[Rcpp-devel] List of matrices mimicking 3d array?

[Dirk Eddelbuettel]

On 5 January 2012 at 10:00, Edward Wallace wrote:
| In the end I wrote an obvious miniature sugary hack to convert 3d array indices
| into numeric vector indices
| 
| int threeDIndex(int j, int k, int l, int J, int K, int L) { 
|     return j*K*L + k*L + l;
| }
| 
| so that where one wants to call A[j,k,l], instead one calls A[ threeDIndex

Just to restate something many of us tripped over at one point or other:
A[j,k,l] is NOT a valid C or C++ statement: the comma is an operator, so you
get something like A[l] here.

Multidimensional indexing _must_ use round parens:  A(j,k,l).

And I do think Rcpp support multi-dim arrays.  Whenever I do "real math work"
I tend to use RcppArmadillo though which has vectors, matrices, cubes
(3-dim), as well as fields (similar to lists or matlab cells).

Dirk

| (j,k,l,J,K,L) ], where dim(A) = (J,K,L). Presumably a programmer with more
| experience than me could write a cpp method to do this for NumericVectors with
| dimension attribute?
| 
| Edward
| 
| On Wednesday, January 4, 2012, Dirk Eddelbuettel wrote:
| 
| 
|     On 4 January 2012 at 19:11, Hadley Wickham wrote:
|     | > Our arrays in Rcpp can be multidimensional just like they can in R.
|     Here is a
|     | > line from one of the unit tests:
|     | >
|     | >   return IntegerVector( Dimension( 2, 3, 4) ) ;
|     | >
|     | > which sets the dim attribute of a given vector to c(2,3,4) just like
|     you
|     | > would in R.  That should all work too but may have seen less exposure
|     than
|     | > other parts of our code.
|     |
|     | Am I reading the Vector documentation correctly
|     | (http://dirk.eddelbuettel.com/code/rcpp/html/classVector.html) in
|     | interpreting that while you can create vectors with dimensions, that
|     | there is no syntactic sugar for  multidimensional indexing?  I'm not
| 
|     I think that is correct. We do have Range objects, and that is about it.
|     This is not yet a level comparable to R.
| 
|     The unitTests/ directories, as always, has the largest collections of
|     working
|     idioms to copy from.  runit.Vector.r is probably what you want to look at.
| 
|     | quite sure how to interpret method signatures like operator() (const
|     | size_t &i, const size_t &j) const. I can guess that means you can
|     | index by two integers - but I don't know what those two integers mean.
| 
|     Well if I had to guess I'd say row and column ...
| 
|     Dirk
| 
|     |
|     | Hadley
|     |
|     | --
|     | Assistant Professor / Dobelman Family Junior Chair
|     | Department of Statistics / Rice University
|     | http://had.co.nz/
| 
|     --
|     "Outside of a dog, a book is a man's best friend. Inside of a dog, it is
|     too
|     dark to read." -- Groucho Marx
| 
| 
| 
| --
| Edward Wallace, PhD
| Harvard FAS center for Systems Biology
| +1-773-517-4009

-- 
"Outside of a dog, a book is a man's best friend. Inside of a dog, it is too
dark to read." -- Groucho Marx