Question

I need to multiply a matrix and a vector.

To accomplish that I have wrote a function with parameters :

float** M The maxtrix of dimensions : m x n.

float* V The vector of length n.

float* R Where I store the result, vector of length m, already allocated.

int m, int n The lengths.

Here is my code :

int i,j;

for (i=0;i<m;i++){
    for (j=0;j<n;j++){
            R[i]+=(M[i][j]*V[j]);
        }
}

The complete function code :

void m_mult_v(float** M, float* V, float* R, int m, int n) {

    int i,j;

    for (i=0;i<m;i++){
        for (j=0;j<n;j++) {
            R[i]+=(M[i][j]*V[j]);
        }
    }
}

The problem is that the result I got is not the right one. :-/ any idea ?

Thanks for your future answers !

EDIT

Solution found thanks for your tips !

I just added this portion of code, to set R to all zeros.

for (i=0;i<m;i++){
    R[i] = 0;
}

Answer 1

You forgot to initialize R? If it’s stack-allocated or allocated with malloc() its initial state is not defined.