Eigen - reshape a vector to matrix - c++

Question

I am trying to reshape a vector to a matrix, but getting the following error
unsigned int Nx = 8;
unsigned int Ny = 7;
Eigen::VectorXi G_temp = Eigen::VectorXi::LinSpaced((Nx + 2) * (Ny + 2),0,(Nx + 2) * (Ny + 2)-1);
Eigen::MatrixXd G = Eigen::Map<Eigen::MatrixXd>(G_temp.data(),Nx+2, Ny+2); // error: no matching constructor for initialization of 'Eigen::Map<Eigen::MatrixXd>'
I followed what is written here, but I do not understand the way I am doing wrong.

Answer 1

There is no implicit conversion from integer-valued to double-valued expressions in Eigen. Either just use VectorXd for G_temp (and the LinSpaced expression):
Eigen::VectorXd G_temp = Eigen::VectorXd::LinSpaced((Nx + 2) * (Ny + 2),0,(Nx + 2) * (Ny + 2)-1);
Or use a MatrixXi-Map and .cast<double>() the result before assigning it to G.
Eigen::MatrixXd G = Eigen::Map<Eigen::MatrixXi>(G_temp.data(),Nx+2, Ny+2).cast<double>();
To avoid any temporary, you can also allocate a MatrixXd and directly assign the proper values inside:
Eigen::MatrixXd G(Nx+2, Ny+2); // allocate matrix
// set values in-place:
Eigen::VectorXd::Map(G.data(), (Nx + 2) * (Ny + 2)).setLinSpaced(0,(Nx + 2) * (Ny + 2)-1);
Or with the master/3.4 branch:
G.reshaped().setLinSpaced(0,(Nx + 2) * (Ny + 2)-1);