[Rcpp-devel] NumericVector

[Dirk Eddelbuettel]

Hi James,

On 16 August 2013 at 11:59, James Li wrote:
| Dear Dirk and Rcpp-devel members,
| 
| I am currently passing a multidimensional (N > 2) array (i.e.
| array(NA, dim = rep(3,5)) ) from R via Rcpp using


How big is 'N' going to be?

| "in C++:"
| 
| //[[Rcpp::export]]
| Rcpp::List check_arrayC (Rcpp::NumericVector x, Rcpp::IntegerVector modes){
| //do stuff to x
|     return Rcpp::List::create(Rcpp::_["data"] = x, Rcpp::_["modes"] = modes);
| }
| 
| 
| "in R:"
| 
| a <- array(1:32, dim=rep(2,5))
| b <- check_arrayC(a, dim(a))
| 
| While I know that a multidimensional array is stored as a contiguous
| array internally, is there currently a more natural/efficient way to
| pass it back and forth within Rcpp?
| 
| Also from Dirk's book, it seems that an instance of
| Rcpp::NumericVector can be instantiated into a multidimensional array
| via
| 
| Rcpp::NumericVector vec3 =Rcpp::NumericVector( Rcpp::Dimension(4, 5, 6));
| 
| In this case, how do we access element vec3[1,2,3]?
| 
| Some background about what I am trying to do: I would like to create a
| multidimensional array wrapper class around the base R multi-way array
| class. I would also like to be able to pass this multidimensional
| array via Rcpp to do all the heavy-lifting in c++. Ideally, I could
| also convert the mda into a Boost::multi_array.

For a moderately-sized project (at work, not open source) I had a very good
experience using Armadillo 'cubes' (3-d matrices) which I occassionally
stored in 'fields' (which I though of as lists of such cubes).  I think in
most (all?) cases I reduces data to 2-d matrices before returning that
R. That worked great.

Beyond that ... you are on your own as there is very little C++ support
already useable by Rcpp.  You'd have to write custom as<>() and wrap()
methods (which is not hard and may well be worth it).

Cheers, Dirk
 
| Thanks in advance for any help.
| 
| -James
| 
| -- 
| James Li | Ph.D. Candidate | http://jamesyili.com/
| Dept. of Statistical Science | Cornell University
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-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com