Print Matrix Diagonally OR Diagonal order of Matrix.

Print Matrix Diagonally. OR
Loop diagonally through two dimensional array. OR
Traverse an array diagonally. OR
Print elements of Matrix in Diagonal order.

Given a Matrix / 2D array, Print all elements of Matrix in Diagonal order. Lets understand Diagonal order of Matrix graphically.

Algorithm

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Lets see algorithm for printing Matrix diagonally.

If we observe the Matrix, there are 2 set of Diagonals,

rowCount diagonals
(colCount - 1) diagonals.

We will first print the rowCount diagonals and then print the remaining columnCount-1 diagonals.

For printing rowCount diagonals:

Iterate "r" from 0 till rowCount
For each row, initialize "row=r" and "col=0" iterate till (row >=0 && col<collength )
Print matrix[row][col]
Decrement row-- and Increment col++

For printing (colCount-1) diagonals:

Iterate "c" from 1 till colCount
For each row, initialize "row=rowCount" and "col=c" iterate till (row >=0 && col < collength)
Print matrix[row][col]
Decrement row-- and Increment col++

Java Program to Print Matrix Diagonally.

package javabypatel.matrix;
 
/*
 Input:
   {1,  2,  3,  4},
   {5,  6,  7,  8},
   {9,  10, 11, 12},
   {13, 14, 15, 16},
   {17, 18, 19, 20},
    
Output:
   1
   5   2
   9   6   3
   13  10  7  4 
   17  14  11 8
   18  15  12
   19  16
   20  
 */
 
public class PrintMatrixDiagonally {
 
 public static void main(String[] args) {
  new PrintMatrixDiagonally();
 }
 
 public PrintMatrixDiagonally() {
  int[][] matrix = {
   {1,  2,  3,  4},
   {5,  6,  7,  8},
   {9,  10, 11, 12},
   {13, 14, 15, 16},
   {17, 18, 19, 20}
  };
   
  printMatrixDiagonally(matrix);
 }
 
 private void printMatrixDiagonally(int[][] matrix){
   
  int rowCount = matrix.length;
  int columnCount = matrix[0].length;
  
  for (int r = 0; r < rowCount; r++) {
   for (int row = r, col = 0; row >= 0 && col < columnCount; row--, col++) {
    System.out.print(matrix[row][col] + " "); 
   }
   System.out.println();
  }
   
  for (int c = 1; c < columnCount; c++) {
   for (int row = rowCount-1, col = c; row >= 0 && col < columnCount; row--, col++) {
    System.out.print(matrix[row][col] + " "); 
   }
   System.out.println();
  }
 
 }
  
}

Another way of looping

package javabypatel.matrix;
 
public class PrintMatrixDiagonally {
 
    public static void main(String[] args) {
        int[][] matrix = {
            {1,  2,  3,  4},
            {5,  6,  7,  8},
            {9,  10, 11, 12},
            {13, 14, 15, 16},
            {17, 18, 19, 20}
        };
 
        printMatrixDiagonally(matrix);
    }
 
    private static void printMatrixDiagonally(int[][] matrix) {
 
        for (int i = 0 ; i < matrix.length ; i++) {
            int row = i;
            int col = 0;
 
            while (row >= 0 && col < matrix[0].length) {
                System.out.print(matrix[row][col] + " ");
                col++;
                row--;
            }
            System.out.println();
        }
 
        for (int i = 1 ; i < matrix[0].length ; i++) {
            int row = matrix.length-1;
            int col = i;
 
            while (row >= 0 && col < matrix[0].length) {
                System.out.print(matrix[row][col] + " ");
                col++;
                row--;
            }
            System.out.println();
        }
    }
 
}