Question

In eigen c++, how do you
map a vectorXf to a matrixXf
(of appropriate dimensions)

(there is good docs on
how to do it for external objects
so i know we can do:

MatrixXf x_cen=Map<MatrixXf>(*x,*n,*p);

but what if xis a VectorXf?

Answer 1

You can use the .data() member function followed by Map:

VectorXf vec(rows*cols);
vec = ...;
Map<MatrixXf> vec_view_as_a_matrix(vec.data(), rows, cols);

Then you can use vec_view_as_a_matrix just like any Eigen objects, modifications to vec_view_as_a_matrix will be reported to vec as well since they are sharing the memory. If you want to copy to a new MatrixXf object, then use the construction you wrote:

MatrixXf x_cen = Map<MatrixXf>(vec.data(), rows, cols);