Adding up the elements of an array via multithreading

Question

Before I go any further, this IS apart of my homework.The part that I am having trouble with however, is not the main point of the assignment.

For the assignment we are just storing numbers in an array, and am adding up the elements of the array via multithreading.

The user enters how many threads that they would like to run, and what the upper bound should be.

For example: Upper Bound: 12 Threads: 2 The application should add up elements 1-6, then 7-12. In this case the lower bound starts out at 1 and the upper bound starts out at 6. Then the second time the loop should iterate the upper bound should be 7 and the upper bound should be 12.

I am having trouble trying to divide the upper bound by the number of threads to create the increments in which the lower and upper bounds are based off of.

It is fairly simple if the number of threads divides evenly into the starting upper bound. But when It doesn't is when I am having the problem.

If the upper bound isn't evenly divided by the number of threads you should just leave out the last element and add it at the end when you add the other sub totals. — evanmcdonnal
– evanmcdonnal, Commented Feb 24, 2012 at 2:49
Can you include a fragment of the code? The portion on upper bound and lower bound is pretty vague. — Deyang
– Deyang, Commented Feb 24, 2012 at 2:53

paulbovbel · Accepted Answer · 2012-02-24 02:50:33Z

3

Bound: 18, Threads: 7

Use division to figure out the nominal amount of elements per thread: Ops per thread = 18 / 7

Use the modulo operator to figure out the number of threads that should get one extra element (for the remainder): Threads with one extra = 18 % 7

answered Feb 24, 2012 at 2:50

paulbovbel

901 silver badge6 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

NightWolf · Accepted Answer · 2012-02-24 04:19:38Z

This might be a bit outside the box of what your assignment wants you to do with the bounds (not sure what you mean here). But this is a very basic example of how you could solve this with a fixed thread pool. Saves you having to manage the threads yourself. You # threads would be the size of the fixedThreadPool. Then you could just create 'jobs' which add two numbers. Here you just add the last number if you have an odd upper bound.

    private static ExecutorService tpool = Executors.newFixedThreadPool(20);
    private static final int upper = 140;

    private static AtomicInteger total = new AtomicInteger(0);

public static void main(String[] args) throws Exception {

    int ar[] = new int[upper];
    for(int i = 1 ; i <= upper; i++){
        ar[i-1]=i;
    }

    for(int i = 1 ; i <= ((upper%2) !=0 ? (upper-1):(upper)); i+=2){
        final int a = ar[i-1];
        final int b = ar[i];
            Thread thread = new Thread(new Runnable(){
                public void run() {
                    int res = add(a, b);
                    total.addAndGet(res);
                    return;
            }});
            tpool.execute(thread);
    }
    if(upper%2!=0)
        total.addAndGet(ar[ar.length-1]);

    tpool.shutdown(); //wait for everything to finish
    System.out.println(total.get()); //get the result.
}

private static int add(final int a, final int b){
    return a+b;
}

Arguably this is less efficient if you had two threads reading off chunks of the array as you would tell thread 1 to add 1-6 and thread 2 to add 7-12. But it essentially does the same thing and this approach would be well suited to a more computationally intensive task.

Collectives™ on Stack Overflow

Adding up the elements of an array via multithreading

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related