AA.remove in foreach && AA = new vs cleaning

[Saaa]

Is there anything with removing the current key in a foreach?

foreach (K k, ; aa)
{
  ..
  aa.remove(k);
}

What if it isn't the current key?
If the iteration is generated on the fly it shouldn't be a problem, right?

&&

Is it better to new an AA or empty(.remove) it on the fly if you loop over 
all the keys anyways?
That is, I need an similar empty AA the next iteration :)
Removing every element takes of course as many operations as keys, but 
filling the array won't take
as many memory allocations, as the gc doesn't return the key allocations. Am 
I making any sense here? :D