[Rcpp-devel] accessing list elements without using names

src <- "

f <- cxxfunction(signature(L="list"), src, plugin = "Rcpp" )
f(list(matrix(1:9,3,3)))

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!
|
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