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

[Roberto Ierusalimschy <roberto@...>]

> >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.

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.

-- Roberto