#include <stdio.h>

int is_primer(int x)
{   int i=2;
    if(x == 1)
        return 0;
    else if (x <= 0)
        return 1;
    else if (x == 2)
        return 1;
    else
    {
        for ( i;i<x;i++)
        {
            if(x%i == 0)
                return 0;
        }
        return 1;
    }
}

int main()
{
    int num;
    while(scanf("%d", &num) != EOF)
    {
        if(is_primer(num))
            puts("YES");
        else
            puts("NO");
    }
    return 0;
}

为什么当x<0时，return=1呢
其它的没有问题

for ( i;i<x;i++)
        {
            if(x%i == 0)
                return 0;
        }
        return 1;
少了个else吧，而且应该在大括号里面吧