Difference between upper_bound and lower_bound in stl

A simple answer is and less confusing WAY to remember this is below

std::lower_bound - returns iterator to first element in the given range which is EQUAL_TO or Greater than val.

std::upper_bound - returns iterator to first element in the given range which is Greater than val.

value a a a b b b c c c
index 0 1 2 3 4 5 6 7 8
bound       l     u

Where l represents the lower bound of b, and u represents the upper bound of b.

So if there are range of values that are "equal" with respect to the comparison being used, lower_bound gives you the first of this, upper_bound gives you one-past-the-end of these. This is the normal pattern of STL ranges [first, last).