使用 C++ 中的指针将两个矩阵相乘

Question

I want to multiply matrices using pointers, which I have seen over the internet. But what I have not seen is how to do that with just two for loops. All solutions I've seen were consisting of three for loops. I have opened another question about this but it got closed due to duplication but my question wasn't answered there.

Maybe I should have given an example of my code:

for(i = 0; i < size; ++i)
    for(j = 0; j < size; ++j)
        {
            *((*PRODUCT)+i) += *((*A)+j) * *((*B)+size*j+i);
        }

This is what I've done. It uses pointers to reach elements of the matrix. But it doesn't work...

Answer 1

Here is a naive matrix multiplication using pointers and two for loops. Note that it does the same amount of work as a typical three loop implementation. It just combines the two outer loops as one and calculates the i and j indexes.

void multiply(const float *A, const float *B, float *result, int N) // square N*N matrixes
{
    // combine two outer loops into one
    for (int n = 0, size = N * N; n < size; ++n) {

        // compute i and j indexes
        int i = n / N;
        int j = n % N;

        // inner loop
        float temp = 0;
        for (int k = 0; k < N; ++k) {
            temp += A[i * N + k] * B[k * N + j];
        }
        result[i * N + j] = temp;
    }
}

int main()
{
    int N = 42;
    float *A = new float[N * N];
    float *B = new float[N * N];
    float *C = new float[N * N];

    // give A and B some values...

    multiply(A, B, C, N);
}

I used array notation because A[i * N + k] is easier to read than *(A + i * N + k)