How to modify a for cycle to Parallel.For? C#

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I created a really simple app to calculate prime numbers and then write them. I want to let it run on all cores, if a user chooses to calculate for example to 1 000 000...

How can I convert my already functioning for cycle to run on all cores? I found out that there is a Parallel.For but I don't really know how to convert to that from my existing for cycle...

    {
        bool isPrime = true;
        int fPrime = Convert.ToInt32(txt_input.Text);
        Application.DoEvents();

        for (int i = 2; i <= fPrime; i++)
        {
            for (int j = 2; j <= fPrime; j++)
            {

                if (i != j && i % j == 0)
                {
                    isPrime = false;
                    break;
                }

            }

            if (isPrime)
            {
                txt_result.Text = txt_result.Text + "..." + i;
            }
            isPrime = true;
        }

        txt_result.Text = txt_result.Text + Environment.NewLine + "Done";
    }

Parallelizing the outer loop should be enough. You are probably not going to run the program in a machine with more processors than outer loops!

Parallel.For(fromInclusive: 2, toExclusive: fPrime + 1, i =>
{
    for (int j = 2; j <= fPrime; j++)
    {

        if (i != j && i % j == 0)
        {
            isPrime = false;
            break;
        }

    }

    if (isPrime)
    {
        if (txt_result.InvokeRequired)
        {
            txt_result.Invoke((MethodInvoker)delegate { txt_result.Text = txt_result.Text + "..." + i; });
        }
        else
        {
            txt_result.Text = txt_result.Text + "..." + i;
        }
    }
    isPrime = true;
});
txt_result.Text = txt_result.Text + Environment.NewLine + "Done";

How to: Write a Simple Parallel.For Loop, NET in which you don't need to cancel the loop, break out of loop iterations, or maintain any thread-local state. Feedback; Edit If you are not familiar with lambda expressions in C# or Visual Basic, see Lambda Stopwatch class to compare the performance of a parallel loop with a non-parallel loop. Parallel For in C# with Examples. In this article, I am going to discuss the static Parallel For in C# with some examples. Please read our previous article before proceeding to this article where we discussed the basics of Parallel Programming in C#. As part of this article, we will discuss the need and use of Parallel For loop comparing with

You will not be able to update the UI using parallel foreach, you could write a method to return all of the primes

        private static List<int> PrimeNumbers(int input)
        {
            var bag = new ConcurrentBag<int>();
            Parallel.ForEach(Enumerable.Range(2, input), x =>
            {
                var isPrime = true;
                Parallel.ForEach(Enumerable.Range(2, input), (y, state) =>
                {
                    if ((x != y) && (x % y) == 0)
                    {
                        isPrime = false;
                        state.Break();
                    }
                });
                if (isPrime)
                    bag.Add(x);
            });

            return bag.AsEnumerable().OrderBy(x => x).ToList();
        }

Parallel.For Method (System.Threading.Tasks), The value of each element is equal to its index. C# Copy. using System; using� For more information, see How to: Write a Parallel.For Loop with Thread-Local Variables. To use a partition-local variable in a ForEach loop, you must call one of the method overloads that takes two type parameters.

Better algorithm usually beats worse (but parallelized) one:

private static IEnumerable<int> Primes(int upTo) {
  if (upTo <= 1)
    yield break;

  yield return 2; // Special case: the only even prime

  List<int> primes = new List<int>() { };

  for (int number = 3; number <= upTo; number += 2) {
    int max = (int)(Math.Sqrt(number) + 0.5);
    bool isPrime = true;

    foreach (var div in primes)
      if (div > max)
        break;
      else if (number % div == 0) {
        isPrime = false;

        break;
      }

    if (isPrime) {
      primes.Add(number);

      yield return number;
    }
  }
}

...

txt_result.Text = string.Join(", ", Primes(1000000));

And you'll have

2, 3, 5, 7, 11, 13, 17, 19, 23, ... 999959, 999961, 999979, 999983

in a fraction of second

How to: Write a Parallel.For Loop with Thread-Local Variables , When you run the example, it rotates each .jpg image in Sample Pictures and saves it to Modified. You can modify the two paths as necessary. C# The Parallel.For version (version 1) finishes about 4 times faster than the sequential version. The computer being used has a quad-core processor. Max: Try increasing the "max" local to 1000 or something. Then run the program and open the Task Manager. And: The CPU (even on a quad-core system) will go to 100% (or 99%) while Parallel.For is running.

How to: Write a simple Parallel.ForEach loop, ForEach loop that uses partition-local variables in . invoked by the loop on each iteration { subtotal += j; //modify local variable return subtotal;� Parallel.For(start,end,delegate); The loop is run from start to end-1 and must run in the forward direction, that is from smaller to bigger index values. There is a range of variations of the Parallel.For comand including parameters to control the parallel iteration and a Parallel.ForEach.

How to: Write a Parallel.ForEach loop with partition-local variables , NET tutorial examines the parallel for loop. loop commands that are parallel versions of the for and foreach looping structures of C#. This change to the ordering of the loop almost always happens when running in parallel� Parallel.For(0, N, New ParallelOptions With {.MaxDegreeOfParallelism = 4}, Function() ' Initialize the local states Return 0 End Function, Function(i, loopState, localState) ' Accumulate the thread-local computations in the loop body Return localState + Compute(i) End Function, Sub(localState) ' Combine all local states Interlocked.Add(result

Parallel For Loop, The Parallel ForEach in C# provides a parallel version of the standard, The parallel version of the loop uses the static ForEach method of the Parallel class. so changing MaxDegreeOfParallelism from the default only limits how many� int counter = 0; Parallel.For(0, 50, i => { int progress = Interlocked.Increment(ref counter); }); If you want to count all the nested iterations, you should remove the count = 0 in the first iteration - or it'll mess with the nested loops count.

Comments
  • Is txt_result a UI component? You can't use UI components (be it WinForms or WPF) from other threads. You should store intermediate results in locals, not fields or statics.
  • You can implement a by far more efficient prime tester (O(sqrt(N)) vs. O(N)) instead of making parallel the inefficient implementation
  • Yes, txt_result is a UI textbox... Yeah I know that I can use a more efficient prime tester, can that version be parallel?
  • the items in bag will not be in order so the right way to update the textbox would be ``` txt_result.Text = string.Join("...",PrimeNumbers(1000)); ```