Matrices in C: solved main and secondary diagonal exercise

Matrices in C: solved exercise

If you are searching for matrices in C solved exercises, this is one of the best tasks to master indexing in 2D arrays.

Problem statement

Given a 3x3 square matrix, compute:

• main diagonal sum,
• secondary diagonal sum.

C solution

#include <stdio.h>

int main(void) {
    int m[3][3] = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    int main_sum = 0;
    int secondary_sum = 0;

    for (int i = 0; i < 3; i++) {
        main_sum += m[i][i];
        secondary_sum += m[i][2 - i];
    }

    printf("Main diagonal: %d\n", main_sum);
    printf("Secondary diagonal: %d\n", secondary_sum);

    return 0;
}

Expected output

Main diagonal: 15
Secondary diagonal: 15

Recommended edge case

Also test a 1x1 matrix:

{ {7} }
Main diagonal: 7
Secondary diagonal: 7

Common mistakes

• Swapping formulas: main is m[i][i] and secondary is m[i][n - 1 - i].
• Hardcoding 2 - i and breaking the logic for non-3x3 sizes.
• Not resetting accumulators to 0 between test runs.
• Using a non-square matrix and expecting diagonal logic to hold.

Time and space complexity

• Time: O(n) to compute both diagonals in an n x n matrix.
• Extra space: O(1) with only accumulator variables.

Practical use

This indexing pattern appears in table analytics, data transforms, and matrix-based optimization tasks.

Recommended next exercise

• Strings in C: solved exercises with strlen, strcpy, and strcmp
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• Bubble sort in C: solved exercise step by step
• All C exercises

Guided practice and next step

If you want a complete path with progressive difficulty:

• Programming in C in 100 Solved Exercises
• View on Amazon (included in Kindle Unlimited)

FAQ

Is this matrices exercise beginner-friendly?

Yes, this is basic-to-intermediate and excellent for loop and index practice.

Where can I find more solved C exercises?

At Programming in C in 100 Solved Exercises and C Exercises. Kindle Unlimited: View on Amazon.

How should I practice this exercise type to improve faster?

Start with small inputs, run edge cases (empty, one item, max capacity), then rewrite the solution from scratch without copying.