difference between x*= and x=x*...?

Question

Look at this function:

int c(int n,int k) {
    if (n<0 || k<0 || n<k) {
        return 0;
    }
    int c=1,p=n-k,i=1;
    while (n>p) {
        c = c * n/i++; // <<<
        n--;
    }
    return c;
}

when I change c=c*n/i++ to c*=n/i++ function don't work correctly. for example c(4,2)=4 (=6 true) I traced with:1.i++ 2./ 3.*= and all other possibilities but no answer. why?

edit: this is not my question as people know. I asked why compiler answer c(4,2)=4 when c*=n/i++. I traced */++, *++/, /*++, /++*, ++*/, ++/* but answer is not 4 at all. what is compiler's logic?(sorry for bad English)

Answer 1

In the case of

c = c * n / i++;

You first multiply c and n, then divide it by i, and lastly i is increased.

When you do

c *= n / i++;

You first to the division and increase of i, then do the multiplication.

---

In other words:

c = c * n / i++;

is equal to

c = (c * n) / i++;

While

c *= n / i++;

is equal to

c = c * (n / i++);