Factorial using Addition

Question

I am attempting to create a C code that finds the factorial of a integer so that I may convert my code to assembly language. My code seems to \'multiply\' the second integer twi

Answer 1

If you add some intermediate debugging output you'll find where you went wrong:

#include 

#define N 5

int main()
    {
    int j = 0;
    int i = 0;

    int num1 = N;
    int num2 = N - 1;
    int sum = 0;

    while (num2 != 0)
        {
        printf("1 -> num1=%d  num2=%d  sum=%d  j=%d\n", num1, num2, sum, j);

        while (j < num2)
            {
            sum += num1;
            j++;
            }

        printf("2 -> num1=%d  num2=%d  sum=%d  j=%d\n", num1, num2, sum, j);

        j = 0;
        printf("%d\n", sum);
        printf("--------------\n");
        --num2;
        num1 = sum;
        }

    printf("--->%d", sum);
    }

This produces:

1 -> num1=5  num2=4  sum=0  j=0
2 -> num1=5  num2=4  sum=20  j=4
20
--------------
1 -> num1=20  num2=3  sum=20  j=0
2 -> num1=20  num2=3  sum=80  j=3
80
--------------
1 -> num1=80  num2=2  sum=80  j=0
2 -> num1=80  num2=2  sum=240  j=2
240
--------------
1 -> num1=240  num2=1  sum=240  j=0
2 -> num1=240  num2=1  sum=480  j=1
480
--------------
--->480

You can see here that the problem is that the sum value is carried forward from each pass through the loop, when it should really be reset to 0 each time. So add

sum = 0;

at the top of the while loop.

So your final code becomes:

#include 

#define N 5

int main()
    {
    int j = 0;
    int i = 0;

    int num1 = N;
    int num2 = N - 1;
    int sum = 0;

    while (num2 != 0)
        {
        sum = 0;

        while (j < num2)
            {
            sum += num1;
            j++;
            }

        j = 0;
        printf("%d\n", sum);
        printf("--------------\n");
        --num2;
        num1 = sum;
        }

    printf("--->%d", sum);
    }

Best of luck.