Simple prime number program - Weird issue with threads C#
- by Para
Hi!
This is my code:
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);
}
}
}
This should be a pretty simple program that is supposed to find and output the first nrNums prime numbers using nrThreads threads working in parallel.
So, I just split nrNums into nrThreads equal chunks (well, the last one won't be equal; if nrThreads doesn't divide by nrNums, it will also contain the remainder, of course).
I start nrThreads threads.
They all test each number in their respective chunk and see if it is prime or not; they mark everything out in a bool array that keeps a tab on all the primes.
The threads all turn a specific element in another boolean array ThreadsFinished to true when they finish.
Now the weird part begins:
The threads never all end. If I debug, I find that ThreadNr is not what I assign to it in the loop but another value.
I guess this is normal since the threads execute afterwards and the counter (the variable i) is already increased by then but I cannot understand how to make the code be right.
Can anyone help?
Thank you in advance.
P.S.: I know the algorithm is not very efficient; I am aiming at a solution using the sieve of Eratosthenes also with x given threads. But for now I can't even get this one to work and I haven't found any examples of any implementations of that algorithm anywhere in a language that I can understand.
