[Rcpp-devel] NumericVector

[James Li]

Hi Romain and Dirk,

Sorry I did not see Romain's earlier response. That is very helpful!

Thank you both for your prompt assistance and for developing such a useful
tool.

-James



On Friday, August 16, 2013, Dirk Eddelbuettel wrote:

>
> Hi James,
>
> On 16 August 2013 at 12:57, James Li wrote:
> | Hi Dirk,
> |
> | N could be anywhere between 3-10.
>
> Eek. 10 is a lot.
>
> | Thanks! I will definitely look into how to do those.
> |
> | Also, if
> |
> | Rcpp::NumericVector vec3 =Rcpp::NumericVector( Rcpp::Dimension(4, 5, 6));
> |
> | In this case, how do we access element vec3[1,2,3]?
>
> Well a) you cannot use [] to index, only () as the [] only allows a single
> index (and , is a special operator for C/C++).  Romain already sent you a
> first cut at something homegrown.
>
> Alternatively, if you find a matrix library dealing with N up to 10 ... you
> may want to consider writing glue code to access it from R via Rcpp.
>
> Cheers, Dirk
>
> | Thanks again,
> | James
> |
> | On Friday, August 16, 2013, Dirk Eddelbuettel wrote:
> |
> |
> |     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
> |     | _______________________________________________
> |     | Rcpp-devel mailing list
> |     | Rcpp-devel at lists.r-forge.r-project.org <javascript:;>
> |     |
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
> |
> |     --
> |     Dirk Eddelbuettel | edd at debian.org <javascript:;> |
> http://dirk.eddelbuettel.com
> |
> |
> |
> | --
> | James Li | Ph.D. Candidate | http://jamesyili.com/
> | Dept. of Statistical Science | Cornell University
>
> --
> Dirk Eddelbuettel | edd at debian.org <javascript:;> |
> http://dirk.eddelbuettel.com
>


-- 
James Li *|* Ph.D. Candidate *|* http://jamesyili.com/
Dept. of Statistical Science *|* Cornell University
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