Java Program to Compute the Sum of Diagonals of a Matrix

In this article, we will understand how to compute the sum of diagonals of a matrix. The matrix has a row and column arrangement of its elements. The principal diagonal is a diagonal in a square matrix that goes from the upper left corner to the lower right corner.

The secondary diagonal is a diagonal of a square matrix that goes from the lower left corner to the upper right corner.

Below is a demonstration of the same −

Suppose our input is −

The input matrix:
4 5 6 7
1 7 3 4
11 12 13 14
23 24 25 50

The desired output would be −

Sum principal diagonal elements: 74
Sum of secondary diagonal elements: 45

Algorithm

Step 1 - START
Step 2 - Declare an integer matrix namely input_matrix
Step 3 - Define the values.
Step 4 - Iterate over each element of the matrix using two for-loops, add the diagonal elements use the condition ‘i’ = ‘j’ to get the sum of principle diagonal elements and the condition ‘i’ + ‘j’ = matrix_size – 1 to get the sum of secondary diagonal elements.
Step 5 - Display the result
Step 5 - Stop

Example 1

Here, we bind all the operations together under the ‘main’ function.

public class MatrixDiagonals {
   static public void main(String[] args) {
      int[][] input_matrix = {
         { 4, 5, 6, 7 },
         { 1, 7, 3, 4 },
         { 11, 12, 13, 14 },
         { 23, 24, 25, 50 }
      };
      int matrix_size = 4;
      System.out.println("The matrix is defined as : ");
      for (int i = 0; i < matrix_size; i++) {
         for (int j = 0; j < matrix_size; j++)
         System.out.print( input_matrix[i][j] + " ");
         System.out.print("\n");
      }
      int principal_diagonal = 0, secondary_diagonal = 0;
      for (int i = 0; i < matrix_size; i++) {
         for (int j = 0; j < matrix_size; j++) {
            if (i == j)
               principal_diagonal += input_matrix[i][j];
            if ((i + j) == (matrix_size - 1))
               secondary_diagonal += input_matrix[i][j];
         }
      }
      System.out.println("\n The sum of principal diagonal elements of the matrix is: " + principal_diagonal);
      System.out.println("\n The sum of secondary diagonal elements of the matrix is: " + secondary_diagonal);
   }
}

Output

The matrix is defined as :
4 5 6 7
1 7 3 4
11 12 13 14
23 24 25 50

The sum of principal diagonal elements of the matrix is: 74

The sum of secondary diagonal elements of the matrix is: 45

Example 2

Here, we encapsulate the operations into functions exhibiting object-oriented programming.

public class MatrixDiagonals {
   static void diagonals_sum(int[][] input_matrix, int matrix_size) {
      int principal_diagonal = 0, secondary_diagonal = 0;
      for (int i = 0; i < matrix_size; i++) {
         for (int j = 0; j < matrix_size; j++) {
            if (i == j)
               principal_diagonal += input_matrix[i][j];
            if ((i + j) == (matrix_size - 1))
               secondary_diagonal += input_matrix[i][j];
         }
      }
      System.out.println("\n The sum of principal diagonal elements of the matrix is: " + principal_diagonal);

      System.out.println("\n The sum of secondary diagonal elements of the matrix is: " + secondary_diagonal);
   }
   static public void main(String[] args) {
      int[][] input_matrix = {
         { 4, 5, 6, 7 },
         { 1, 7, 3, 4 },
         { 11, 12, 13, 14 },
         { 23, 24, 25, 50 }
      };
      int matrix_size = 4;
      System.out.println("The matrix is defined as : ");
      for (int i = 0; i < matrix_size; i++) {
         for (int j = 0; j < matrix_size; j++)
         System.out.print( input_matrix[i][j] + " ");
         System.out.print("\n");
      }
      diagonals_sum(input_matrix, matrix_size);
   }
}

Output

The matrix is defined as:
4 5 6 7
1 7 3 4
11 12 13 14
23 24 25 50

The sum of principal diagonal elements of the matrix is:74

The sum of secondary diagonal elements of the matrix is: 45