Simple numerical integration algorithm.

NumIntegral.cc

// https://helloacm.com/c-function-to-compute-numerical-integral-using-function-pointers/

double integral(double(*f)(double x), double a, double b, int n) {
    double step = (b - a) / n;  // width of each small rectangle
    double area = 0.0;  // signed area
    for (int i = 0; i < n; i ++) {
        area += f(a + (i + 0.5) * step) * step; // sum up each small rectangle
    }
    return area;
}

int main()
{
    cout.precision(7);
    cout << integral(cos, 0, M_PI / 2, 10);
    return 0;
}