Java Multithreading Array Separating

Question

I need to separate an array considering count of threads. For example i have the array [1][2][3][4][5][6][7][8][9][10] and user print count of thread which will be work with elements in this array. My task is distribute the work between arrays' elements and threads.

Count of threads = 2 -> thread1:[1][2][3][4][5] thread2:[6][7][8][9][10]. But what i need to do if threads will be for instance 7. Ho to separate work in this case?

[1][2][3][4][5][6][7][8][9][10] is a 10-dimensional array, not a single array of 10 elements. {1,2,3,4,5,6,7,8,9,10} is a single array with 10 elements. — Andreas
– Andreas, Commented May 3, 2016 at 20:42
How about using an ExecutorService with a Threadpool that have the required number of threads? Like ExecutorService pool = Executors.newFixedThreadPool(poolSize). Then, you can submit the tasks by iterating though the array and submitting each element as a parameter to a runnable task. Take a look at docs.oracle.com/javase/7/docs/api/java/util/concurrent/… — mdewit
– mdewit, Commented May 3, 2016 at 20:59

Iłya Bursov · Accepted Answer · 2016-05-03 20:57:42Z

2

here is sample application which splits input array to threads:

final int threadCount = 7;
final int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

int from = 0;
for (int i = 0; i < threadCount; i++) {
    final int to = (from + (int) Math.ceil((a.length - from) / (double) (threadCount - i)));
    System.out.println("Thread #" + i + " will use " + Arrays.toString(Arrays.copyOfRange(a, from, to)));

    from = to;
}

actually (thanks to @Andreas), instead of ceiling, you can use simply integer math:

final int to = from + ((a.length - from) / (threadCount - i));

output:

Thread #0 will use [1, 2]
Thread #1 will use [3, 4]
Thread #2 will use [5, 6]
Thread #3 will use [7]
Thread #4 will use [8]
Thread #5 will use [9]
Thread #6 will use [10]

edited May 3, 2016 at 20:57

answered May 3, 2016 at 20:39

Iłya Bursov

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6 Comments

Andreas Over a year ago

I like Math.round() better. ;-)

Iłya Bursov Over a year ago

@Andreas math.round will not work, for example 11 elements and 10 threads

Andreas Over a year ago

It works. Try it. You can even do Math.floor(). Why should the first threads get all the fun?

Iłya Bursov Over a year ago

@Andreas oh, you're right, you can do floor or round

Andreas Over a year ago

You can see the difference between ceil(), round(), and floor() in this IDEONE demo.

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