简单的素数程序 - 线程C#的奇怪问题

Question

这是我的代码:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace FirePrime
{
    class Program
    {
        static bool[] ThreadsFinished;
        static bool[] nums;

        static bool AllThreadsFinished()
        {
            bool allThreadsFinished = false;
            foreach (var threadFinished in ThreadsFinished)
            {
                allThreadsFinished &= threadFinished;
            }
            return allThreadsFinished;
        }

        static bool isPrime(int n)
        {
            if (n < 2) { return false; }
            if (n == 2) { return true; }
            if (n % 2 == 0) { return false; }
            int d = 3;
            while (d * d <= n)
            {
                if (n % d == 0) { return false; }
                d += 2;
            }
            return true;
        }

        static void MarkPrimes(int startNumber,int stopNumber,int ThreadNr)
        {
            for (int j = startNumber; j < stopNumber; j++)
                nums[j] = isPrime(j);
            lock (typeof(Program))
            {
                ThreadsFinished[ThreadNr] = true;
            }
        }

        static void Main(string[] args)
        {
            int nrNums = 100;
            int nrThreads = 10;
            //var threadStartNums = new List<int>();

            ThreadsFinished = new bool[nrThreads];

            nums = new bool[nrNums];
            //var nums = new List<bool>();
            nums[0] = false;
            nums[1] = false;
            for(int i=2;i<nrNums;i++)
                nums[i] = true;

            int interval = (int)(nrNums / nrThreads);
            //threadStartNums.Add(2);
            //int aux = firstStartNum;
            //int i = 2;
            //while (aux < interval)
            //{
            //    aux = interval*i;
            //    i=i+1;
            //    threadStartNums.Add(aux);
            //}

            int startNum = 0;

            for (int i = 0; i < nrThreads; i++)
            {

                var _thread = new System.Threading.Thread(() => MarkPrimes(startNum, Math.Min(startNum + interval, nrNums), i));
                startNum = startNum + interval;
                //set the thread to run in the background
                _thread.IsBackground = true;
                //start our thread
                _thread.Start();
            }

            while (!AllThreadsFinished())
            {
                Thread.Sleep(1);
            }

            for (int i = 0; i < nrNums; i++)
                if(nums[i])
                    Console.WriteLine(i);
        }
    }
}

这应该是一个非常简单的程序,应该nrNums使用nrThreads并行工作的线程来查找和输出第一个素数.

所以,我只是分裂nrNums成nrThreads相等的块(当然,最后一个是不相等的;如果nrThreads不被分裂nrNums,它还将包含课程的其余部分).

我开始nrThreads线程.

他们都在各自的块中测试每个数字,看看它是否是素数; 它们在bool数组中标记所有内容,并在所有素数上保留一个标签.

这些线程在ThreadsFinished完成时都会将另一个布尔数组中的特定元素转换为true.

现在奇怪的部分开始了:

线程永远都不会结束.如果我调试,我发现这ThreadNr不是我在循环中分配给它的另一个值.我想这是正常的,因为线程后来执行并且计数器(变量i)已经增加,但我无法理解如何使代码正确.

有人可以帮忙吗？

先感谢您.

PS:我知道算法效率不高; 我的目标是使用Eratosthenes的筛子和x给定的螺纹.但是现在我甚至无法让这个工作起来,我也没有找到任何在我能理解的语言中任何实现该算法的例子.

Answer 1

线程接收的值是startNum线程运行时保存的值.要解决它,请将值复制到局部变量中:

for (int i = 0; i < nrThreads; i++)
{
 var localStartNum = startNum; // save value in local variable
                                  // and use in the thread start
    var localIndex = i;

 var _thread = new System.Threading.Thread(() => 
                      MarkPrimes(localStartNum,
                                 Math.Min(localStartNum + interval, nrNums),
                                 localIndex));
 startNum = startNum + interval;
 _thread.IsBackground = true;
 _thread.Start();
}

代码中的另一个错误是等待所有线程:

static bool AllThreadsFinished()
{
    bool allThreadsFinished = true; // Initialize to true instead of false
                                    // Otherwise, all ANDs will result false
    foreach (var threadFinished in ThreadsFinished)
    {
        allThreadsFinished = threadFinished;
    }

    return allThreadsFinished;
}

一个可以帮助一点同步线程的提示:您可以保存列表中的所有线程并从主线程连接它们.

var threads = new List<Thread>();

for (int i = 0; i < nrThreads; i++)
{
 var localStartNum = startNum; // save value in local variable
                                  // and use in the thread start
 var _thread = new System.Threading.Thread(() => 
                      MarkPrimes(localStartNum,
                                 Math.Min(localStartNum + interval, nrNums), i));
 startNum = startNum + interval;
 _thread.IsBackground = true;
 _thread.Start();
    threads.Add(_thread);
}

foreach(var thread in threads)
{
    thread.Join();
}