How to get an Eigen vector from an Eigen matrix column/row?

Question

The thing I want to do is this:

MatrixXd M;
// Whatever assigned to `X`
VectorXd V = M(1);

Eigen does not support this. Is there a way to retrieve an Eigen vector from an Eigen matrix?

Answer 1

Eigen Slicing and Indexing is of much use here.

To retrieve an i -th row vector:

VectorXd V = M(i, all);

To retrieve an i -th column vector:

VectorXd V = M(all, i);