Re: [eigen] Custom expression type API stability

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

On Fri, Feb 19, 2016 at 12:07 AM, Dan Čermák <dan.cermak@xxxxxxxxxxxxxxxxxxx> wrote:

Maybe a small follow-up to that, is it possible to append/prepend a value to
(cond_vec>0).select(choice_vec.head(n-1), choice_vec.tail(n-1)) ? I want to
extend this vector of length n-1 to length n, but not create an additional
vector along the way, so not something like this:
new_vec << 1, (cond_vec>0).select(choice_vec.head(n-1), choice_vec..tail(n-1))

Actually its a tad more complicated than that, since I want to take the choice
result, put it into head(n-1) add subtract it from tail(n-1) and add values to
the first and last element (while doing the same again with another
_expression_).
Is there an elegant/efficient way how to do this? Or would it be best to give
the function a reference to a result vector where to write the values into?

such manipulations cannot be efficiently abstracted through coefficient-wise expressions (you would need extra branches within the evaluation of a single coeff), so the best is to write your own function doing the the work inplace.

gael