While working on some code this evening, I came across an interesting problem that was entirely the result of my own doing and thought that I would share this with others.

The problem manifest itself with the failure of information stored in an array within a hash to be updated - For example, the following example illustrates the basic intention of my code, that is, to manipulate an array within a hash element, in this contrived example, to remove web page links from a user profile if the associated checkbox is ticked on a 'delete' form: 

my %user;
eval {
    my $session_id = $cgi->cookie( -name => 'mycookie', -path => '/' )
+;
    tie %user, 'Apache::Session::DB_File', $session_id, { 'FileName' =
+> '.sessions' };
};
croak( 'Cannot retrieve user session ID from client-side cookie' ) if 
+$@;

foreach (0..$#{$user{'links'}}) {
    if ($cgi->param('link_'.$_)) {
        splice(
            @{$user{'links'}},
            $_,
            1
        )
    }
}

[download]

As the code above demonstrates, there is little in the code itself to raise any red flags, however, it simply wasn't working in the larger script - The array within the hash element remained unmodified with execution, when all logic and passed parameters suggested that array elements should be spliced from the data structure.

I freely admit, I was somewhat puzzled by this and pondered on it for some time. Then, as I stared vacantly at my computer screen, I remembered something that I read about tied-hashes long ago in the MLDBM man page - You cannot directly modify an existing reference within a tied hash interface; it must first be retrieved and stored in a temporary variable for further modifications. This limitation, according to the MLDBM man page, is because the Perl TIEHASH interface currently has no support for multi-dimensional ties.

Therein lies the problem with my code - One cannot directly edit data structures stored within a tied-hash. Sure enough, once I updated this section of code to copy the hash element to a temporary array prior to splicing modification, the code worked perfectly. As such, this node, while not illustrative enough on its own to form a tutorial (or necessarily pensive and/or thought-provoking enough to warrant a true meditation), may serve as a solution for others who come across a similar problem.

The lesson learnt - Context is everything when it comes to debugging! :-)

 

In reply to The importance of context in debugging by rob_au

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