📊⚡Beginner

Compute Sum of Diagonals of Matrix

Sum diagonal elements

A square matrix has two diagonals: primary diagonal (top-left to bottom-right) and secondary diagonal (top-right to bottom-left).

📊 Diagonal Elements (3×3 Matrix)

123456789

Primary: 1+5+9 | Secondary: 3+5+7

C Program to Find Diagonal Sum

matrix_diagonal_sum.c

1#include <stdio.h>
2
3int main() {
4    int matrix[3][3] = {
5        {1, 2, 3},
6        {4, 5, 6},
7        {7, 8, 9}
8    };
9    int n = 3;
10    int primarySum = 0, secondarySum = 0;
11    
12    for (int i = 0; i < n; i++) {
13        primarySum += matrix[i][i];           // Primary: [0][0], [1][1], [2][2]
14        secondarySum += matrix[i][n - 1 - i]; // Secondary: [0][2], [1][1], [2][0]
15    }
16    
17    printf("Matrix:\n");
18    for (int i = 0; i < n; i++) {
19        for (int j = 0; j < n; j++) {
20            printf("%d ", matrix[i][j]);
21        }
22        printf("\n");
23    }
24    
25    printf("\nPrimary diagonal sum: %d\n", primarySum);
26    printf("Secondary diagonal sum: %d\n", secondarySum);
27    printf("Total diagonal sum: %d\n", primarySum + secondarySum - matrix[n/2][n/2]);
28    
29    return 0;
30}

Output

Matrix:

1 2 3

4 5 6

7 8 9

Primary diagonal sum: 15

Secondary diagonal sum: 15

Total diagonal sum: 25

📖 Code Explanation

Code	Explanation
matrix[i][i]	Primary diagonal: row index = column index
matrix[i][n-1-i]	Secondary diagonal: column = n - 1 - row
- matrix[n/2][n/2]	Subtract center element (counted twice)