Fill random numbers in a 2D array for column/row addition

Question

Hi folks new to Java and am making slow progress. I am trying to populate a 2D array with random numbers before I add the rows and columns. So far I can create the array, display it and I'm pretty sure I have the addition bit sorted. But I'm getting an outofboundsexception when I try to fill it with random numbers. Where am I going wrong?

public static void main(String[] args)
{
    //create the grid
    final int row = 9;
    final int col = 9;
    int [][] grid = new int [row][col];

    //fill the grid
    for (int i=0; i<grid.length; i++)
    grid[i][grid[i].length] = (int)(Math.random()*10);

    //display output
    for(int i=0;i<grid.length; i++)
    {
        for(int j=0; j<grid[i].length; j++)
        System.out.print(grid[i][j]+"");
        System.out.println();
    }

    int sum = 0;
    for (int i = 0; i < grid.length; i++) {
        System.out.println("This row sums up to: " + sum);


        for (int j = 0; j < grid[i].length; j++) {
            sum += grid[j][i];
        }
        System.out.println("This column sums up to: " + sum);
    }
}

Answer 1

grid[i][grid[i].length] = (int)(Math.random()*10);

This will be an out-of-bounds exception. The maximum index of an array a is a.length - 1 (since arrays are 0-indexed) -- you're trying to access an index of a.length. Here a is grid[i].

In any case, if you want to fill the array fully, you'll need two for-loops:

for (int i = 0; i < grid.length; i++) {
    for (int j = 0; j < grid[i].length; j++) {
        grid[i][j] = (int)(Math.random()*10);
    }
}

The outer for-loop loops over all the 1D arrays contained in the 2D array grid, and the inner for-loop fills each one of these inner 1D arrays with random values.

Oh, and one last thing. When you calculate the sum, in the innermost loop, you have sum += grid[j][i]. You likely want i to be the array index and j to be the element index of the array at index i, i.e. grid[i][j].

Also note that if you're not writing to the array (e.g. printing or finding the sum) you can use Java's enhanced for-loop as well:

int sum = 0;

for (int[] row : grid)
    for (int n : row)
        sum += n;

It's slightly less verbose and perhaps more legible.