Unintialised value?

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

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Re: Unintialised value?
by Vautrin (Hermit) on Jan 24, 2004 at 22:41 UTC

If you use strict; use warnings; whenever you try to compare an undefined value it will yelp. This is great to do, but if you want to fix that warning you should let perl know you expect the possibility of an undef value by testing for an undef value (and only performing concatanation, regular expressions, or anything else with it) if it's defined, i.e.:

# note that we have to check if its defined
# because it's possible it could be 0, and false
#62 exit 0 unless ((defined ($page))
  and (defined($pages{$page}))
  and ($pages{$page} == '1'));
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You should note that you are shifting from @_ to set $page == undef; especially on this little bit of test code. So if @_ is undefined then you'll get those warnings. You can turn off warnings for uninitialized values by using:

NOWHINING: {
  no warnings qw (uninitialized);
  my %pages = ("index" => '1',
               "me"    => '1',
               "stuff" => '1',
               "links" => '1');
  my $page = shift;
  exit 0 unless $pages{$page} == '1'; 
}
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Just be careful with the above code because:

The no warnings qw(uninitialized); is declared within the NOWHINING: block so as to keep its scope only for the offending code. You could extend this to your entire program by using no warnings qw(uninitialized); at the top of your script, but this becomes dangerous because you want to know when uninitialized values are being used.
The %pages and the $page variables are only valid within the {}s. This may or may not be an issue. Adjust your scope accordingly.
Is @_ supposed to contain undef values? If it isn't you've got bigger problems.
because @_ is "magical" as soon as you use any function that modifies @_ it can on change you. I recommend you never use it unless where the @_ comes from is clear from the code.
Use something like my @arguments = @_; at the beginning of wherever you get the @_ from so that any sub that is called doesn't modify @_ on you.
perl -e 'use strict; use warnings; my $foo; print $foo;' and perl -e 'use strict; use warnings; undef =~ m/foo/;' will show you some ways you can get and reproduce that error.

Good luck!

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Re: Unintialised value?
by blue_cowdawg (Monsignor) on Jan 24, 2004 at 22:06 UTC

I just tried to duplicate your problem as follows:

use strict;
use warnings;
 
 
&foo('gronk');
 
sub foo {
    my %pages = ("index" => '1',
              "me"    => '1',
               "stuff" => '1',
               "links" => '1');
  my $page = shift;
  exit 0 unless $pages{$page} == '1';
}
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[peter@kennel tmp]$ perl test_eq.pl
Use of uninitialized value in numeric eq (==) at test_eq.pl line 13.
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does

==

What value is $page set to?

Peter L. Berghold -- Unix Professional Peter at Berghold dot Net
	Dog trainer, dog agility exhibitor, brewer of fine Belgian style ales. Happiness is a warm, tired, contented dog curled up at your side and a good Belgian ale in your chalice.

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Re: Unintialised value?
by neuroball (Pilgrim) on Jan 25, 2004 at 01:11 UTC

Hello, you might want to learn about the ins and outs of debugging.

Most of the information can be found in the perl documentation by using perldoc perldebug. If you want to invest a bit more time and effort into debugging you might want to get a copy of Perl Debugged.

Now back to your problem: You might want to print $page between line 61 and 62 and then exit (So that you actually see the result of your CGI).

The only problem that I see with your script, not knowing the input, is that $page might have been set to undef and then used as hash key, which returns also undef and breakes the if-statement.

Btw. The line numbers returned by errors and warnings are more of a semi-intelligent guess-timate of perl. It just gives you a point to start... but it isn't like an X on a treasure map.

/oliver/

Hans: Not ze problem!

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Re: Unintialised value?
by davido (Cardinal) on Jan 25, 2004 at 07:40 UTC

exists

my % pages = ( 'index' => 1,
               'me'    => 1,
               'stuff' => 1,
               'links' => 1 );
my $page = shift;
exit 0 unless exists $pages{$page} and $pages{$page} == 1;
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This will not generate the uninitialized value warning, because the right hand side equality test only gets evaluated if the left side of the logical short-circuit 'and' evaluates to truth first (if the hash key exists).

If you happen to be in a situation where the key might exist but without any initialized value, you could chain a defined in there...