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Tk with map - Perl newbie

by stickleback (Novice)
on Mar 31, 2011 at 13:25 UTC ( #896626=perlquestion: print w/replies, xml ) Need Help??
stickleback has asked for the wisdom of the Perl Monks concerning the following question:

I am new to Perl and even newer to Tk. I'm trying to write a login dialog box for a database and learning along the way. I get unexpected behaviour in the following script ...
#/usr/bin/perl use warnings; use strict; use Tk; use Tk 800.000; use Tk::Dialog; my $user = "Hello"; my $password = "World"; my $mw = MainWindow->new(); my $dlgLDL = $mw->Dialog( -title=>'Login', -buttons=>['Ok','Cancel'], -default_button=>'Ok'); my ($fLeft,$fRight) = map {$dlgLDL->Frame(-padx=>4,-borderwidth=>0,-relief=>'groove')- +>pack(-side=>$_,-pady=>5)} qw/left left/; my ($l1,$l2) = map {$fLeft->Label( -text => $_, -anchor=>'e', -width=>12)->pack +(-side=>'top',-pady=>3)} qw/User: Password: /; my ($dfHost, $dfPort, $dfUser,$dfPassword) = map {$fRight->Entry( -textvariable=>$_ )->pack(-side=>'top',- +pady=>3)} qw/\$user \$password/; my $selected = $dlgLDL->Show(); MainLoop;
The -textvariable substitution in the third map statement above doesn't do what I hoped, i.e., set up the link to the associated variable. Instead it seems to just place the quoted text into the Entry field. What am I missing?

Please note this may be a very simple mistake as I am still trying to pick up basic Perl syntax.

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Re: Tk with map - Perl newbie
by vkon (Curate) on Mar 31, 2011 at 13:40 UTC
    by adding unneeded complexity you've lost what is happening

    why you wrote this way:

    my ($dfHost, $dfPort, $dfUser,$dfPassword) = map {$fRight->Entry( -textvariable=>$_ )->pack(-side=>'top',- +pady=>3)} qw/\$user \$password/;
    you assign 2-element list to 4 variables, so last two are undefined.

    But more to the point, if you'll write

    $fRight->Entry( -textvariable=>\$user )->pack(-side=>'top',-pady=>3 +); $fRight->Entry( -textvariable=>\$password )->pack(-side=>'top',-pad +y=>3);
    your code works as expected.
    I can not explain it, but I know it

    (addition) qw/\$user .../ does not do what you want

      Thanks for the quick reply.

      The 2 element list to 4 variables was a mistake in extracting the dialog from the larger program. Apologies for that.

      I your suggested broken out code works fine. The following also works ...

      $dfUser->configure( -textvariable=>\$user); $dfPassword->configure( -textvariable=>\$password);
      I can get my code to work and the dialog to display correctly but I wanted to understand why the original code does not work. I am trying to learn Perl along the way.
        (update) deleted wrong content,
        The quoting operator as in qw/\$user \$password/ doesn't work because what is getting translated in to the $_ variable is the TEXT '\$user', not a reference to $user!

        I show this below with a simple example. Using parens and comma for a normal list allows the \$user to be translated into a value that is the reference. The code will expect to deference the hash value and that will work if what is stored is really a reference to something else rather some text. So in this case, the qw// is not doing what (..,...) does.

        #!/usr/bin/perl -w use strict; use Data::Dumper; my ($user,$password); my $a=0; my %hash1 = map{$a++=>$_}qw/\$user \$password/; #just text my %hash2 = map{$a++=>$_}(\$user, \$password); #real reference print Dumper \%hash1, \%hash2; $user = 234; $password = 'xyz'; print Dumper \%hash1, \%hash2; __END__ $VAR1 = { '1' => '\\$password', '0' => '\\$user' }; $VAR2 = { '3' => \undef, #ref to an undefined val '2' => \undef }; ################# $VAR1 = { '1' => '\\$password', '0' => '\\$user' }; $VAR2 = { '3' => \'xyz', #see changing $user or $password '2' => \234 #did update the hash #because these are real references. };
Re: Tk with map - Perl newbie
by Sinistral (Prior) on Mar 31, 2011 at 15:12 UTC

    As a newbie to both Perl and Tk, I highly recommend the TkDocs site. The tutorial housed there covers multiple OS and multiple programming language issues and comparisons. The Mastering Perl/Tk book is still relevant for the Tk module, but programming best practices have changed and newer Tk libraries/wrappers have been written to take advantage of native operating system widgets to produce better looking and working results.

    Good luck with your programming!

      Thanks for the tips.
      Hi, I would like to know about libraries and wrappers that produce better looking results. Could you provide any links? TkDocs doesn't seem to offer much for Perl TK users.
Re: Tk with map - Perl newbie
by Anonymous Monk on Mar 31, 2011 at 13:41 UTC
    Do you notice anything about how the Tk demo program "widget" makes use of textvariable?

    textvariable takes a reference :)

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