Parallel Programming Assignment in java

Question

I am busy on a parallel programming assignment, and I am really stuck. To be honest I am not entirely sure how each method works, but I think I have an idea.

I need to sum an array of consecutive values (in parallel). Seems easy enough, but I get 0 as an answer every time I try. I really don't know why.

class SumThreaded extends RecursiveTask<Integer> {
    static int SEQUENTIAL_THRESHOLD = 10000;
    double lo=0.0;
    double hi=0.0;

    long[] arr;

    public SumThreaded(long[] array, double a, double b) {
        arr=array;
        lo=a;
        hi=b;


    }

    public Integer compute() {

        //System.out.println(mid);
        if(hi - lo <= SEQUENTIAL_THRESHOLD) {
            int ans = 0;
            for(int i= (int) lo; i < hi; ++i)
                ans += arr[i];
            return ans;
        }
        else {
            SumThreaded left = new SumThreaded(arr,lo,(hi+lo)/2.0);
            SumThreaded right = new SumThreaded(arr,(hi+lo)/2.0,hi);
            left.fork();
            int rightAns = right.compute();
            int leftAns = left.join();
            return leftAns+rightAns;

        }
    }


    public static void main(String args[]){

        int size = 1000000;
        long [] testArray=new long[size]; 

        for(int i=0;i<size;i++){
            testArray[i]=i+1;
        }

        SumThreaded t = new SumThreaded(testArray,0.0,testArray.length); 
        ForkJoinPool fjPool = new ForkJoinPool();
        int result =fjPool.invoke(t);
        System.out.println(result);

    }
}

Any help would be greatly appreciated.

To start with, properly formatting your code makes it much easier to read and debug. — chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-, Commented Sep 16, 2013 at 1:53
This sure seems like a whole lotta code for what should take just a few lines. Try separating out your concerns so that each class does the minimum it can. — Bohemian
– Bohemian ♦, Commented Sep 16, 2013 at 2:11

chrylis -cautiouslyoptimistic- · Accepted Answer · 2013-09-16 02:01:58Z

1

Your problem appears to be that you have two separate constructors for SumThreaded, only one of which sets the class's fields. When you're feeding in the long[] array from the new in sumArray, you throw the array away. You need to pick whether you're using ints or longs (and the sum of a big array is likely to need a long) and then make sure your values are getting set appropriately. Debugging and setting a breakpoint on compute would have shown you this.

edited Sep 16, 2013 at 2:01

answered Sep 16, 2013 at 1:56

chrylis -cautiouslyoptimistic-

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9 Comments

Seeker Over a year ago

I'm very sorry about that, I completely missed that extra contructor ( it's pretty late :P). I edited the OP, but I am getting a ClassCastException.

chrylis -cautiouslyoptimistic- Over a year ago

Which ClassCastException? It prints out plenty of detail that usually tells you exactly where and what the problem is.

Seeker Over a year ago

Exception in thread "main" java.lang.ClassCastException:

java.lang.Thread cannot be cast to java.util.concurrent.ForkJoinWorkerThread 	 at java.util.concurrent.ForkJoinTask.fork(Unknown Source) 	at SumThreaded.compute(SumThreaded.java:24) 	at Main.sumArray(SumThreaded.java:36) 	at Main.main(SumThreaded.java:49)

code` I am really not familiar with the fork/join framework :/

Theodore Norvell Over a year ago

See stackoverflow.com/questions/13903429/…

Seeker Over a year ago

Thanks Theodore, I edited the OP again, but now its a logical error ( the answer is wrong)

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