Thread: Someone explain this to me

how does this work:

Code:

#include <stdio.h>

int fact(int n){
  static int num = 0;
  if(n == 1)
    return 1;
  else{
    num++;
    printf("&#37;d \n", num); 
    return fact(n-1)*n;
  }
}

int main(void){
  printf("fact(%d) is %d\n", 5, fact(5));
  return 0;
}

i dont understand it to good, the return fact(n-1)*n; part

It's an example of recursion. A function calls itself, or it could have several levels: function A calls function B which calls function A.

If you call fact() with 0 or a negative value you're going to have a runaway condition. Otherwise the code looks OK on quick inspection. I haven't compiled it to check.

i dont understand how it works though, does it do n-1 = 4 * 5 = 20 the first time? if so what does it do the second time?

i put the static int num to see how much times the function is used but i still dont entirely understand how it works

Output:

Code:

1 
2 
3 
4 
fact(5) is 120

Yes. It is in fact doing 5 * 4 * 3 * 2 * 1
which is 120.

That's the definition of factorial.
Mathematically written it's 5! = 120

oh, makes sense now

Excellent! You get the idea.

I was wrong with the above - because it's not 100% accurate reading it left-to-right.

In fact it's doing it in this order
(((1 * 2) * 3) * 4) * 5

Because factorial is defined as n x (n - 1) in short-form mathematical terms, it's used often to introduce recursive function calls in programming. Ironically it's probably the worst example of the need for recursion because a simple loop with a count-down is much more efficient.

Still, it expands the mind as to possibilities wherein recursion may serve elegantly with other problems.