[Rcpp-devel] accessing list elements without using names
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
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