Quadratic form in c

QuadraticForm

for(i = 0; i < n_rows; i++)
 {
    tmp = 0;
    for(j = i + 1; j < n_rows; j++)
    {
        tmp += prA[i*n_rows + j]*prX[j];
    }
    y += prX[i]*2.0*tmp;
    y += prX[i]*prX[i]*prA[i*n_rows + i];
}