Submatrices and indices using Eigen

Question

I\'m currently working on a MATLAB project and I\'d like to re-implement the most computational-heavy parts using C++ and Eigen. I\'d like to know if there\'s a way to perform t

Answer 1

The latest development available on master branch of Eigen allows to work with numerical indices.

Here is a similar request, that shows an example of numerical indexing

Answer 2

You can perform operations on selected elements only with select(), which is the equivalent for the ternary ?: operator. This is not exactly what you wanted, but should work in many cases.

MatrixXd B = (A.array() < 3).select(operation_on(A), MatrixXd::Zero(A.rows(), A.cols()));

This will fill B with zeros if A<3 and the result of any required operation on A otherwise.

Answer 3

There currently is a feature request for selecting sub-matrices by indexing filed at the Eigen BugTracker system. Therefore, I doubt it will be possible that way.

The only workaround I could think of is to copy the data manually. Not very nice though.

Answer 4

libigl has many wrappers for Eigen to make it feel more like MATLAB. In particular, there is a slice function so that you can call:

igl::slice(A,indices,B);

which is equivalent to MATLAB's

B = A(indices)