和素数相关的两个小问题

1.判断一个数是不是素数，效率还行的一段小代码

#include    
#include    
int IsPrime(int x)   
{      
    if (x == 1)return 0;   
    else if (x == 2)return 1;   
    else if (x % 2 == 0) return 0;   
    else    
    {    
        int i,bound = sqrt((double)x) + 1;   
        for( i = 3; i < bound; i += 2)   
        {   
            if (x % i == 0)   
                return 0;   
        }   
        return 1;   
    }   
}   

int main()   
{   
    int n;   
    printf("Please input a positive integer:");   
    while (scanf( "%d",&n) && n) {   
        if (n < 0) printf("Please input a positive integer:");   
        else    
        {   
            if(IsPrime(n))   
                printf("Yes\n");   
            else    
                printf("No\n");    
            printf("Please input a positive integer:");   
        }   
    }   
    return 0;   
}  

2.如果你想计算[0,n]中所有素数的和，那可以一边寻找，一遍构造素数数组，这个数组对后面整数的素数判断有好处，这样检查起来速度会更快。当然你可以先构造好数组，再计算，但是个人觉得，两者差不多。

#include    
#include    
int Ps[100];   
int index = 2;   
int IsPrime(int x)   
{      
    if (x == 1)return 0;   
    else if (x == 2)return 1;   
    else if (x % 2 == 0) return 0;   
    else    
    {    
        int i,bound = sqrt((double)x) + 1;   
        for( i = 1; Ps[i] < bound; i ++)   
        {   
            if (x % Ps[i] == 0)   
                return 0;   
        }   
        Ps[index++] = x;   
        return 1;   
    }   
}   

int SumPrime(int n)   
{   
    int i, sum = 0;   
    for(i = 2; i <= n; i++)   
        if(IsPrime(i))   
            sum += i;   
    return sum;   
}   

int main()   
{   
    int n;   
    Ps[0] = 2;Ps[1] = 3;   
    printf("Please input a positive integer:");   
    while (scanf( "%d",&n) && n) {   
        if (n < 0) printf("Please input a positive integer:");   
        else    
        {   
            printf("The Sum of primers between 0 and %d is: %d\n",n,SumPrime(n));              
            printf("Please input a positive integer:");   
        }   
    }   
    return 0;   
}