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- Subject: Re: Changing tables during traversal: clearing and re-adding a field
- From: Dirk Feytons <dirk.feytons@...>
- Date: Wed, 29 Jun 2011 22:09:45 +0200
On Wed, Jun 29, 2011 at 18:43, Roberto Ierusalimschy
>> >But what happens if I clear an existing field and then assign to it
>> >again during one iteration? In pseudocode:
>> >for k,v in pairs(t) do
>> > t[k] = nil
>> > if (<some condition>) then
>> > t[k] =<some other value>
>> > end
>> >Is this allowed? We've seen some intermittent errors in our code that
>> >I suspect are coming from a pattern like described above.
>> According to the letter of the manual, it is not allowed.
>> When you clear a field that field becomes non-existent. So if you
>> assign to it afterward, you violate the rule.
>> I cannot say, though, if the current implementation is tolerant
>> about that and if the failures you describe actually depends on
> This should not be a problem with the current implementation. When you
> create a new key, the table may rehash, and then all keys may change
> positions and the traversal goes weird. (This is the time where nil
> fields are really removed from the table, too.) As long as you do
> not create a new key, the "erased" field is still there, and so the
> assignment does not create a new key.
> But Lorenzo is right that it is not "formally" allowed. You should
> try to rewrite it.
Yeah, from past messages about table rehashing I guessed it wouldn't
cause badness but since you now confirmed it's an implementation
detail I'll surely rewrite the code.
Which also means our investigation continues...