n";

    while (1) {
        #print "MOD: " . $a{'modified'} . "\n";
        print "MOD: " . $$a_ref2{'modified'} . "\n";
        sleep 1;
    }
}

sub resetCounters {

    $a{'added'} = 0;
    $a{'removed'} = 7;
    $a{'modified'} = 0;

    print $a{'modified'} . "\n";
}

sub add {

    my $a_ref2 = shift;
    print "add REF2: $a_ref2\n";

    while (1) {

        print "add\n";
        $$a_ref2{'modified'}++;
        foreach (keys %$a_ref2) {
            print $_ . "  " . $$a_ref2{$_} . "\n";
        }
        sleep 5;
    }
}

      [download]
[reply]
[d/l]
        Look more closely at "perldoc threads::shared". You have to declare variables as shared in order for different threads to see a given variable as referring to the shared data: 
        my %a : shared;

        [download]
        or, alternatively: 
        my %a;
share( %a );

        [download]
        When you stringify and print the values of different references to the same underlying hash, they will still show up with different memory address values (and I can't say as I understand why that is).

        In any case, your test script does behave differently with the "share" statement included -- possibly it does what you intend.

[reply]
[d/l]
[select]
      Ahh, yes. Natrually and ofcource! :)
[reply]
Re: References on hashes!
by artist (Parson) on Jul 08, 2005 at 03:45 UTC
    In your code, within 'add' sub, %temp is a local hash and it doesn't modify the passed reference for the main program.
    --Artist
[reply]

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