Help with factorial problem please

Hi there. I'm trying to write up a simple factorial code yet i'm finding it difficult to get around.

Here is the code i have so far

Code:

---

#include <stdio.h>

int main() {
    int num, sum, dum;

    num = 1;
        printf("Enter number: ");
        scanf("%d", &sum);
        dum = sum;
    while(num <= dum) {
        sum = sum * num;
        num = num + 1;
    }
    printf("Sum = %d\n", sum);
    return 0;
}

---

Now it goes through it in the right way, just that when it comes to the answer it should output, it multiplies it again by the initial sum number. So if i put in to find 3! it produces 18 (ie, the answer should be 6, yet it multiplies by 3 one last time). Sorry if it is obvious, i'd stay around to figure it out but i have to leave. Thanks.

That's because sum starts at 3 (for instance); so you're doing 3*1*2*3. Since you don't need to put the last number in again, your while loop goes one too far.

What would you suggest?

or

sum = 1;

As Mr. Law says, start your accumulator off at 1, or go one less time through the loop.

^^I tried that and i get a 0.

Edit - Mr Law was right. I made sum = 1, deleted the dum=sum assignment and changed the input to dum instead of sum.

Code:

---

num = 1;
        sum = 1;
        printf("Enter number: ");
        scanf("%d", &dum);
    while(num <= dum)

---

Thanks heaps guys!