Fill diagonal elements of array with Numbers-CodePudding

I.ve got 2D array amount of raws and columns are the same.

I need to fill diagonal elements like this:

1 0 1
0 1 0
1 0 1

Here is my code:

   private static void fillDiagonal() {
        int[][] arr = new int[3][3];
        //  System.out.println(Arrays.deepToString(arr));
        for (int i = 0; i < arr.length; i  ) {
            for (int j = 0; j < arr[i].length; j  ) {
                arr[i][i] = 1;
                arr[0][2] = 1;
                arr[2][0] = 1;
            }
        }
        System.out.println(Arrays.deepToString(arr));
    }

It's worked, but I need to change it from Hardcode values.

arr[0][2] = 1;
arr[2][0] = 1;

To fill it with loop

How to achieve it?

CodePudding user response：

There is no need for two for loops. You can do this in O(n), as for values on the diagonal y and x will be identical.

To get both diagonals you need to start once on the left using i and once on the right using arr.length - i - 1. As i is increasing you will gradually move from left to right for i and at the same time move from right to left for arr.length - i - 1.

This will work for any n x n matrix.

private static void fillDiagonal() {
    int[][] arr = new int[5][5];
    for (int i = 0; i < arr.length; i  ) {
        arr[i][i] = 1;
        arr[arr.length - i - 1][i] = 1;
    }
    Arrays.stream(arr).map(Arrays::toString).forEach(System.out::println);
}

Expected output:

[1, 0, 0, 0, 1]
[0, 1, 0, 1, 0]
[0, 0, 1, 0, 0]
[0, 1, 0, 1, 0]
[1, 0, 0, 0, 1]

CodePudding user response：

First, notice that you set those two entries for every iteration of the loops, which is bad. Second, you have a double-nested loop where you don't even use j, so that's bad. Think about this: You want to set 3 entries on the main diagonal and 3 entries on the reverse diagonal. So that should just require one or two loops that execute 3 times, not a double-nested loop. Third, you realized that the important indices are [0][0],[1][1], and [2][2], which you can index as [i][i] if you make i go from 0 to 2. Now what are the indices in the reverse diagonal? Write them down, and then create a formula based on i and 3 (or in general n) that will be the correct ones.

CodePudding user response：

private static void fillDiagonal() {
        int[][] arr = new int[3][3];
        //  System.out.println(Arrays.deepToString(arr));
        for (int i = 0; i < arr.length; i  ) {
            for (int j = 0; j < arr[i].length; j  ) {
                if(i == j) arr[i][j] = 1;
                if(i j == arr.length - 1) arr[i][j] = 1;
            }
        }
        System.out.println(Arrays.deepToString(arr));
    }