Eigen extracting submatrix from vector of indices

Question

I have been googling for a while now, but cant find the answer to this simple question.

In matlab i can do this:

rows = [1 3 5 9];
A = rand(10);
B = A(rows, : );

How do i do this in eigen? It does not seem like it is possible. The closest thing i have found is

MatrixXd a(10,10);
a.row(1); 

,but I want to get multiple rows/cols. Another user has also asked the question here: How to extract a subvector (of a Eigen::Vector) from a vector of indices in Eigen? , but I think there must some built in way of doing this because it is a really common operation I think.

Thanks.

Answer 1

While this was not possible at the time this question was asked, it has since been added in the development branch!

It's very straight forward:

Eigen::MatrixXf matrix;
Eigen::VectorXi columns;
Eigen::MatrixXf extracted_cols = matrix(Eigen::all, columns);

So I'm guessing this will be in the 3.3.5 3.4 stable release. Until then the development branch is the way to go.

Answer 2

Unfortunately, this is still not directly supported even in Eigen 3.3. There has been this feature request for a while: http://eigen.tuxfamily.org/bz/show_bug.cgi?id=329

Gael linked to an example implementation in one of the comments there: http://eigen.tuxfamily.org/dox-devel/TopicCustomizing_NullaryExpr.html#title1