I got bit by precedence today, and as it turned out the nature of my data complicated my debugging efforts. In the end nothing I discovered was all that surprising, but the road to get there seemed dark and twisty enough to merit a 'lesson learned' posting.

I had the following data:

%dest = ();

%source = ( very_long_key_name => 1,
            another_key => 2 );

%names = ( 1 => 'name1', 2 => 'name2' );
[download]

I simply wanted to copy $source{very_long_key_name} to the %dest hash, doing a lookup in %names along the way. Specifically, I wanted to do this:

$dest{name} = $names{ $source{very_long_key_name} };
[download]

I ran the code and (thanks to warnings) got several 'Use of uninitialized value' warnings. A few strategically-placed print statements indicated that some of the data is missing. No problem - I'll just check for that before doing the lookup in %names.

$dest{name} = exists $source{very_long_key_name} ?
              $names{ $source{very_long_key_name} } :
              '';
[download]

My caffeine-starved brain thought the ''; looked lonely on a line all by itself, so I reversed the condition and moved it up. (In retrospect this was excessively silly - I said I was in need of caffeine.)

$dest{name} = not exists $source{very_long_key_name} ? '' :
              $names{ $source{very_long_key_name} };
[download]

But when I ran it, $dest{name} = 1 instead of 'name1':

print "name code = $source{very_long_key_name}\n";            # 1
print "converted = $names{ $source{very_long_key_name} }\n";  # name1
print "dest name = $dest{name}\n";                            # 1
[download]

In reality, the data structures were much more complex and I thought I screwed up one of the dereferencing steps. I tested each one and everything seemed to be working fine in isolation. I could not figure out why the original value in $source{very_long_key_name} was getting copied rather than the lookup value in %names, until it dawned on me that the source of the '1' might not be what I thought it was.

Sure enough - a quick glance at perlop confirmed my suspicions: the conditional operator has higher precedence than 'not', so the code was being parsed like this (output from B::Deparse):

$dest{'name'} = (! (exists($source{'very_long_key_name'}) ? '' :
                   $names{$source{'very_long_key_name'}}        ) );
[download]

Since exists($source{'very_long_key_name'} evaluates to true, the right hand side simplifies to not ''. The empty string evaluates to false, so not '' is true, or 1. (FWIW, not 'scalar' = '' and not undef = 1.)

A set of parentheses did the trick:

$dest{name} = (not exists $source{very_long_key_name}) ? '' :
              $names{ $source{very_long_key_name} };

print $dest{name}; # name1
[download]

That's my 'duh' moment for the day. Nothing fancy, nothing complicated, and nothing that couldn't have been prevented by simply remembering precedence.

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