Re: [eigen] How to resize a partially fixed matrix

[Tim Hutt <tdhutt@xxxxxxxxx>]

> I tried to do that change and now I think that it's very nontrivial:
> not sure anymore that it's a good move!
> Here's what makes me change my mind. Look at what happens when you do:
>
> Matrix<float,3,4> m;
> m.resize(4);
>
> How to interprete this? Should this be a NOP or a failed assertion?
> There are many such issues in Matrix.h alone and I am afraid that any
> way to allow that will result in a much more complex Matrix.h and less
> consistent API....
>
> Benoit

Hmm fair enough. If it really is too hard to change tho API then maybe
good documentation is the best approach. I've made some for you to
copy/paste:

At the end of 'Matrix and vector types' in the tutorial:

-------------
For dynamically sized matrices, where the size of one or both
dimensions is unspecified at compile time use the special value
Eigen::Dynamic. For example, VectorXd is a typedef for

 Matrix<double, Dynamic, 1>

As with statically sized matrices the static dimensions can be any
size. The following are all valid:

Matrix<double, 6, Dynamic> // Dynamic number of columns
Matrix<double, Dynamic, 2> // Dynamic number of rows
Matrix<double, Dynamic, Dynamic> // Fully dynamic
Matrix<double, 13, 3> // Fully static

Partially dynamic matrices use all the same API calls as fully dynamic
matrices, but the fixed dimension must remain constant or an assertion
will fail.

Matrix<double, 2, Dynamic> A; // OK, 2x0 matrix.
Matrix<double, 2, Dynamic> B(3); // Error, wrong constructor.
Matrix<double, 2, Dynamic> C(5, 7); // Error, number of rows is inconsistent.
Matrix<double, 2, Dynamic> D(2, 7); // OK.

The same is true of the resize() function.

A.resize(3); // Error, wrong function.
A.resize(5, 7); // Error, number of rows is inconsistent.
A.resize(2, 7); // OK.
------------

For Matrix::resize(int):
------------
Resizes *this to a vector of length size. This only works for vectors
(Matrix<?, Dynamic, 1> or Matrix<?, 1, Dynamic>). It does not work for
partially dynamic matrices when the static dimension is anything other
than 1. For example it will not work with Matrix<double, 2, Dynamic>.
To resize matrices of this type use resize(int, int).
------------

For Matrix::resize(int, int):
-----------
Makes sense for dynamic- or partially dynamic-size matrices only.

For partially dynamic sized matrices the static dimension must be
(redundantly unfortunately) given. For example:

Matrix<double, Dynamic, 2> A(10, 2); // Create a 10x2 matrix.
A.resize(20, 2); // Expand it to 20x2.
A.resize(10, 3); // This is an error.
----------

For Matrix(int, int):
---------
....default constructor Matrix() instead. For partially dynamic sized
matrices you must pass both the static and dynamic sizes, for example

Matrix<double, Dynamic, 2> A(10, 3); // Error.
Matrix<double, Dynamic, 2> A(10, 2); // OK, Creates a 10x2 matrix.
----------

Even if you change it in future it would be good to add this
documentation. And I've written it all for you so no excuses! :-)

Tim