c programming pointers (a factorial calculation)

Randall · 02-22-2003, 09:50 PM

Hi everyone,
I'm trying to write a program to calculate the factorial of a given integer. I would like to get some pointers on how to go about doing it. I was thinking of using a for loop but i haven't been able to put the right expressions in. here is where i am at so far:

#include <stdio.h>

int main(void)
{
int x;
printf("Enter number for factorial calculation: ");
scanf("%d", &x);

/*and here is where i am stuck*/

i well definitely appreciate any pointers in the right direction,
Thanks,
Randall

wapcaplet · 02-22-2003, 09:57 PM

Think about the way you would calculate a factorial by hand. Say, 5 factorial. "Okay, that's 5x4x3x2x1." In other words, it's n times n-1 times n-2 ... all the way down to 1 (or 2, if you want to save the redundant step of multiplying by 1).

So you are on the right track with a for loop. How about, for i from n to 1, i = i - 1... multiply the total by i.

Randall · 02-22-2003, 10:28 PM

that's where i was headed,
and here's what i have so far

#include <stdio.h>

int main(void)

{
int n,i, factorial;
printf("Enter number for factorial calculation: ");
scanf("%d", &n);

for (i = n; i <= 0 ; i --)
factorial = n*(n-1);
printf("%d\n", factorial);

printf("%c", getchar());
return (0);
}

but that just returns garbage
i get 134513294

thanks for the help so far!

Randall

GtkUser · 02-23-2003, 06:06 AM

Code:

#include<stdio.h>

int Factorial(int x) {
  if (x > 1)
    x *= Factorial (x - 1);
  return x;
}

int main() {
  int x;
  printf("Enter a number: ");
  scanf("%d",&x);
  printf("The factorial is %d\n", Factorial(x) );
  return 0;
}

GtkUser · 02-23-2003, 06:33 AM

Or else:

Code:

#include<stdio.h>

int main() {
  int num;
  int factorial;

  printf("Enter a number: ");
  scanf("%d",&num);

  for (factorial = num; num > 1; --num) {
    factorial *= (num - 1 );
  }

  printf("The factorial is: %d\n", factorial);
  return 0;
}

Note that:

Code:

factorial *= (num - 1);

Expands (by the compiler) into:

Code:

factorial = factorial * (num - 1);

Randall · 02-23-2003, 12:47 PM

Thanks a lot!!
That really helped, i was getting really confused about the middle statement that terminated the loop, and i believe i made an error with the decrementation. i don't think i would have figured out the assignment of factorial *= (n -1); though

anyway, thanks a lot for the help, i really appreciate it,
i added a if statement to check for negetaive numbers so ... it works pretty well now.

best regards,
Randall