How to randomly fill two-dimensional array in a loop that ends when array is filled

Question

I've taken this piece of code out of my bigger project, basically I want to pick a random point within the array and remember that it's been picked (by setting its value to 1) and repeat it till the whole array is filled. What's wrong with this code? Why doesn't it fill every single field?

    //initialize
    int arraysizex = 5;
    int arraysizey = 5;
    int x = 0, y = 0;
    int filledcount = 0;
    int[][] array = new int[arraysizex][arraysizey];

    //fill
    for (int i = 0; i < arraysizex * arraysizey; i++) {
        do {
            x = (int) (Math.random() * arraysizex);
            y = (int) (Math.random() * arraysizey);
            if (array[x][y] == 0) {
                array[x][y] = 1;
            //do something
            }
        } while (array[x][y] == 0);
    }

    //display
    for (int i = 0; i < arraysizey; i++) {
        System.out.println();
        for (int j = 0; j < arraysizex; j++) {
            System.out.print(array[j][i]);
            if (array[j][i] == 1) {
                filledcount++;
            }
        }
    }
    System.out.print("\n" + filledcount+"\n");

Joseph Young · Accepted Answer · 2016-01-23 03:29:27Z

1

In the 'worst case', if it picked x = 0, y = 0 every time, your code would apply the operation to only that pixel. It'd then try arraysizex * arraysizey times but skip due to the check for being zero.

Try

   int i = 0;
   while(i < arraysizex * arraysizey) {
        x = (int) (Math.random() * arraysizex);
        y = (int) (Math.random() * arraysizey);
        if (array[x][y] == 0) {
            array[x][y] = 1;
            i++;
            //do something
        }
    }

This guarantees that you did the operation arraysizex * arraysizey times, as it only increments when you do the operation

answered Jan 23, 2016 at 3:29

Joseph Young

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