Since we've been doing linked lists in computer science
class recently but are unfortunately using the ol' Pascal
(which I've grown to dislike) I decided to code the same
simple program we finished in class - this time in Perl to see how it works.
After reading a little on that topic I made an implementation
like this : 
use strict;

my $list        = undef;
my $lastnode    = \$list;

while (1)
{
    print '<i>nput, <o>utput : ';
    chomp (my $input = <STDIN>);
    
    if ($input eq  'i')
    {
        ($list, $lastnode) = add ($list, $lastnode);
        next;
    }        

    if ($input eq 'o')
    {
        show ($list);
        next;
    }
}

sub add
{
    my ($list, $lastnode) = @_;
    chomp (my $input = <STDIN>);

    my $newnode = [undef, $input];
    $$lastnode  = $newnode;
    $lastnode   = \$newnode -> [0];

    return ($list, $lastnode);    
}    

sub show
{
    my $list = shift;

    for (my $node = $list; $node; $node = $node -> [0])
    {
        print $node -> [1] . "\n";
    }
}

[download]
Interestingly I found out that this wasn't working.
Debugging it with Komodo 1.1 I found that after 
$$lastnode  = $newnode;

[download]
$list wasn't being set, which kinda defeated it's purpose.
Then I changed the implementation to this : 
use strict;

my $list        = undef;
my $lastnode    = \$list;

while (1)
{
    print '<i>nput, <o>utput : ';
    chomp (my $input = <STDIN>);
    
    if ($input eq  'i')
    {
        &add;
        next;
    }        

    if ($input eq 'o')
    {
        &show;
        next;
    }
}

sub add
{
    chomp (my $input = <STDIN>);

    my $newnode = [undef, $input];
    $$lastnode  = $newnode;
    $lastnode   = \$newnode -> [0];  
}    

sub show
{
    for (my $node = $list; $node; $node = $node -> [0])
    {
        print $node -> [1] . "\n";
    }
}

[download]
I removed the variable passing and accessed $list and $lastnode directly.
And well, now it worked. But I don't get why passing the variable
makes example 1 don't work.

I am thankful for enlightenment.

In reply to Linked List Strangeness by Mandor

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