iOS Removing items from a dictionary and related array

[idelovski]

I have a dictionary and two arrays. All three objects are mutable and first array holds all the keys from the dictionary and second array holds only a few of the keys.

So I need to remove all items from the dictionary (and the first array) except those present in the second array too. Naively the code would look something like this:

for (NSString *key in allKeys)  {
   if (![importantKeys containsObject:key])  {
      [allKeys removeObject:key];
      [theDict removeObjectForKey:key]
   }
}

The question is, how would I do it for real? Create a third array with the keys that should be removed?

EDIT:

I need to do it in response to a notification - UIApplicationDidReceiveMemoryWarningNotification, and allocating another mutable array at that point seemed as pushing my luck and inviting even more trouble, especially with multitasking.