Armadillo - fill a matrix from the values in a column vector

Question

I would like to go back and forth between an arma::mat of size M x N and an arma::vec of size MN (which is a column-major linearization of the matrix).

I can easily go from matrix to vector using arma::vectorise, i.e.

arma::vec vector = arma::vectorise(matrix);

However, I cannot find a simple way to go the other way around. I would like to insert the first M values of the vector in the first column of the matrix, the second M values in the second column and so on. Is there a way to do so efficiently?

Answer 1

Make the memory from the matrix to be shared with the vector using advanced constructors:

mat X(4,5);

vec V(X.memptr(), X.n_elem, false, false);

// changing elements in X or V will affect both

As long as your operations don't cause aliasing or change the size of either X or V, the two objects will keep sharing the memory.