Question

I was trying to use the accumulate function for vectors

vector <double> A;
double B = 0;

A.reserve(100);
for(itr = 0; itr < 210; itr++)
{
    term1 = pow(r[itr], 12);
    term1 = 1/term1;
    term2 = pow(r[itr], 6);
    term2 = 2/term2;
    A.push_back(term1 - term2);
}
B = accumulate(A.begin(), A.end(), 0);

however, I always got B = 0, while A had nonzero values

Answer 1

std::accumulate is a bit sneaky in the sense that the type of the result is the type of the initial value, and not the type of the container elements! So your accumulator produces ints.

To fix this, accumulate into a double:

accumulate(A.begin(), A.end(), 0.0);
//                             ^^^^^^^ literal of type double