Re: [eigen] sub-matrix extensions

[Jitse Niesen <jitse@xxxxxxxxxxxxxxxxx>]

On Fri, 6 Feb 2009, Gael Guennebaud wrote:

> c) here I have a use case where runtime strides are needed, but do
> you think we also need compile-time strides ? (probably yes)
> d) do you think bidirectional strides might be useful ? eg:
> mat.stride(2,3) (picks the intersections of all even rows and k*3
> columns)

I have some Matlab code that uses A(1:2:end, 1:2:end) = ... which would be 
A.stride(2,2) = ... in your notation. In the end, we got rid of it by 
redefining A to be a permutation of the original A, so it was not 
essential, but it was useful in developing the code / maths.

As for c), I'm not sure what the question is. I guess with compile-time 
strides you mean all the template tricks so that you write A.stride<2,2> . 
In my case, 2 is a compile-time constant (obviously), so this could be 
done. As I understand it, the use of the templates is that you can do 
optimizations at compile time. Is there a possible optimization here? In 
my case, it wouldn't be very useful since we could get rid of the double 
strides (indeed, the fact that double strides are slow in Matlab was one 
of the reasons we re-defined A).

Cheers,
Jitse