error in c++ matrix*vector multiplication

Question

I need a function that multiply a matrix and a vector (Matrix*vector)

It takes in a matrix A and a vector B, with int describing the dimensions. Somehow it isn't running correctly. Any help??

void Multiply(double *res, double **A, double *B, int ARows, int ACols, int BRows)
{

    if (ACols !=BRows)

    {

        return;
    }
    
    for (int i = 0; i < ACols; i++)
    {
        res[i] = 0;
        for (int j = 0; j < BRows; j++)
        {
            res[i] += A[i][j]*B[j];
        }
    }
}

It should be Arows in the first for, not Acols. Note that res will have the same number of rows as A, has it been allocated correctly? — dmuir, Jun 14 '21 at 14:04
Perhaps [`std::valarray`](https://en.cppreference.com/w/cpp/numeric/valarray) could help. — Ted Lyngmo, Jun 14 '21 at 14:06
[Row major order](https://en.wikipedia.org/wiki/Row-_and_column-major_order) — Ted Lyngmo, Jun 14 '21 at 14:08

Vlad from Moscow · Accepted Answer · 2021-06-14T14:10:31.090

It seems you mean

for (int i = 0; i < ARows; i++)
{
    res[i] = 0;
    for (int j = 0; j < ACols; j++)
    {
        res[i] += A[i][j]*B[j];
    }
}

It would be better if the function returns a boolean value that signals whether the function execution was successful for example

bool Multiply(double *res, double **A, double *B, int ARows, int ACols, int BRows)
{    
   bool success = ACols == BRows;

    if ( success )
    {
        for (int i = 0; i < ARows; i++)
        {
            res[i] = 0;
            for (int j = 0; j < ACols; j++)
            {
               res[i] += A[i][j]*B[j];
            }
        }
    }

    return success;
}

Instead of the manually written loops you could use the standard algorithm std::inner_product declared in the header <numeric>.

A.k.a `res[i] = std::inner_product(A[i], A[i] + ACols, B, 0);` — Caleth, Jun 14 '21 at 14:08

error in c++ matrix*vector multiplication

1 Answers1