When I had to implement a binary search, it wasn't exactly the version you'd find in a textbook or library. I'm not sure I recall the details now, but I think it needed to return the two closest values if the search key wasn't present. And maybe there was something special about handling duplicate keys, but you get the idea. Sure, it's always good not to re-implement the wheel, but as a professional there are a million mundane reasons you get stuck doing it anyway.