#include <stdio.h>
    #include <math.h>

    #define YES 1
    #define NO 0

    int isprime(int num)
    {
        int i, j;

        if (num == 2)
            return YES;
        else if (num < 2 || num % 2 == 0)
            return NO;
        else
        {
            j = (int)sqrt(num + 1);
            for (i = 3; i <= j; i = i + 2)/*这句。。。如果我输入的数据是4.求个解释，这个程序的步骤怎么运行。*/
                if (num % i == 0)
                    return NO;
        }

        return YES;
    }

    int main(int argc, char *argv[])
    {
        int N;
        scanf("%d", &N);

        if (isprime(N) == YES)
            printf("yes\n");
        else
            printf("no\n");

        return 0;
    }

输入4时，j = (int)sqrt(num + 1)=2，for循环不执行。

num等于4，不就是进第二个else if么？