Question

I need to check if a std::set contains element/elements in a range. For example, if the set is a set<int> {1, 2, 4, 7, 8}, and given an int interval [3, 5] (inclusive with both endpoints), I need to know if it has elements in the set. In this case, return true. But if the interval is [5, 6], return false. The interval may be [4, 4], but not [5, 3].

Looks like I can use set::lower_bound, but I am not sure whether this is the correct approach. I also want to keep the complexity as low as possible. I believe using lower_bound is logarithmic, correct?

Answer 1

You can use lower_bound and upper_bound together. Your example of testing for elements between 3 and 5, inclusive, could be written as follows:

bool contains_elements_in_range = s.lower_bound(3) != s.upper_bound(5);

You can make the range inclusive or exclusive on either end by switching which function you are using (upper_bound or lower_bound):

s.upper_bound(2) != s.upper_bound(5); // Tests (2, 5]
s.lower_bound(3) != s.lower_bound(6); // Tests [3, 6)
s.upper_bound(2) != s.lower_bound(6); // Tests (2, 6)

Logarithmic time is the best you can achieve for this, since the set is sorted and you need to find an element in the sorted range, which requires a dichotomic search.