Eigen extracting submatrix from vector of indices - c++

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

Answer 3

Ok, say for example you have a 3x3 matrix:
m = [3 -1 1; 2.5 1.5 6; 4 7 1]
and say you want to extract following rows from m matrix:
[0 2], // row 0 and row 2
essentially giving out following matrix:
new_extracted_matrix = [3 -1 1; 4 7 1] // row 0 and row 2 of matrix m
Main thing here is, let's create a vector v having contents [0 2], means we would extract following row indices from matrix m.
Here is what i did:
#include <iostream>
#include <Eigen/Dense>
using namespace Eigen;
using namespace std;
int main()
{
Matrix3f m;
m(0,0) = 3;
m(0,1) = -1;
m(0,2) = 1;
m(1,0) = 2.5;
m(1,1) = m(1,0) + m(0,1);
m(1,2) = 6;
m(2,0) = 4;
m(2,1) = 7;
m(2,2) = 1;
std::cout << "Here is the matrix m:\n" << m << std::endl; // Creating a random 3x3 matrix
VectorXf v(2);
v(0) = 0; // Extracting row 0
v(1) = 2; // Extracting row 2
MatrixXf r(1,v.size());
for (int i=0;i<v.size();i++)
{
r.col(i) << v(i); // Creating indice vector
}
cout << "The extracted row indicies of above matrix: " << endl << r << endl;
MatrixXf N = MatrixXf::Zero(r.size(),m.cols());
for (int z=0;z<r.size();z++)
{
N.row(z) = m.row(r(z));
}
cout << "Extracted rows of given matrix: " << endl << N << endl;
}
This would give us following output:
Here is the matrix m:
[3 -1 1; 2.5 1.5 6; 4 7 1]
The extracted row indicies of above matrix:
[0 2]
Extracted rows of given matrix:
[3 -1 1; 4 7 1]