证明任意一个偶数为两个素数之和

一、案例工程

1、案例代码

#include <stdio.h>
#include <stdbool.h>
#include <math.h>
#define YES 1
#define NO  0

int isprime(int x)
{
    int i = 2;
    bool flag = false;
    for(; i < sqrt(x); i++)
    {
        if(x % i == 0)
        {
            flag = false;
            return NO;
        }
        else
            flag = true;
    }
    return YES;
}

void prove_1_even_has_2_prime(int x)
{
    int i = 2;
    for (; i < x/2; i++)
    {
        if(isprime(i))
        {
            if(isprime(x-i))
                printf("%d is a even,the prime are : %d,%d\n",x,i,x-i);
        }
    }
}

int main()
{
    int a;
    printf("Input one even :\n");
    scanf("%d",&a);
    if(a%2 == 0)
        prove_1_even_has_2_prime(a);
    else
        printf("%d is not a ever number\n",a);

    return 0;
}

2、编译过程

[root@localhost arithmetic]# gcc -o evenAndprime evenAndprime.c -lm

注意要加上-lm

 

3、实数验证

[root@localhost arithmetic]# ./evenAndprime
Input one even :
50
50 is a even,the prime are : 3,47
50 is a even,the prime are : 7,43
50 is a even,the prime are : 9,41
50 is a even,the prime are : 13,37
50 is a even,the prime are : 19,31

 

二、算法分析

 

 

    算法思想:如果需验证的数置于变量X中，则可依次从X中减去i，其中i从2变化到X/2。