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This is the sieve of Eratosthenes implemented in C++.

C++, 57 lines
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/*
 Copyright (c) 2008 Florian Mayer

 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights
 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 copies of the Software, and to permit persons to whom the Software is
 furnished to do so, subject to the following conditions:

 The above copyright notice and this permission notice shall be included in
 all copies or substantial portions of the Software.

 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
*/

#include <iostream>
#include <vector>
#include <string.h>

using namespace std;

vector<unsigned long> get_primes(unsigned long max){
    vector<unsigned long> primes;
    char *sieve;
    sieve = new char[max/8+1];
    // Fill sieve with 1  
    memset(sieve, 0xFF, (max/8+1) * sizeof(char));
    for(unsigned long x = 2; x <= max; x++)
        if(sieve[x/8] & (0x01 << (x % 8))){
            primes.push_back(x);
            // Is prime. Mark multiplicates.
            for(unsigned long j = 2*x; j <= max; j += x)
                sieve[j/8] &= ~(0x01 << (j % 8));
        }
    delete[] sieve;
    return primes;
}

int main(void){
    vector<unsigned long> primes;
    primes = get_primes(10000000);
    // return 0;
    // Print out result.
    vector<unsigned long>::iterator it;
    for(it=primes.begin(); it < primes.end(); it++)
        cout << *it << " ";
    
    cout << endl;
    return 0;
}

Not wasting memory by using a bool array anymore.

2 comments

Florian Mayer (author) 13 years, 1 month ago  # | flag

Calculates in 14.08s when skipping the output. Note that this is for all the prime numbers <= 100000000.

David Lambert 13 years ago  # | flag

How shall I use my pretty new primes?

Created by Florian Mayer on Sat, 8 Nov 2008 (MIT)
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