Divide for loop between tasks

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I want to divide for loop between Tasks.

Sample code that should describe my idea:

C#

for(int i = 0; i < array.Length; i++) 
{
   array[i].Function(); //void method
}

And I can't solve two issues:
1. How to divide it knowing that depending on chunk count, chunk size can be even or odd.
2. How to declare Tasks knowing the above.

What I have tried:

I tried a bunch of things but none of it actually worked as expected. Someone definitely knows how to handle it, and I would really appreciate the help. Already got banned on stackoverflow for asking this, but maybe here people are not afraid of answering questions.

Posted 16-Jan-21 3:27am

Member 15047625

Updated 16-Jan-21 4:17am

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Comments

Richard MacCutchan 16-Jan-21 9:32am

What exactly are you trying to achieve?

Member 15047625 16-Jan-21 9:34am

That void method does some complicated things. For example if array.Length is 10, i can divide it to 2 Tasks. First task will handle i 0 - 4, second 5 - 9. And it will be faster than sequence.
I have a feeling it should be easy, but for some reason i can't even divide it properly. I want to create tasks dynamically depending on size, and not declare X manually because it's terrible solution.

Richard MacCutchan 16-Jan-21 9:42am

It sounds like you need to calculate the size of each sub-array. So for array of size 10, you just get two 5 element chunks. You then create a task for each chunk and pass it the aray, the start offset of the chunk and its length. Something like:

int chunkSize = array.Length / 2;
Task first = new Task();
first.start(array, 0, chunkSize);
Task second = new Task();
second.start(array, chunkSize, chunkSize);

That is an over simplification but should give you the idea.

Member 15047625 16-Jan-21 9:46am

I know, i have manually made two Tasks already, but what if array.Length is 100 000, or 100 000 000. It begs to use more Tasks, but I'm not sure how to make it dynamically.

Richard MacCutchan 16-Jan-21 9:59am

You need to decide some maximum number of tasks and adjust the count up or down between 1 and the maximum based on the number of elements to process.

2 solutions

Solution 1

Take a look at: Parallel.For Method (System.Threading.Tasks) | Microsoft Docs[^]

And: Batch.Parallel[^]

Posted 16-Jan-21 3:50am

RickZeeland

Updated 16-Jan-21 3:56am

v2

Comments

Member 15047625 16-Jan-21 9:53am

Parallel.For is somewhat equal to 1 Task per one iteration, and I had an idea to divide it to chunks.

RickZeeland 16-Jan-21 9:57am

Updated the solution with a "chunky" alternative :)

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OriginalGriff · Accepted Answer · 2021-01-16T04:17:00

Set up a function which accepts a start index and a items count, and use it to run a for loop.
Then call that function inside a thread, and start several threads to complete the task.
Something like this:

C#

    int[] myArray = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    int chunkSize = 5;
    for (int startIndex = 0; startIndex < myArray.Length; startIndex += chunkSize)
        {
        int sIndex = startIndex;
        System.Threading.Thread t = new System.Threading.Thread(() => DoChunk(myArray, sIndex, chunkSize));
        t.Start();
        }

public void DoChunk(int[] arr, int i, int s)
    {
    Console.WriteLine($"{i}:{s}");
    while (i < arr.Length && s-- > 0)
        {
        arr[i++] += 100;
        }
    }

You do need the temporary variable inside the thread creating loop, or you won't get the results you expect!

Divide for loop between tasks

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Divide for loop between tasks

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