Re: [eigen] Using Eigen::Map to use eigen operations on my data

[Robert Lupton the Good <rhl@xxxxxxxxxxxxxxxxxxx>]

> >    float data[4] = { 1, 2, 3, 4 };
> >    MatrixXf mat2x2 = MatrixXf(Map<MatrixXf>(data, 2, 2));
> Here you are constructing a new MatrixXf, mat2x2, from the map.
>
> after this line, mat2x2 is just another matrix and doesn't even
> remember that it was copied from the Map.

I realised that that must be the problem, but the documentation  
implied (to me) that it ought to work:

> Map
> Any memory buffer can be mapped as an Eigen expression:
....
> int data[4] = 1, 2, 3, 4;
> MatrixXi mat2x2 = Map<MatrixXi>(data,2,2);

Whereas this is in fact performing a copy.

> If you want to initialize an array as the identity matrix, do:
> Map<MatrixXf>(data, 2, 2).setIdentity();

No, that's just the example.  I'm actually working on a swig interface  
from numpy, based on the one in rikrd-loudia

> if you want to write your own function, make it like this:
> template<typename Derived>
> void identity(MatrixBase<Derived> *x)

Now I'm confused.  I want a MatrixXf that shares memory with data[]  
that I can pass to a pre-existing library.  How does this help?

					R