Binary search obviously works on searching for elements in a sorted array. But if you think about the reason why it works is that the array itself is monotonic ( either increasing or decreasing ). So, if you are in a particular position, you can make a definite call whether the answer lies in the left part of the position or the right part of it.

A similar thing can be done with monotonic functions ( monotonically increasing or decreasing ) as well.
Let's say we have f(x) which increases when x increases.

So, given a problem of finding x so that f(x) = p, I can do a binary search for x.

In any instance,
1. if f(current_position) > p, then I will search for values lower than current_position.
2. if f(current_position) < p, then I will search for values higher than current_position
3. if f(current_position) = p, then I have found my answer.

Problems for practice
SQRT