How to print all the diagonals of a matrix in Java

Question

I'm trying to figure out how to print an NxN matrix diagonally. Right now I can print the left to right diagonal but not the right to left. So let's say the matrix is:

1 2 3 
4 5 6
7 8 9

Now I can print left to right:

But I want also to print right to left:

Here is the code for left to right:

public static void printLeftToRightDiagonal(int[][] matrix) {
    int length = matrix.length;
    int diagonalLines = (length + length) - 1;
    int itemsInDiagonal = 0;
    int midPoint = (diagonalLines / 2) + 1;

    for (int i = 1; i <= diagonalLines; i++) {
        int rowIndex;
        int columnIndex;

        if (i <= midPoint) {
            itemsInDiagonal++;
            for (int j = 0; j < itemsInDiagonal; j++) {
                rowIndex = (i - j) - 1;
                columnIndex = j;
                System.out.print(matrix[rowIndex][columnIndex] + " ");
            }
        } else {
            itemsInDiagonal--;
            for (int j = 0; j < itemsInDiagonal; j++) {
                rowIndex = (length - 1) - j;
                columnIndex = (i - length) + j;
                System.out.print(matrix[rowIndex][columnIndex] + " ");
            }
        }
        System.out.println();
    }
}

I tried to figure out the pattern but I was left without ideas

user_11713777 · Answer

The problem can be divided into two tasks:

print one reverse diagonal starting from a generic matrix[i, j] element, basically decrementing i and j by 1 in every cycle if both i and j are >= 0, otherwise ending the cycle.
identifying the starting matrix[i, j] elements and print the reverse diagonal starting from them.

The first rule is equivalent to write a method like below:

//example for matrix[2, 2] it will print 9 5 1
private static void printReverseDiagonal(int[][] matrix, int i, int j) {
    System.out.print(matrix[i][j]);
    for (int row = i - 1, column = j - 1; row >= 0 && column >= 0; --row, --column) {
        System.out.print(" " + matrix[row][column]);
    }
    System.out.println();
}

The second rule is equivalent to write one cycle iterating over elements matrix[0, n - 1], matrix[1, n - 1] ... matrix[n - 1, n - 1] and another one iterating over elements matrix[n - 2, n - 1], matrix[n - 3, n - 1] .... matrix[0, n - 1] like below:

public static void printRightToLeftDiagonal(int[][] matrix) {
    int n = matrix.length;

    int j = n - 1;
    for (int i = 0; i < n; ++i) {
        printReverseDiagonal(matrix, i, j);
    }
        
    int i = n - 1;
    for(j = n - 2; j >= 0; --j) {
       printReverseDiagonal(matrix, i, j);
    }           
}

Combining the two functions you obtain your expected result.

How to print all the diagonals of a matrix in Java

1 Answers