Question

I have started a project trying to create a Ken Ken puzzle. If you are not sure what Ken Ken is, it is similar to Sudoku in the way that there can be no duplicated integer values in a row or column.

I am trying to fill a 2D Array with numbers from an Array List that is created for every new row. I make checks to see whether or not the number taken from the Array List does not match any numbers within its own row and column.

When I run my code I get an “Index Out Of Bounds” exception when I try removing the integer value from the list. I’m not sure why this is happening because I think I am getting the right element.

Here is my code:

int GRID_SIZE = 4;
int[][] grid = new int[GRID_SIZE][GRID_SIZE];
List<Integer> nums = new ArrayList<Integer>();

private void populateGrid() {

    for (int row = 0; row < GRID_SIZE; row ++) {

        // Creates an array of values from 1 to grid size.
        for (int i = 1; i <= GRID_SIZE; i++) nums.add(i);

        for (int col = 0; col < GRID_SIZE; col++) {

            while (nums.size() > 0) {

                // Gets a random number from the Array List
                int ranNum = nums.get(numGen.nextInt(GRID_SIZE));

                // Checks to see if the number is placeable.
                if (canPlace(ranNum, row, col)) {

                    // Places the number in the 2D Array
                    grid[row][col] = ranNum;
                    break;

                } else {

                    // Removes duplicate element from the Array List.
                    nums.remove(ranNum); <------{Index Out Of Bounds Exception]
                }
            }
        }
    } 
}

private boolean canPlace(int ranNum, int row, int col) {

    for (int i = 0; i < GRID_SIZE; i++) {

        // Checks if the specified number is already in the row/column.
        if (grid[col][i] == ranNum) return false;
        if (grid[i][row] == ranNum) return false;
    }

    return true;
}

I have a few questions about this:

First of all, why am I getting the error that I am?

Secondly is there anything better to use than a 2D Array for the grid and the way I place my numbers?

Lastly, am I using the break correctly?

Thanks in advance for your answers.

Answer 1

How about a different approach to the problem? Start with a valid square and transform it. The two operations, ‘exchange two rows’ and ‘exchange two columns’ preserve the properties of the square. This allows you to do two Fisher-Yates shuffles, one on the rows and one on the columns which would give you a valid randomised square as long as you start from a valid square. Constructing an initial valid square is trivial:

123456
234561
345612
456123
561234
612345