Eigen3 Flatten Matrix as Vector

Question

In MATLAB, I can do the following

A = [1 2 3; 4 5 6];
A(:)

to get:

ans =
1
4
2
5
3
6

How would I do this with an Eigen3 Matrix?

Answer 1

The best way is to use Map:

Map<VectorXd> v(A.data(),A.size());

because in this case Eigen knows at compile time that you now have a 1D vector.

Of course, the result will depend on the storage order of A, that is, for a column major matrix (the default):

[1 4 2 5 3 6]^T

and for a row-major one:

[1 2 3 4 5 6]^T