java concurrency problem

[Nick7]

Hi ! I want to make a simple math operations on a vector(array) using two cores of my CPU. The program doesn't work correctly. Please explain me how to solve my problem.

public class MyRunnable implements Runnable {
 
    private int startIndex;
    private int endIndex;
    private float[] tab;
 
    public MyRunnable(int startIndex, int endIndex, float[] tab)
    {
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.tab = tab;
    }
 
    @Override
    public void run() 
    {
        for(int i = startIndex; i < endIndex; i++)
        {
            tab[i] = i * 2;
        }
 
        System.out.println("Finished");
    }
}

public class Test {
 
    public static void main(String[] args) {
 
        int size = 10;
        int n_threads = 2;
        float tab[] = new float[size];
 
        for(int i = 0; i < size; i++)
        {
            tab[i] = i;
        }
 
        for(int i = 0; i < size; i++)
        {
            System.out.println(tab[i]);
        }
 
        Runnable r1 = new MyRunnable(0, size / n_threads, tab );
        Runnable r2 = new MyRunnable(size / n_threads, size, tab );
 
        Thread t1 = new Thread(r1);
        Thread t2 = new Thread(r2);
 
        t1.start();
        t2.start();
 
        for(int i = 0; i < size; i++)
        {
            System.out.println(tab[i]);
        }
   }
 
}

[wookyoo]

doesn't work what??

[Norm]

What does your code attempt to do?

[Nick7]

I want to multiply an array using two threads (which should be done using two cores of a C2D CPU).


The program prints, for example: "0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, Finished, 0.0, 2.0, 4.0, 6.0, 8.0, 5.0, 6.0, 7.0, 8.0, 9.0, Finished" or something similar. I except a result like this:
0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, Finished, Finished, 0.0, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, 16.0, 18.0 and that make sense.

Maybe there should be:

t1.start();
t2.start();
t1.join();
t2.join();

[Norm]

What happens when you add the join() calls?
The threads only have to do 10 loops. The first thread could finish before the second thread was started.
If you want overlaps between the two threads, try looping a lot more times.

Add some printlns in the run to show when the threads start.

[Nick7]

Ok, I added println to the run:

public void run() 
    {
        System.out.println(Thread.currentThread());
 
        for(int i = startIndex; i < endIndex; i++)
        {
            tab[i] = i * 2;
        }
 
        System.out.println("Finished");
    }

With join() calls the program prints: 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, Thread[Thread-0,5,main], Finished, Thread[Thread-1,5,main], Finished, 0.0, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, 16.0, 18.0 and that make sense.