indexing of multi-dimensional array (3D and beyond)

[Tor Arne Øigård]

Hi,

I've looked in the documentation and these questions on the Google group about accessing rows and columns in arrays of order 3 and beyond but could not find much on this. Maybe because it is so trivial? :-). As far as I understand arrays of dimension > 2 is not developed in the Eigen library but specifically developed for TMB. I also looked in the matrix_arrays_8cpp-example which has been very useful in many other cases but could not find anything on it there.

Say for example I have a matrix M of dimension I x J, I can access row i by M.row(i).

But what if I have an array A of dimension 3. Say dim of A is I x J x K and I would like  to extract a vector as a function of j given row i and the 3rd dimension k and get a vector as a function of j.

My initial guess was newvector = A.row(i,,k) but that does not seem to work. Have tried several other variations without success.

Kind regards,
Tor

[Kasper Kristensen]

The relevant documentation is

https://kaskr.github.io/adcomp/structarray.html

Due to the memory layout of arrays there's only a column extractor. To get the R equivalent of A[i,,k] one would have to do:

A.col(k).transpose().col(i)

There's some overhead in this construction so it's worth permuting the array if slicing has to be done for many i,j combinations. Use either the 'perm' or 'rotate' method.

---

From: tmb-...@googlegroups.com <tmb-...@googlegroups.com> on behalf of Tor Arne Øigård <torarne...@gmail.com>
Sent: Wednesday, February 7, 2018 3:05:21 PM
To: TMB Users
Subject: [TMB users] indexing of multi-dimensional array (3D and beyond)

 

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[Tor Arne Øigård]

Thanks for this, it worked! Also thanks for the link to the documentation. I saw that by using A.matrix() it keeps the first dimension and collapses the others. I might get away with that somehow and extract a row from the collapsed matrix. Would that be a better solution?