CODEWITHSUNDEEP
c#.netmultithreading
Clock
Clock

Reputation: 984

Control execution in a Parallel.For

In this parallel execution:

         Parallel.For(0, 100, i =>
         {
             Console.WriteLine(i);
         });

I am trying to get the same values of the below usual loop, so the "i" is always gave in an ascendent order, 0, 1, 2, 3, ... , 100

        for(int i=0; i <=100; i++)
        {
            Console.WriteLine(i);
        }

Is this possible, how could I control this parallel for execution regarding the order of the index ?

Upvotes: 1

Views: 407

Answers (3)

willaien
willaien

Reputation: 2797

Once threading is involved, the order of execution is no longer guaranteed. You can technically get the results you're asking for by synchronizing everything, like so:

int actualIndex = 0;
var lockObject = new object();
Parallel.For(0, 100, i =>
{
    lock(lockObject) Console.WriteLine(actualIndex++); 
});

But, doing so simply adds the overhead of parallelism, then ensures that only one thread is doing something at a time - in other words, it's much worse than simply doing a regular for loop!

If you need things to happen serially, the best you can do is either serialize via synchronization like lock, or... simply do it serially.

What are you trying to accomplish, exactly?

Upvotes: 1

csharpfolk
csharpfolk

Reputation: 4280

If you want only results in ascending order, then this may work for you:

var results = new Dictionary<int>();
object lock = new object();
Parallel.For(0, 100, i =>
         {
             // here code runs in parallel
             lock(lock) { 
                // here code runs sequentially
                results[i]; 
             }
         });

// results from 0..100
var r = results.OrderBy(x => x.Key).Select(x => x.Value).ToList();

lock creates what is known as critical section - inside lock block of code may be only one thread at time, other threads must wait.

Parallel programming is a huge and complicated topic, here is a best course for C# programmers I ever read http://www.albahari.com/threading/part5.aspx

Upvotes: 0

usr
usr

Reputation: 171178

If you want that the execution of i starts after i - 1 is done then there is no parallelism left and using a parallel loop is pointless.

A clever answer would be to build a string that has the 101 numbers separated by newlines and then print it. But that does not help you with your real use case I assume.

Upvotes: 1

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