Re^3: need explanation of @foo{@bar} = (); (hash slice)

I did not mean to imply that exists causes autovivification. What I was trying to get at is that autovivification has made me wary of keys that exist spuriously. These can occur in a number of ways, and I'd rather avoid the issue altogether by using defined instead of exists as a matter of policy.

The point is not whether spurious autovivification can be avoided (it is not hard to do so). The point is that I want to write my code so that, if I find myself debugging it weeks or months later, I don't have to worry over every exists expression that I run across. In other words, the possibility of unintended autovivification renders exists suspect in my eyes, and I'd rather deal with such suspicions as little as possible. A consequence of this is that I don't use undef as a hash value unless it is really necessary, or in very narrowly circumscribed blocks of code, in which one can tell at a glance that unintended autovivification is not a problem. It's all defensive programming, like avoiding globals, for example.

the lowliest monk

Comment on Re^3: need explanation of @foo{@bar} = (); (hash slice)

Replies are listed 'Best First'.
Re^4: need explanation of @foo{@bar} = (); (hash slice) by davido (Cardinal) on May 09, 2005 at 01:52 UTC
But defined doesn't do anything to prevent autovivification either. Look at this: `my %hash = ( Key1 => { john => 1, pete => 2 }, Key2 => { frank => 3, howard => 4 } ); print "Exists!\n" if defined $hash{Key7}{ted}; print "Key7 exists!\n" if exists $hash{Key7};` [download] Here you're testing the definedness of a key that doesn't exist: $hash{Key7}{ted}, and then you're going to check to see if $hash{Key7} autovivified. Guess what? It did, just the same as when we used exists to check for the existance of $hash{Key7}{ted}. The same conditions that will cause exists to autovivify a hash element will also cause defined to autovivify a hash element. exists doesn't have any more possibility to do that than any other function. Dave	[reply] [d/l]
Re^5: need explanation of @foo{@bar} = (); (hash slice) by tlm (Prior) on May 09, 2005 at 02:24 UTC
As I said, I don't claim that `exists` is a more efficacious way to induce autovivification than `defined` (or than anything else for that matter), but rather that it is a bad test to use, since there is the possibility of unintended autovivification happening anywhere else, and when this happens `exists` gives misleading results. In other words, using a slight modification of your example (`s/defined/exists/`): `my %hash = ( Key1 => { john => 1, pete => 2 }, Key2 => { frank => 3, howard => 4 } ); print "Exists!\n" if exists $hash{Key7}{ted}; print "Key7 exists!\n" if exists $hash{Key7};` [download] it is the second `exists` that I find problematic is not the first one, because it is the second `exists` that gives the misleading result (yes "Key7" exists but for the wrong reason). I'd just as soon have both tests fail, by using `defined` instead of `exists` for both. the lowliest monk	[reply] [d/l]