You can do it by controlling whether your iterator + 3 exceeds you vector (vect_add) limits so that you won't exceed vector bounds, also for each iteration you have to increase iterator by 4 to obtain your designed scheme.
Don't know if this is related but clearing vectors invalidates iterators. From std::vector clear.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.