Trapping a warning

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

Fellow Monks, I recently ran into a seemingly simple problem, which to my complete surprise managed to stumped me.
Scenario:

for my $row (@{$dates}) {
    $seasons{'low'} = $row->[0] if ($row->[0] < $seasons{'low'});
    $seasons{'high'} = $row->[1] if ($row->[1] > $seasons{'high'});
}
[download]

At some point the vars $row->[0] and $row->[1] were empty, this eventually broke a few other parts of the code which depended on this information. What I was wanting to do is act on the variables if the $row vars were empty, rather than checking for a value I assumed I could act if $! had a value.

for my $row (@{$dates}) {
    $seasons{'low'} = $row->[0] if ($row->[0] < $seasons{'low'});
    print "$row->[2]\n" if $!;
    $seasons{'high'} = $row->[1] if ($row->[1] > $seasons{'high'});
}
[download]

The theory behind it was that if an uninitialized var was being used a warning would pop up and I would print the corresponding id to let me know if it was empty.

I was trying to save myself a little bit of work by not testing the variable for a value. Of course my thoughts on how to do this were wrong since $! never had any value. I've scoured 'perldoc warnings & perlvar & perllexwarn ' to no avail, I am aware that I could use an eval to wrap the code in, but this is the innermost loop of a resource hungry piece of code and I was trying to avoid taking as many extra steps as possible to gain the advantage of speed.

Please note that this is not production code and the value of $row->[2] obtained by testing would be used via another method to populate $row

Any thoughts?

BlackJudas

Comment on Trapping a warning Select or Download Code

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Re: Trapping a warning by chromatic (Archbishop) on Jul 03, 2002 at 22:04 UTC
Don't work so hard. `warn "Empty row ($row->[2])\n" unless defined $row->[0];`	[reply] [d/l]
Re: Trapping a warning by zaimoni (Beadle) on Jul 03, 2002 at 22:40 UTC
Welcome to autovivification. I've fried myself quite a few times on this as well. Even if those reference-array entries `$row->[0]` and `$row->[1]` didn't exist before with your original code, they do exist (and are `undef`) after that code runs. The subsequent code is not likely to appreciate this. To test whether `$row->[0]` and `$row->[1]` are defined without risking autovivification, use something like `if (defined($row) and defined($row->[0]) and defined($row->[1]) { # .... }` The first clause tests for whether the $row variable is defined. If that variable is defined, we proceed to test whether the 0 and 1 entries of the referenced array are defined. This also prevents being flummoxed by 0 (which is Perl-false). Of course, do this before you start using `$row->[0]` and `$row->[1]`. After that, it's too late.	[reply] [d/l] [select]
Re: Trapping a warning by Abigail-II (Bishop) on Jul 04, 2002 at 11:15 UTC
Well, if you want to check whether `$row->[0]` and `$row->[1]` exists without autovivification, just check whether `$row` contains any elements. `$row->[0]` can only exist if and only if `@$row > 0`. Similar, `$row -> [1]` only exists if and only if `@$row > 1`. Note that from 5.6.0 onwards, `delete` and `exists` work on arrays as well. Abigail	[reply] [d/l] [select]
Re: Trapping a warning by Jim Morrison (Novice) on Jul 03, 2002 at 21:55 UTC
perlfaq7.pod has a very related advice: =head2 How can I catch accesses to undefined variables/functions/metho +ds? The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to undefined functions and methods. When it comes to undefined variables that would trigger a warning under C<-w>, you can use a handler to trap the pseudo-signal C<__WARN__> like this: $SIG{__WARN__} = sub { for ( $_[0] ) { # voici un switch statement /Use of uninitialized value/ && do { # promote warning to a fatal die $_; }; # other warning cases to catch could go here; warn $_; } }; [download] Adopt that code, and you will be able to "intercept" your warnings and use a value of "$!" at your will Also, do not forget to protect your code by `try { } if ($@) { ... }` [download] Jim	[reply] [d/l] [select]
Re: Trapping a warning by blackjudas (Pilgrim) on Jul 04, 2002 at 17:49 UTC
Thank you all for your input, I don't think I've fully explained my question. I know about autovivification and testing for truth on a variable, but in this case I was looking for a solution to use Perl's built-in warnings and how I would test for that situation. BlackJudas	[reply]