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Why does my C++ code for matrix multiplication give incorrect results when using pointer arithmetic?

What I'm Trying:

I’m trying to implement matrix multiplication in C++, and I’m using pointer arithmetic to access matrix elements. However, my program is producing incorrect results. Here's the code I’ve written so far:

#include <iostream>
#include <vector>

void multiply_matrices(int* A, int* B, int* C, int n) {
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            *(C + i * n + j) = 0; // Clear the element in C
            for (int k = 0; k < n; ++k) {
                *(C + i * n) += *(A + i * n + k) * *(B + k * n + j);
            }
        }
    }
}

int main() {
    int n = 3;
    int A[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    int B[] = {9, 8, 7, 6, 5, 4, 3, 2, 1};
    int C[9] = {0};  // Resultant matrix

    multiply_matrices(A, B, C, n);

    std::cout << "Matrix C (result):\n";
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            std::cout << *(C + i * n + j) << " ";
        }
        std::cout << std::endl;
    }

    return 0;
}

Issue:

I’m using pointer arithmetic to access the elements of the matrices, but the multiplication result is incorrect. Instead of producing the correct matrix product, the result is wrong.

Incorrect Output:

Matrix C (result):
72 0 0 
207 0 0
342 0 0

Expected Output:

The expected output for multiplying the matrices:

Matrix A:
1 2 3
4 5 6
7 8 9

Matrix B:
9 8 7
6 5 4
3 2 1

Should be:

Matrix C (result):
30 24 18
84 69 54
138 114 90

Question:

Can anyone explain what I’m doing wrong and why the multiplication result is incorrect? I suspect there might be an issue with how I’m accessing matrix elements using pointers.

Why does my C++ code for matrix multiplication give incorrect results when using pointer arithmetic?

What I'm Trying:

I’m trying to implement matrix multiplication in C++, and I’m using pointer arithmetic to access matrix elements. However, my program is producing incorrect results. Here's the code I’ve written so far:

#include <iostream>
#include <vector>

void multiply_matrices(int* A, int* B, int* C, int n) {
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            *(C + i * n + j) = 0; // Clear the element in C
            for (int k = 0; k < n; ++k) {
                *(C + i * n) += *(A + i * n + k) * *(B + k * n + j);
            }
        }
    }
}

int main() {
    int n = 3;
    int A[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    int B[] = {9, 8, 7, 6, 5, 4, 3, 2, 1};
    int C[9] = {0};  // Resultant matrix

    multiply_matrices(A, B, C, n);

    std::cout << "Matrix C (result):\n";
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            std::cout << *(C + i * n + j) << " ";
        }
        std::cout << std::endl;
    }

    return 0;
}

Issue:

I’m using pointer arithmetic to access the elements of the matrices, but the multiplication result is incorrect. Instead of producing the correct matrix product, the result is wrong.

Incorrect Output:

Matrix C (result):
72 0 0 
207 0 0
342 0 0

Expected Output:

The expected output for multiplying the matrices:

Matrix A:
1 2 3
4 5 6
7 8 9

Matrix B:
9 8 7
6 5 4
3 2 1

Should be:

Matrix C (result):
30 24 18
84 69 54
138 114 90

Question:

Can anyone explain what I’m doing wrong and why the multiplication result is incorrect? I suspect there might be an issue with how I’m accessing matrix elements using pointers.

• c++
• matrix

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1 Answer 1

Sorted by: Reset to default 5

You have a bug related to the matrix indices calculation when you accumulate the output values.

This line:

*(C + i * n) += *(A + i * n + k) * *(B + k * n + j);

Should be changed to:

//----------vvv-----------------------------------------
*(C + i * n + j) += *(A + i * n + k) * *(B + k * n + j);

I.e. you need to add the second index (j), exactly like you did when you zeroed the output elements before the innermost loop (*(C + i * n + j) = 0;).

Live demo 1

A note about style:
For an array a and index i, *(a+i) is equivalent to a[i], but the latter is considered by many to be more readable (as @catnip also commented below).
Therefore you can change *(C + i * n + j) to C[i * n + j] (and apply a similar change to the other array accesses).
This is demonstrated in the 2nd live demo below:
Live demo 2

A side note:
In order to "catch" such bugs, it is recommended to use a debugger.
See: What is a debugger and how can it help me diagnose problems?.

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