why isn't it endless loop of while?(c)-CodePudding

#include <stdio.h>
int main(void) {
     unsigned long long f;
     int n;
     printf("factorial:");
     scanf("%d", &n);
     printf("%d! = ",n);
     f = n;
     while (--n)
         f*=n;
     printf("%llu\n",f);
     return 0;
 }

I thought it would work endless loop in while-loop. However when I put negative number, it works. --> factorial : -1 -1! : 0 Why this thing happened? I wonder why zero was printed to the end. Pleaseeeeee help me.

CodePudding user response：

The while condition will execute till the condition is false. This means after the while loop !n which in C is the same as n == 0. If you subtract 1 from n = -1 sufficiently many times it will signed underflow and become positive and then eventually 0. Hence it is not an infinite loop.

f is unsigned and you multiply it by a negative number which then is interpreted as very large number. It turns out that with the input -1 at n == -67 f becomes 0 due to unsigned overflow:

n  f
-2 18446744073709551615
-3 2
-4 18446744073709551610
...
-65 9223372036854775808
-66 9223372036854775808
-67 0
...
2 0
1 0