herby1620 has asked for the wisdom of the Perl Monks concerning the following question:

In diagnosing some error problems, I made up a TIE module for printing. It works quite well, but does "wierd things" in one instance. The routine that is called says:

Use of uninitialized value in split at <routine/line> during global destruction.

The code is pretty simple (reconstructed a bit): 

sub fool
  {
  my @vals;
  my $n;
  my $i;
  while (@_)
    {
    @vals = split /(\n)/, (shift); # Error happens here
    $n = @vals;    
    for ($i = 0; $i < $n; $i++)
      {
      print "The value is: $vals[$i] \n"; # Or similar action
      }
    }
  }

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My question: how do I detect that this error is GOING to happen with the called arguments (in this case to the 'fool' routine) so I can print out a more intelligent error message?

Is there any way I can "produce" this same error message using the 'fool' routine as shown above. It would "enlighten" me quite a bit.

Replies are listed 'Best First'.
Re: Wierd error message
by graff (Chancellor) on Aug 16, 2006 at 03:00 UTC
    Here is one way to generate that sort of warning message (there are many other ways): 
    my @array=qw/foo bar/;
$array[3] = "baz";

foo1( @array );  # $array[2] is undefined

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    One way to avoid the warning would be: 

       for my $param (grep {defined} @_) {
      for my $val ( split /\n/, $param ) {
         print "The value is: $val\n";  # or whatever you want to do
      }
   }

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    (I took the liberty of eliminating unnecessary variables, and localizing the scope of needed variables a little more tightly.)

    The other methods suggested above would also work, I expect. There might be reasons for preferring one approach over the other, depending on the rest of your application.

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      Reply most helpful. I was able to make the proper tests, and incorporate the detection of "undefined" stuff in my code.

      Sorry if the example isn't that good. I tried to extract the pertinant stuff for the example. The intended application is a "tie" method for output (prints). It replaces the output method to stdout, and adds the time, and module/line as a prefix. It seems to work quite well, as we've put it in a bunch of scripts here. Efficient? I don't know, it does the job, and really isn't called that much. I suspect that being my first module, it refelcts my background ("...you can program in Fortran in any language.").

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Re: Wierd error message
by Zaxo (Archbishop) on Aug 16, 2006 at 02:19 UTC

    Your line eight should be something like, 

        @vals = split /(\n)/, $_;

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    I don't understand what the "fool" routine is. You have a very straightforward loop, done in C-style. Perhaps if you included some of the Tie code?

    After Compline,
    Zaxo

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      This use of while doesn't seem to set $_ for me. Which version of Perl are you using?

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        I was thinking of while (<$fh>) { }, which does.

        After Compline,
        Zaxo

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Re: Wierd error message
by roboticus (Chancellor) on Aug 16, 2006 at 02:18 UTC
    herby1620--

    I'd be similarly "enlightened" if I could see a real example--a small complete example--that I could just run to see the error message. So please bear with me, as my ESP::Clairvoyance module doesn't compile at the moment.

    If I had to guess, (and I do must!), I'd suspect that shift is returning an undef when it runs out of items, and that is the source of your error...

    UPDATE: perhaps changing

        @vals = split /(\n)/, (shift); # Error happens here

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    to something like: 
        @vals = split /(\n)/, (shift)||'';

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    might help. (I don't know if it'll even compile, but the intent is to provide split *something* to work with if shift returns undef...)

    UPDATE: As ikegami states below, my attempted "fix" is quite bogus. Luckily, he also had a better suggestion. (Had I refreshed before posting my prior update, I'd've probably been spared the embarassment! Thanks Icky! 8^)

    --roboticus

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Re: Wierd error message
by ikegami (Patriarch) on Aug 16, 2006 at 02:21 UTC
    Perhaps adding next if not defined $_[0]; as the first line of the while loop body? It's hard to tell if there's a better approach or not without more complete (working?) example.
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Re: Wierd error message
by jdporter (Paladin) on Aug 16, 2006 at 04:25 UTC

    If you have 'some weird error', the problem is probably with your frobnitzer. — #11938

    :-)

    We're building the house of the future together.
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Re: Wierd error message
by ysth (Canon) on Aug 16, 2006 at 15:22 UTC
    In general, you can avoid that kind of error message by not putting objects into global variables (or tieing globals, which is a sideways way of putting in an object), at least without manually clearing/untieing them at some point.
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