[eigen] Using Matrix in conjunction with Matrix?
• To: <eigen@xxxxxxxxxxxxxxxxxxx>
• Subject: [eigen] Using Matrix<AutoDiff> in conjunction with Matrix<double>?
• From: <Daniel.Vollmer@xxxxxx>
• Date: Thu, 31 Mar 2016 15:21:25 +0000
• Accept-language: de-DE, en-US
• Thread-topic: Using Matrix<AutoDiff> in conjunction with Matrix<double>?

```Hi,

I'm working with the (unsupported) AutoDiff module and I was wondering whether it might be worthwhile to support mixing expressions containing Matrix<AutoDiff<T> > with Matrix<T>, e.g.

typedef Eigen::Matrix<double, 3, 1> Vec;

Vec bar(Vec::Ones());
foo += bar; // fail: need to use cast, `bar.cast<AD>()` works
foo = bar * AD(0.5); // fail: error: invalid operands to binary expression ('Vec' (aka 'Matrix<double, 3, 1>') and'AD' (aka 'AutoDiffScalar<Matrix<double, 3, 1> >'))
AD v = foo.dot(bar); // fail: error: implicit instantiation of undefined template Eigen::internal::conj_helper<Eigen::AutoDiffScalar<Eigen::Matrix<double, 3, 1, 0, 3, 1> >, double, false, false>'

I can work around this by using `cast<AD>()`, but that is sub-optimal because the derivatives are then known to be zero (and could thus be implemented more efficiently).

Can I just extend the supported expressions to do this without too much trouble? How would I go about this?

As an additional note, in order to stream Matrices containing AD, I had to do the following:

namespace Eigen {
namespace internal {

// so we can print matrices of AutoDiffScalar
template<typename DerType>
struct significant_decimals_impl<AutoDiffScalar<DerType> > : significant_decimals_impl<typename DerType::Scalar> { };

} // end namespace internal
} // end namespace Eigen

Best regards

Daniel Vollmer

--------------------------
Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)
German Aerospace Center
Institute of Aerodynamics and Flow Technology | Lilienthalplatz 7 | 38108 Braunschweig | Germany

Daniel Vollmer | AS C²A²S²E
www.DLR.de
```