C++ STL set: Can I use upper_bound() to find the minimum difference of a number with the elements of the set?

Question

I am working on a problem. I have a number a and a set of elements (say myset ). I want to find the number b in myset such that |ba| is minimum. I am using it=upper_bound(a) and then check |*it-a| and |(*it-1)-a| and take the minimum of these two. Is this the correct approach?

Answer 1

That's correct except for the edge cases: what if upper_bound returns end() and what if it returns begin() ?

// pre-conditions: contains at least 1 elem, is sorted

template <typename It, typename Value>
const Value& min_dist(It first, It last, const Value& a)
{
    It it = std::upper_bound(first, last, a);
    if (it == last) {
        return *(std::prev(last, 1));
    }
    else if (it == first) {
        return *it;
    }
    else {
        // somewhere in the middle, so both it and prev are valid
        It prev = std::prev(it, 1);
        return (*it - a) < (a - *prev)
            ? *it : *prev;
    }
}