struct Pt{
 double x;
 double y;
 double z;
};

std::vector<Pt> ptVector;
arma::mat B(numPts,numUnk);
for(size_t i = 0, m = ptVector.size(); i<m ; i++)
{
     // does not work
     // want something similar to B(:,i) = [1 2 3 4];
     B(arma::span::all,i) = { ptVector[i].x*ptVector[i].x , ptVector[i].x*ptVector[i].y , ptVector[i].y*ptVector[i].y , ptVector[i].x*ptVector[i].z , ptVector[i].y*ptVector[i].z , ptVector[i].z*ptVector[i].z , ptVector[i].x , ptVector[i].y , ptVector[i].z};
}