[Rcpp-devel] accessing list elements without using names

[Douglas Bates]

Or if you want to show off you could collapse it to

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

On Fri, Feb 10, 2012 at 3:01 PM, Douglas Bates <bates at stat.wisc.edu> wrote:
> 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