[Rcpp-devel] accessing list elements without using names

[Douglas Bates]

Another approach is to force the evaluation of the list element with as<SEXP>

> src <- "
+   List inputList(L) ;
+   NumericMatrix tmpMatrix(as<SEXP>(inputList[0])) ;
+
+   return wrap(tmpMatrix.ncol());
+ "
> f <- cxxfunction(signature(L="list"), src, plugin = "Rcpp" )
> f(list(matrix(1:9,3,3)))
[1] 3

In answer to Sameer's follow-up question of why it doesn't work
directly to form the NumericMatrix from the list element, try reading
the code.  It's complicated.  R is an untyped language with an
underlying generic structure called an SEXPREC whereas C++ is a
strongly typed language.  It is not always possible to map an
amorphous untyped object to a particular data structure without going
through a lot of template magic.  Template meta-programming is not for
the faint of heart.

On Fri, Feb 10, 2012 at 2:10 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
>
> On 10 February 2012 at 14:49, Sameer Soi wrote:
> | Hello Rcpp'ers,
> |
> | I want to access a List of matrices in an Rcpp/C++ function that will passed to
> | me via R...
> |
> | src <- "
> |   using namespace Rcpp ;
> |   List inputList(L) ;
> |   NumericMatrix tmpMatrix ;
> |
> |   tmpMatrix = NumericMatrix(inputList[0]) ;
> |   return(wrap(tmpMatrix.ncol())) ;
> | "
> | test <- cxxfunction(signature(L="list"), src, plugin = "Rcpp" )
> |
> | This gives me the following error-
> |
> | file30b9664194fc.cpp: In function ‘SEXPREC* file30b9664194fc(SEXPREC*)’:
> | file30b9664194fc.cpp:34: error: call of overloaded ‘Matrix
> | (Rcpp::internal::generic_proxy<19>)’ is ambiguous
> | /usr/share/R/include/Rcpp/vector/SubMatrix.h:57: note: candidates are:
> | Rcpp::Matrix<RTYPE>::Matrix(const Rcpp::SubMatrix<RTYPE>&) [with int RTYPE =
> | 14]
> | /usr/share/R/include/Rcpp/vector/Matrix.h:67:
> | note:                 Rcpp::Matrix<RTYPE>::Matrix(const
> | Rcpp::Matrix<RTYPE>&) [with int RTYPE = 14]
> | /usr/share/R/include/Rcpp/vector/Matrix.h:65:
> | note:                 Rcpp::Matrix<RTYPE>::Matrix(const int&)
> | [with int RTYPE = 14]
> | /usr/share/R/include/Rcpp/vector/Matrix.h:46:
> | note:                 Rcpp::Matrix<RTYPE>::Matrix(const
> | Rcpp::Dimension&) [with int RTYPE = 14]
> | /usr/share/R/include/Rcpp/vector/Matrix.h:39:
> | note:                 Rcpp::Matrix<RTYPE>::Matrix(SEXPREC*)
> | [with int RTYPE = 14]
> | make: *** [file30b9664194fc.o] Error 1
> |
> | In looking at the error, it's not clear to me how these candidates will help.
> | What am I missing in telling Rcpp what class of object it should be expecting
> | coming out of inputList?
>
> Template code gets complicate. The subsetting (of the List) confuses here.
> And one more statement and all is well -- you were very close!
>
> R> library(inline)
> R> src <- '
> +   Rcpp::List inputList(L) ;
> +   Rcpp::NumericMatrix tmpMatrix ;
> +   SEXP tmp = inputList[0];
> +   tmpMatrix = Rcpp::NumericMatrix(tmp) ;
> +   return(wrap(tmpMatrix.ncol())) ;
> + '
> R> f <- cxxfunction(signature(L="list"), src, plugin = "Rcpp" )
> R> f(list(matrix(1:9,3,3)))
> [1] 3
> R>
>
>
> Hope this helps,  Dirk
>
> |
> | I know this kind of question (i.e. I want a 3d cube etc) pops up frequently and
> | the answer is typically "use arma's 3d cubes" (e.g. http://stackoverflow.com/
> | questions/8159872/r-list-of-numeric-vectors-c-2d-array-with-rcpp), but I don't
> | intend on doing linear algebra. My data really is just a series of matrices of
> | arbitrary sizes.
> |
> | Also, I looked into multi-dimensional arrays, but how to access them seems
> | unclear; I did see a post on this list to that nature but the solution seemed
> | inelegant.
> |
> | Thanks in advance for any help!
> |
> | ----------------------------------------------------------------------
> | _______________________________________________
> | Rcpp-devel mailing list
> | Rcpp-devel at lists.r-forge.r-project.org
> | https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
> --
> "Outside of a dog, a book is a man's best friend. Inside of a dog, it is too
> dark to read." -- Groucho Marx
> _______________________________________________
> Rcpp-devel mailing list
> Rcpp-devel at lists.r-forge.r-project.org
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel