Re: [eigen] A vector indexed by another vector

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On 24.08.2012 16:31, Matthieu Brucher wrote:

This has been requested several times, essentially this is directly what
bug 329 wants to have:**show_bug.cgi?id=329<>
There is a prototype implementation, I'm not sure how well it worked/works
with the current Eigen version.

If, however, the offset array is a permutation, i.e. each index occurs
exactly once, you should be able to solve it using:**dox-devel/classEigen_1_**

Thanks for the pointers. Unfortunately, I know that there are indices that
will not be used. We do skip some values depending on a parameter at the
beginning of the process, so perhaps duplicating data would be an
acceptable solution (although a memory- and time-costly one, there are
dozens of arrays that may or may not be needed).

No, duplicating the data definitely is not an option, both for reasons of performance and elegance-of-code.

The cleanest solution, obviously, would be to fix bug 329 ;).
Until you or someone else does so, I would recommend keeping the old code.

If you really need to go for performance, it might be an option to extract all data which is indexed via offset[j] manually into temporaries. That way the remaining expression could even benefit from SSE-vectorization. However, this obviously might be memory-costly.


Dipl.-Inf. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
Enrique-Schmidt-Straße 5
28359 Bremen

Tel: +49 (421) 218-64252

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