C++ set find value smaller but closer to x

Question

I looked at lower_bound and upper_bound in the C++ STL for <set>. I can't find a method, however, to get the value closest (from below) to another one in the set. Is there a simple way to get that or do I have to iterate the set?

As an example, say my set contains the following integers 3 4 7 9, then closest(6) = 4 and closest(4) = 4.

Answer 1

std::upper_bound returns an element greater than the given value, so in your case, your would have to decrement it to get the value before it.

//Returns value of element before 'num'
int closest = *--set.upper_bound(num);

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I would have thought that closest(6) = 7, because 7 is closer to 6 than 4. If you want to get 7, you would have to calculate the difference between the adjacent values and compare them.

//Calculate closest value to 'num' in a std::set<int>
int closest(std::set<int>& set, int num)
{
    //Get iterator to element greater than 'num'
    auto it = set.upper_bound(num);

    //Check if 'it' is the 'end' iterator
    if (it == std::end(set))
        return 0;

    int valueLeft = *it--; //Get value of the greater element
    int valueRight = *it; //Get value of the element before (due to post-decrement)

    //Compare diffence between value less and num, and value greater and num
    if (valueLeft - num > num - valueRight)
        return valueRight;
    else
        return valueLeft;
}

std::set<int> set{ 3, 4, 7, 9 };

int a = closest(set, 6); //Returns '7'
int b = closest(set, 4); //Returns '4'