vector ----- size function Notes

function returns the size of the vector size vector, return type size_type, Member type  size_type  IS AN unsigned Integral type, i.e. an unsigned integer;

vector<int> A;

A.size () - 1 because the size returned value is an unsigned type so A.size () - 1 out of range, a large number of

Proper use (int) (A.size () - 1)