poj2909

素数筛法，然后判断有几个即可。

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;

#define maxn 32005

int prm[maxn];
bool is[maxn];

int getprm(int n)
{
    int i, j, k = 0;
    int s, e = (int) (sqrt(0.0 + n) + 1);
    memset(is, 1, sizeof(is));
    prm[k++] = 2;
    is[0] = is[1] = 0;
    for (i = 4; i < n; i += 2)
        is[i] = 0;
    for (i = 3; i < e; i += 2)
        if (is[i])
        {
            prm[k++] = i;
            for (s = i * 2, j = i * i; j < n; j += s)
                is[j] = 0;
            // 因为j是奇数，所以+奇数i后是偶数，不必处理！
        }
    for (; i < n; i += 2)
        if (is[i])
            prm[k++] = i;
    return k; // 返回素数的个数
}

int main()
{
    //freopen("D:\\t.txt", "r", stdin);
    int n;
    getprm(maxn - 4);
    while (scanf("%d", &n) != EOF && n != 0)
    {
        int e = n / 2;
        int ans = 0;
        for (int i = 0; prm[i] <= e; i++)
            if (is[n - prm[i]])
                ans++;
        printf("%d\n", ans);
    }
    return 0;
}