Return random element from array excluding elements from subarray

Basically, I need something like this:

f(a[], b[]) = random element from a[] which is not in b[]

Idea I have is this one:

while true:
   e = element_from_uniform(a[])
   if e not in b[]: return e

Problem is, I want this function to be fast, since it will be calculating many many times (every frame on multiple objects). I know that since we are choosing from uniform distribution, eventually some element from a[] will pop up that isn't in b[].

Any ideas? Oh, if it helps, a[].length is always 6 and b[] is always some subset of a[] and b.length < a.length.

EDIT:

I want this to be done without allocating any of new memory, only with pointers.