Re: [eigen] Inverse of an array through .matrix()

[Carlos Becker <carlosbecker@xxxxxxxxx>]

Ok great, so I should point out that .matrix() and .array() can be lvalues and rvalues, and also point out that conversion between matrix and array is not always automatic (or at least remove it, since i was saying this was true).

Any comments on this?

On Sun, Jun 27, 2010 at 2:41 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:

2010/6/27 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:

> On Sun, Jun 27, 2010 at 2:21 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
>> 2010/6/27 Carlos Becker <carlosbecker@xxxxxxxxx>:
>>> Hi everyone again. I am still writing some examples for the tutorials and
>>> came out with the following piece of code:
>>> #include <Eigen/Dense>
>>> #include <iostream>
>>> using namespace Eigen;
>>> using namespace std;
>>> int main()
>>> {
>>>   ArrayXXf m(2,2);
>>>   m << 1,2,3,4;
>>>   // this compiles OK
>>>   MatrixXf ww = m.matrix().inverse();
>>>   // ERROR: no matching function for call to
>>> ‘Eigen::TriangularView<Eigen::Matrix<float, 33331, 33331, 0, 33331, 33331>,
>>> 2u>::solveInPlace(....
>>>   ArrayXXf xx = m.matrix().inverse();
>>
>> That can't work: m.matrix().inverse() wants to return a matrix
>> _expression_, so you can't assign that to an array _expression_. This
>> works:
>>
>>  ArrayXXf xx = m.matrix().inverse().array();
>
>
> on the other hand:
>
> MatrixXf m1, m2;
>  ArrayXXf a = m1 + m2;
>
> is legal and works fine.

Thanks for the heads up, i had forgotten that this was legal. Makes sense!

Benoit

> What is not allowed is to mix matrix and
> array in the right and side (e.g., array+matrix is forbidden). What
> happens here is that .inverse() returns a proxy object which is
> compiled as:
>
> m.matrix().lu().compute_the_inverse_into(xx);
>
> (the function compute_the_inverse_into does not exist, it is just to
> get the idea)
>
> => we are mixing matrix and arrays.
>
> However, I think it is fine that this example does not compile,
> because it does not make sense to do pure linear algebra on arrays.
>
> gael
>
>
>>
>> Notice that eigen3's .matrix() and .array() are very different from
>> eigen2's .cwise(): while .cwise() was just a prefix for the next
>> method call, .matrix() and .array() are permanently switching your
>> _expression_ between the Matrix and Array worlds.
>>
>> Please don't mention .inverse() in the Array page, as
>>  - it is nontrivial linear algebra so should not be mentioned before
>> the linear algebra page
>>  - implementation-wise it is very nontrivial and special, whence the
>> weird error messages that you got.
>>
>> Benoit
>>
>>
>>> }
>>> I just wanted to ask if this is the way it should work or not, since I
>>> supposed that the last line should be fine.
>>> Thanks.
>>
>>
>>
>
>
>