c++ - 带线程的质数

Question

我写了一段代码来打印用户写的从头到尾的所有数字。我想用线程来做到这一点。例如，开始是 1，结束是 100。我要求用户输入一个 N 数，它是程序创建的线程数。比如他输入10，程序就会创建10个线程。第一个线程将打印从 1 到 10 的质数。第二个线程将打印从 10 到 20 的质数。第三个从 20 到 30 等等。

但是我有一个问题。事实上，我的程序在文件中打印了很多不是质数的数字，而且我在代码中多次打印了相同的数字。

这是我的代码:

void writePrimesToFile(int begin, int end, ofstream&  file)
{
    for (int i = begin; i <= end; i++)
    {
        for (int j = begin; j < end / 2; j++)
        {
            if (i % j != 0)
            {
                file << i << endl;
            }

        }
    }
}

void callWritePrimesMultipleThreads(int begin, int end, string filePath, int N)
{
    ofstream myfile(filePath);
    clock_t startTimer, stopTimer;

    startTimer = clock();

    vector<thread> arr;

    for (int i = 0; i < N; i++)
    {
        int start = begin;
        int finish = N;
        arr.emplace_back(writePrimesToFile, start, finish, ref(myfile));
        start = finish;
        finish += N;
    }
    for (auto& thread : arr)
    {
        thread.join();
    }

    stopTimer = clock();
    cout << "The time that takes is: " << (double)(stopTimer - startTimer) / CLOCKS_PER_SEC << endl;
}

主要代码:

    callWritePrimesMultipleThreads(1, 100, "primes2.txt", 10);

Answer 1

代码中有很多问题需要修复，素数将从 1 开始，而不是 0，而且你应该开始除以 2 而不是 1 或 0(你不能除以 0)，在你得到其余的 0 之后一，它不是素数，它总是以你要计算的数结束(10 % 20 是无意义的)

#include <stdio.h>
#include <iostream>
#include <thread>
#include <mutex>
#include <vector>
#include <functional>
#include <fstream>
#include <math.h>

using namespace std;
mutex mtx;

void writePrimesToFile(unsigned int begin, unsigned int end, ofstream& f)
{
    for (unsigned int i = begin; i <= end; i++)
    {
        for (unsigned int j = 2; j < i; j++)
        {
            if (i % j == 0)
            {
                break;
            }
            else if(j + 1 == i)
            {
                mtx.lock();
                f << i << endl;
                mtx.unlock();
            }
        }
    }
}

void callWritePrimesMultipleThreads(unsigned int begin, unsigned int end, string filePath, unsigned int N)
{
    ofstream myfile(filePath);
    clock_t startTimer, stopTimer;

    startTimer = clock();

    vector<thread> arr;
    unsigned int each = end/N;
    unsigned int start = begin;
    unsigned int finish = start + each - 1;
    for (unsigned int i = 0; i < N; i++)
    {
        arr.emplace_back(writePrimesToFile, start, finish, ref(myfile));
        start += each;
        finish += each;
    }
    for (auto& thread : arr)
    {
        thread.join();
    }

    stopTimer = clock();
    cout << "The time that takes is: " << (double)(stopTimer - startTimer) / CLOCKS_PER_SEC << endl;
}


int main()
{
    callWritePrimesMultipleThreads(1, 110, (string)"primes.txt", 10);
    return 0;
}

此外，在写入文件时添加了一个互斥量。