Re: Changing tables during traversal: clearing and re-adding a field

[Lorenzo Donati <lorenzodonatibz@...>]

On 29/06/2011 18.15, Dirk Feytons wrote:
> Hi,
>
> The reference manual clearly describes what can and cannot be done to
> a table during traversal: modifying and clearing an existing field is
> OK, assigning to a non-existing field is bad.
> 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
> 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 that.

Try to rewrite the code in order to clear a field only if it must not be 
reassigned.

----------------------------
for k,v in pairs(t) do
   if (<some condition>) then
     t[k] =<some other value>
   else
     t[k] = nil
   end
end
----------------------------

cheers
-- Lorenzo