CODEWITHSUNDEEP
c#multithreading
Mockingbird
Mockingbird

Reputation: 1031

Use array of threads

I'm new to threads so it might be an easy one for you, but I've spent some hours trying to figure it out.

Let's say I have a function

public double Gain(List<int> lRelevantObsIndex, ushort uRelevantAttribute)

which needs some time to finish, but is a read only func.

I have an array of ushort[] values, and I want to get the ushort value that achieves the minimum value of the Gain function.

Here is what I've got so far, but it's not working:

lRelevantObsIndex is a read only index.

lRelevantAttributes is the list of ushort values.

        //Initialize the threads
        double[] aGains = new double[lRelevantAttributes.Count];
        Thread[] aThreads = new Thread[lRelevantAttributes.Count];
        for (int i = 0; i < lRelevantAttributes.Count; i++)
        {
            aThreads[i] = new Thread(() => aGains[i] = Gain(lRelevantObsIndex, lRelevantAttributes[i]));
            aThreads[i].Start();
        }

        //Join the threads
        for (int i = 0; i < lRelevantAttributes.Count; i++)
            aThreads[i].Join();

        //The easy part - find the minimum once all threads are done
        ushort uResult = 0;
        double dMinGain = UInt16.MaxValue;
        for (int i = 0; i < lRelevantAttributes.Count; i++)
        {
            if (aGains[i] < dMinGain)
            {
                dMinGain = aGains[i];
                uResult = lRelevantAttributes[i];
            }
        }

        return uResult;

I know this is a simple multithreading question - but still need your brains since I'm new to this.

Upvotes: 1

Views: 113

Answers (3)

GSerjo
GSerjo

Reputation: 4778

You can do the same on Task

    [Fact]
    public void Test()
    {
        List<Task<int>> tasks = Enumerable.Range(0, 5) //- it's equivalent how many threads
                                          .Select(x => Task.Run(() => DoWork(x)))
                                          .ToList();
        int[] result = Task.WhenAll(tasks).Result; //- Join threads

        result.ToList().ForEach(Console.WriteLine);
    }

    private int DoWork(int taskId)
    {
        return taskId;
    }

Result output:

3
0
1
2
4

Upvotes: 1

TypeIA
TypeIA

Reputation: 17258

I'm not sure if this is your problem, but it is a problem:

for (int i = 0; i < lRelevantAttributes.Count; i++)
{
    aThreads[i] = new Thread(() => aGains[i] = Gain(lRelevantObsIndex, lRelevantAttributes[i]));
    aThreads[i].Start();
}

When a lambda refers to a loop variable, the binding is delayed, so that when your lambda actually runs, it takes the value of i at the time the lambda runs, not the value it had when the lambda was created. To fix this, declare a secondary variable inside the loop, and use that in the lambda:

for (int i = 0; i < lRelevantAttributes.Count; i++)
{
    int j = i;
    aThreads[i] = new Thread(() => aGains[j] = Gain(lRelevantObsIndex, lRelevantAttributes[j]));
    aThreads[i].Start();
}

Upvotes: 2

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 727097

This one is somewhat tricky: your for loop uses a modified value here (a so-called access to modified closure)

for (int i = 0; i < lRelevantAttributes.Count; i++)
{
    aThreads[i] = new Thread(() => aGains[i] = Gain(lRelevantObsIndex, lRelevantAttributes[i]));
    aThreads[i].Start();
}

At the time the thread starts, i will be different in your lambda, accessing a wrong item. Modify your loop as follows:

for (int ii = 0; ii < lRelevantAttributes.Count; ii++)
{
    var i = ii; // Now i is a temporary inside the loop, so its value will be captured instead
    aThreads[i] = new Thread(() => aGains[i] = Gain(lRelevantObsIndex, lRelevantAttributes[i]));
    aThreads[i].Start();
}

This will fix the problem, because lambdas will capture the current value of the temporary variable i on each iteration of the loop.

Upvotes: 4

Related Questions