# Grover algorithm when element not present in the array/database

I've been reading about the Grover algorithm for finding elements in an unstructured database by the means of quantum computing, and even done some exercises.

What I don't understand (and can't find anywhere) is what is supposed to happen if the element is NOT present in the database. The oracle would never flip any qubit, and then I don't know what would happen with the inversion about the mean operator. Would the algorithm keep iterating forever?

• Grover's algorithm won't loop forever, because it always performs $O(\sqrt{N})$ iterations. – Dmitri Urbanowicz Dec 18 '18 at 13:03
• That's right, I misunderstood the algorithm. – Jose Dec 18 '18 at 19:42

As mentioned in the wikipedia article on Grover's algorithm, the algorithm can be extended to work when there are an unknown number of marked items while still running in $$O(\sqrt{N})$$ time. There's just more details to work out.