Summary: In this programming example, we will learn to write a C++ program to find the sum of diagonals elements of a matrix (2D array).

A square matrix has two diagonals i.e. left and right diagonal.

The left diagonal elements have equal row and column indexes i.e. i==j and the sum of indexes of the right diagonal elements is one less than the size of the matrix i.e. i+j == size-1.

Using these relations we can easily get the diagonals elements and find their sum.

Steps to find the sum of diagonal elements of a matrix:

1. Create a 2D array.
2. Take inputs in the array.
3. Loop from `i=0` to `i<(size-1) `
4. Add all left diagonal elements (i.e. elements satisfying `i==j`) to `sum_left`.
5. Add all right diagonal elements (i.e. elements satisfying `i+j<size-1`) to `sum_right`.
6. End loop.
7. Output the sum values.

Here is the implementation of the steps in C++:

Output:

In this program, we have explicitly defined the size of the matrix i.e. 2D array using Marco `#define SIZE 5`.

Inside the nested for loop, after the user inputs the array element we check whether it is a diagonal element. If yes, then we add it to the respective sum variable.

In this tutorial, we learned to find the sum of diagonals elements in C++ programming language.