Re: [eigen] Custom expression type API stability

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On Thu, Feb 18, 2016 at 1:46 PM, Dan Čermák <dan.cermak@xxxxxxxxxxxxxxxxxxx> wrote:
I need basically two different expressions:

1. given two vectors:
        cond_vec and choice_vec -> result(i) = choice_vec(i) if cond_vec(i) > 0
                                                                        else choice_vec(i+1)
this is rather simple and maybe doable otherwise (and I have already managed
that using the 3.3 development version, although it's probably ugly)

what about:

choice_vec.head(n-1) = (cond_vec>0).select(choice_vec.head(n-1) , choice_vec.tail(n-1));
2. calculate reductions of 3d matrices times 2d matrices (they are not stored
fully, since they can be calculated fairly easily from e.g. a matrix and a
vector, or are highly sparse): result(i) = Sum_j,k M_ijk * K_jk (as a full
reduction) or result(i,j) = Sum_k M_ikj * K_kj
I would like to create some abstraction where I only need to provide a
function which calculates the elements or row/col of the contraction result..

Since M and K are not even Eigen matrices, then you can workaround with a nullary-functor, e.g.:

res = MatrixXd::NullaryExpr(rows,cols,your_fancy_functor(M,K));

where your_fancy_functor has to implement an operator()(Index i, Index j) method.


And of course you can hide the construction of the nullary-_expression_ using a free function, so that you simply has to call:

res = fancy_product(M,K);




On Thursday 18 February 2016 12:10:07 Gael Guennebaud wrote:
> Indeed, writing custom expressions still requires to play with
> Eigen::internal, meaning that it is subject to change in the future. As you
> noticed, the current 3.2 way is different than the future 3.3 way. If you
> tell us a bit more about your need for a custom _expression_ we might be able
> to guide you towards the best approach. Maybe you don't need a new one at
> all, maybe ReturnByValue if good for, etc.
> cheers,
> gael
> On Thu, Feb 18, 2016 at 11:06 AM, Dan Čermák <dan.cermak@xxxxxxxxxxxxxxxxxxx
> > wrote:
> >
> > Hi Folks,
> >
> > I have a question concerning adding custom _expression_ types as described
> > here:
> > . I am
> > currently developing a program that would require some custom _expression_
> > types
> > to be efficient.
> > Is the way how they are implemented expected to change during the next
> > versions? Since I could not compile the example with Eigen 3.2.7 and
> > 3.2.8,
> > only with the latest snapshot from mercurial.
> >
> >
> > Cheers,
> >
> > Dan

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