C :: Simple Calculation - Function To Do Factorization

I wrote a function to do factorization (n!). while this function works perfectly:

Code:
int fact (double x){
register int z=1;
if (x==0)
return 1;
else
for(x;x;x--)
z*=x;
return z;
}

this doesn't:

Code:
int fact (double x){
if (x==0)
return 1;
else
for(x;x;x--)
x*=x;
return x;
}

what makes the second one different from the first one? can't see the difference..