Recursive Functions

"Recursion occurs when a function calls itself" - you know that famous sentence ;-). For example, we calculate the factorial of the number 4: 4 = 4*3*2*1. Here we can do the multiplication recursively: Multiply the number 4 with the result of the factorial of 3; to get the factorial of 3 multiply 3 with the factorial of 2; to get the factorial of 2 multiply 2 with the factorial of 1 which is just 1. Here the recursion ends...

Look at the following code:

#include <iostream.h>

double f(int n);

int main() {
  int input = 4;
  cout << input << "! = " << f(input) << endl;
  return 0;
}

double f(int n) {
  if (n>1) return n*f(n-1);
  return 1;
}

At first you define a so called "function prototype" so you can use the function within main(). The function f(int n) calls itself until 1 is reached...

nus@mail:~/ccpp > ./factorial
4! = 24