Problem of the Day
A new programming or logic puzzle every Mon-Fri

We haven't done too much with threading here at Problemotd so let's change that. Today's objective is to create a program that spawns several random number generators. Each random number generator will generate a random number between 1 and 100 and add it to a shared variable which is initialized at 0. Once that shared variable reaches a prime number greater than 10,000 stop all threads and print out the number.

Good luck!

Comments:

• Anonymous - 10 years, 1 month ago

  C# .Net Solution

  class Program
  {
      private static int randomTotal = 0;
      private static List<Thread> threadList = new List<Thread>();
      private static bool keeprunning = true;
  
      static void Main(string[] args)
      {
          string numThreads = "";
          do
          {
              Console.WriteLine("Please enter the number to threads you wish to run :");
              numThreads = Console.ReadLine();
          }
          while (!Regex.IsMatch(numThreads, @"^\d*$"));
  
          int threads = Convert.ToInt32(numThreads);
  
          Console.WriteLine("Please wait while we find your random prime number > 10,000");
  
          for (int i = 0; i < threads; i++)
          {
              Thread  t = new Thread(new ThreadStart(GetPrime));
              threadList.Add(t);
          }
  
          foreach(var t in threadList)
          {
              t.Start();
              t.Join();
          }
  
          Console.ReadLine();
      }
  
      private static bool IsPrime(int number)
      {
          if (number <= 1)
              return false;
  
          for (int i = 2; i * i <= number; i++)
          {
              if (number % i == 0)
                  return false;
          }
  
          return true;
      }
  
      private static void GetPrime()
      {
          do
          {
              Random rng = new Random();
              int rnd = rng.Next(1, 100);
  
              randomTotal += rnd;
  
              var totalSnapshot = randomTotal;
  
              if (totalSnapshot > 10000 && IsPrime(totalSnapshot) && keeprunning)
              {
                  keeprunning = false;
                  Console.WriteLine(totalSnapshot.ToString());  
              }
  
          }
          while (keeprunning);
      }
  }
  

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• Jason Brady - 10 years, 1 month ago

  Wouldn't this start each thread and wait for it to end sequentially? Doesn't this defeat the purpose of threaded code?

  I think changing this:

    foreach(var t in threadList)
    {
      t.Start();
      t.Join();
    }
  

  to separate loops (or using LINQ) would fix this. It would start all the threads, and then, while they are all running, wait for them to finish.

    threadList.ForEach(t => t.Start());
    threadList.ForEach(t => t.Join());
  

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