factorial function

double factorial(double x)

calculate factorial of x

Implementation

double factorial(double x) {
  if (x < 3) {
    if (x < 2)
      return 1;
    else
      return 2;
  } else {
    double res = 1;
    for (double i = 2; i <= x; i++) {
      res *= i;
    }
    return res;
  }
}