rationale for std::lower_bound and std::upper_bound?

Question

STL provides binary search functions std::lower_bound and std::upper_bound, but I tend not to use them because I\'ve been unable to remember what they do, because their contract

Answer 1

Imagine what you would do if you want to find the first element equal to val in [first, last). You first exclude from first elements which are strictly smaller than val, then exclude backward from last - 1 those strictly greater than val. Then the remaining range is [lower_bound, upper_bound]