埃拉托色尼筛子筛选素数

/********************************************************************
    created:    2013/10/22
    created:    22:10:2013   12:51
    file base:    埃拉托色尼筛子
    file ext:    cpp
    author:        Justme0 (http://blog.csdn.net/Justme0)
    
    purpose:    高效筛选素数
*********************************************************************/
#include <iostream>
#include <vector>
#include <cassert>
using namespace std;

/*
** initially all set to true
*/
void initiate(vector<bool> &A) {
    assert(A.size() >= 3);
    for (unsigned i = 2; i < A.size(); ++i) {
        A[i] = true;
    }
}

/*
** the size of A is n+1
** Let A be an array of Boolean values, indexed by integers 2 to n
*/
void sieveOfEratosthenes(vector<bool> &A) {
    assert(A.size() >= 3);

    initiate(A);

    int n = A.size() - 1;
    for (int i = 2; i * i <= n; ++i) {
        if (A[i]) {
            for (int j = i * i; j <= n; j += i) {
                A[j] = false;
            }
        }
    }
}

void outputPrime(const vector<bool> &A) {
    int cnt = 1;
    for (unsigned i = 2; i < A.size(); ++i) {
        if (A[i]) {
            cout << cnt++ << '	' << i << endl;
        }
    }
}

int main(int argc, char **argv) {
    int n;
    while (cout << "请输入数字：
", cin >> n) {    
        vector<bool> A(1 + n);
        sieveOfEratosthenes(A);
        outputPrime(A);
    }

    system("pause");
    return 0;
}

/*

#include <iostream>
#include <algorithm>
#include <cassert>
using namespace std;

const int LENGTH = 26;

void getPrimeNum(int *a, int len) {
assert(len >= 3 && "长度不足");

a[0] = -1;
a[1] = -1;

// 1. 小于sqrt(n)即可
for (int primeIndex = 2; primeIndex * primeIndex < len - 1; ++primeIndex) {
while (-1 == a[primeIndex]) {
++primeIndex;
if (primeIndex >= len) {
return ;
}
}

// 2. 从primeIndex^2开始
for (int i = primeIndex * primeIndex; i < len; i += primeIndex) {
a[i] = -1;
}
}
}

int main(int argc, char **argv) {
int a[LENGTH];
for (int i = 0; i < LENGTH; ++i) {
a[i] = i;
}

getPrimeNum(a, LENGTH);

for (int i = 0, cnt = 1; i < LENGTH; ++i) {
if (a[i] != -1) {
cout << cnt++ << '	' << a[i] << endl;
}
}

return 0;
}

*/