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

Greatings monks. Here's my problem. I'm trying to write a launcher for my NES, SNES, and Genesis emulators but my code isn't working right. When I try to run it, I get 'Can't call method "delete" on an undefined value'. The line number it points at is the first line of the changeList subroutine. Anyway, here's the code. Can anyone tell me what's wrong with it? 
#!/usr/bin/perl
use Tk;
use Switch;

# Variables for emulators

$snesCommand="zsnes";
$genesisCommand="dgen";
$nesCommand="nestra";

$snesDirectory="~/roms/nes/";
$genesisDirectory="~/roms/genesis/Roms/";
$snesDirectory="~/roms/snes/Roms/";

$mw=MainWindow->new(-title=>"Launcher");

#Main window elements
$systemPick=$mw->Optionmenu(-command=>\&changeList,
                -variable=>\$system);
$systemPick->addOptions("SNES","NES","Genesis");
$opt=$mw->Scrolled("Listbox",-selectmode=>'single',
                -scrollbars=>"se");
$go=$mw->Button(-text=>"Launch Game",
                -command=>\&gogo);

# Geometry
$systemPick->pack();
$opt->pack(-expand=>1,-fill=>"both");
$go->pack();


sub changeList {
    $opt->delete(0,'end');
    switch ($system) {
        case "SNES" {
            $command=$snesCommand;
            $directory=$snesDirectory;
            @dir=glob($snesDirecotry);
            foreach my $file(@dir) {
                $file=~s/$directory//;
                $opt->insert('end',$file);
            }
        }

        case "NES" {
            $command=$nesCommand;
            $directory=$nesDirectory;
            @dir=glob($nesDirecotry);
            foreach my $file(@dir) {
                $file=~s/$directory//;
                $opt->insert('end',$file);
            }
        }

        case "Genesis" {
            $command=$genesisCommand;
            $directory=$genesisDirectory;
            @dir=glob($genesisDirecotry);
            foreach my $file(@dir) {
                $file=~s/$directory//;
                $opt->insert('end',$file);
            }
        }
    }
}


sub gogo {
    my $selected=$opt->get($opt->curselection());
    system("$command $directory$selected");
}

&changeList;
MainLoop;

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Re: Tk Listbox problem
by Erez (Priest) on Feb 19, 2009 at 08:17 UTC

    You're calling &changeList before the ListBox is actually rendered (with MainLoop). There is nothing *to* delete when it's running. Same error will be thrown on the insert method.

    Also, use eq instead of == when you're comparing strings (e.g. if ($system eq "SNES"))

    "A core tenant of the greater Perl philosophy is to trust that the developer knows enough to solve the problem" - Jay Shirley, A case for Catalyst.

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Re: Tk Listbox problem
by Anonymous Monk on Feb 19, 2009 at 04:06 UTC
    1) you're using Switch module (the author himself says you shouldn't use it).
    2) you're not using strict/warnings
    3) you're not using <code></code> tags
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      Ok, I rewrote it using strict and warnings and replaced the switch with if/elsif statements. I have $opt declared as a global, but it's throwing the exact same error it was before For some reason I had a brain fart and forgot to use the <code> tags. Sorry about that. It's fixed now. 
      #!/usr/bin/perl
use Tk;
use warnings;
use strict;

# Variables for emulators
use vars qw($snesCommand $genesisCommand $nesCommand $snesDirectory $n
+esDirectory $genesisDirectory $opt $system $directory $command);

$snesCommand="zsnes";
$genesisCommand="dgen";
$nesCommand="nestra";

$snesDirectory="~/roms/nes/";
$genesisDirectory="~/roms/genesis/Roms/";
$snesDirectory="~/roms/snes/Roms/";

my $mw=MainWindow->new(-title=>"Launcher");

my $systemPick=$mw->Optionmenu(-command=>\&changeList,
                -variable=>\$system);
$systemPick->addOptions("SNES","NES","Genesis");
$opt=$mw->Scrolled("Listbox",-selectmode=>'single',
                -scrollbars=>"se");
my $go=$mw->Button(-text=>"Go for it",
                -command=>\&gogo);

# Geometry
$systemPick->pack();
$opt->pack(-expand=>1,-fill=>"both");
$go->pack();

sub changeList {
    $opt->delete(0,'end');
    if ($system=="SNES") {
        $command=$snesCommand;
        $directory=$snesDirectory;
        my @dir=glob($snesDirectory);
        foreach my $file(@dir) {
            $file=~s/$directory//;
            $opt->insert('end',$file);
        }
    } elsif ($system=="NES") {
        $command=$nesCommand;
        $directory=$nesDirectory;
        my @dir=glob($nesDirectory);
        foreach my $file(@dir) {
            $file=~s/$directory//;
            $opt->insert('end',$file);
        }
    } elsif ($system=="Genesis") {
        $command=$genesisCommand;
        $directory=$genesisDirectory;
        my @dir=glob($genesisDirectory);
        foreach my $file(@dir) {
            $file=~s/$directory//;
            $opt->insert('end',$file);
        }
    }
}


sub gogo {
    my $selected=$opt->get($opt->curselection());
    system("$command $directory$selected");
}

&changeList;
MainLoop;

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