Re: [eigen] A vector indexed by another vector

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 24.08.2012 16:04, Matthieu Brucher wrote:
> Hi,
>
> I'd like to replace some of my code that consits of a indexed for loop by a
> pure Eigen statement.
>
> In fact, I have several of these loops, but the most simple one is the
> following:
>
>    Vector vector;
>    vector.resize(size);
>
>    for(long j = 0; j < size; ++j)
>    {
>      long i = offset[j];
>      float tmp = t02 + h2[i] * val[0];
>      vector[j] =  tmp > 0.0f ? std::sqrt(tmp) + tsdiff[i] : -1000.f;
>    }
>
> The issue is that h2 is not accessed in order, but with values from the
> offset vector. Is there a way to efficiently write this kind of loop with
> Eigen?

This has been requested several times, essentially this is directly what 
bug 329 wants to have:
http://eigen.tuxfamily.org/bz/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:
http://eigen.tuxfamily.org/dox-devel/classEigen_1_1PermutationMatrix.html


Christoph


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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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