C++ vector insert with Iterator is not working as I expected

Question

C++11, Input is nums = [1,2,3,4,5]

void rotate(vector<int>& nums, int k) {
    nums.insert(nums.begin(), nums.end() - k, nums.end());
}

When k = 2 , I expect this function should make nums to [4,5,1,2,3,4,5]
but it becomes [2,3,1,2,3,4,5]
When k = 1 , nums is [4,1,2,3,4,5]
but when k = 4 , nums is [2,3,4,5,1,2,3,4,5] , which is what I wanted.
What am I doing wrong? Please help.

Answer 1

The std::vector::insert overload that you are using has a precondition that neither the second nor the third argument are iterators into the vector itself.

You are violating that precondition and therefore your program has undefined behavior.