Rotating vector elements by value greater than vector size?

Question

Somewhat of a simple question, I'm rotating the order of elements in a vector using std::rotate:

std::vector<int> V {1,2,3,4};
std::rotate(V.rbegin(), V.rbegin() + K, V.rend());

//output: {4,1,2,3}

I'm getting a segmentation fault where K is >= V.size(), and I'm struggling to understand: 1) why this error is occurring? and 2) how I can perform a rotate operation with such a K value?

For example, a K of 6 should output {2,3,4,1}

Answer 1

If K is greater than V.size() , then V.rbegin()+K is an invalid iterator. You need to reduce K by mod size. Something like:

if (V.size() != 0) {
    std::rotate(V.rbegin(), V.rbegin() + (K % V.size()), V.rend());
}

Note the test for size != 0 first - otherwise you'll get division by zero.

Answer 2

V.rebegin() + 6 is not a valid iterator. Which element would it point to? I will assume you want to rotate the vector by K elements. What you need to do is apply the modulo operator to find out which value should be at the front of the vector.

std::vector<int> V {1,2,3,4};
const auto K = 4 % V.size();
std::rotate(V.rbegin(), V.rbegin() + K, V.rend());

I'm not sure why you expect that K of 6 should result in {2,3,4,1}, it gives {3,4,1,2}. Perhaps I've misunderstood the question.

Edit: See this answer about checking the size first.