link factoring

Question

I am stubbing the book 6th edition of C as programming of the Deitel brothers and I am doubtful in an exercise, the exercise is the 4.14 of chapter 4 which is about factorization, it asks a program that the user type a number and that number is factored, for example if the user type the number 5, the program will do 5*4*3*2*1 resulting in 120. If anyone can give me some tip and not reply I’ll be very grateful thank you:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int i, num, aux1=0, fat=0, aux=0;

    printf("digite um numero para ser fatorado:\n");
    scanf("%i", &num);
    printf("\nvamos fatorar entao o numero %i:\n\n", num);

    for(i=num;i>=1;i--)
    {
        aux=i;
        fat=aux*i;
        printf("%i! ", fat);
    }
    return 0;
}

Answer 1

Notice what you’re doing:

Suppose in a = 5;

   for(i=num;i>=1;i--)
    {
        aux=i;
        fat=aux*i;
        printf("\n%d ", fat);
    }

Take the test table:

i=5         |    i=4            |  ...
aux=5       |   aux=4           |
fat=5*5???  |  fat=4*4          |

What you’re looking for is this:

i=5         |    i=4            |  ...
aux=5       |   aux=fat   (5)   |
fat=5       |  fat=aux*i (5*4)  |

Note that there are several unnecessary variables and do not forget: 0! = 1.

Tip: You don’t need the aux variable, much less the Aux1 that you didn’t even use.

Answer 2

My tips:

1. What is the factor of 0? This would be a better value to boot fat, since then you print the correct answer if the loop does not run at all.

2. Note as the final result of fat depends only on the last value of aux , which depends only on the last value of i. I don’t think that’s why you created the fat...

3. I prefer to iterate from 1 to N instead of N to 1. This makes it easier to write an invariant for the value of fat. If you don’t want to think about the invariant then whatever, obviously)

---

Hint of C:

You can declare variables after any {. It doesn’t have to be all at the top of the job:

{int i; for(i=n; i >= 1; i--){
   int aux = /* ... */
}}

Also, if you’re not using Visual Studio, you can compile your program using the C99 standard that lets you declare variables in the middle of the function

int num;
scanf("%d", &num);

for(int i=n; i >= 1; i--);