判断一个正整数是否为质数?不知道哪里错了?求助
```c
#include <stdio.h>
int main() {
int a = 0;
int num = 0;
scanf_s("%d", &num);
for (int i = 2; i < num; i++) {
if (num % i == 0) {
a++;
}
}
if (a == 0) {
printf("Yes");
}
else {
printf("No");
}
return 0;
}
```