[Rcpp-devel] accessing list elements without using names

On Fri, Feb 10, 2012 at 3:30 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

On 10 February 2012 at 15:05, Douglas Bates wrote:
| 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

Nice. The as<> cast is a good idea, it may even work with

as<NumericMatrix>

which would avoid the ugly SEXP in user code ...

IIRC it doesn't work because you need to "de-proxy" the returned
element from the list before you can pass it to some instantiation of
"as".

By the way, this use of the as<SEXP> actually has a practical use.  I
have a class C++ glmFamily which corresponds to the R "family" class
for glm and related functions.  It is a good practice to initialize
class data members before the body of the constructor function and it
took me a while to work out

   class glmFamily {
   protected:
       std::string    d_family, d_link; /**< as in the R glm family */
                               //@{ R functions from the family, as a
fall-back
       Rcpp::Function d_devRes, d_linkfun, d_linkinv, d_muEta, d_variance,
d_aic;
                               //@}
       Rcpp::Environment d_rho;
   public:
       glmFamily(Rcpp::List);
...
   };

   glmFamily::glmFamily(List ll)
       : d_family(  as<std::string>(as<SEXP>(ll["family"]))),
         d_link(    as<std::string>(as<SEXP>(ll["link"]))),
         d_devRes(  as<SEXP>(ll["dev.resids"])),
         d_linkfun( as<SEXP>(ll["linkfun"])),
         d_linkinv( as<SEXP>(ll["linkinv"])),
         d_muEta(   as<SEXP>(ll["mu.eta"])),
         d_variance(as<SEXP>(ll["variance"])),
         d_aic(     as<SEXP>(ll["aic"])),
         d_rho(     d_devRes.environment()) {
       if (!ll.inherits("family"))
           throw std::runtime_error("glmFamily requires a list of (S3)
class
\"family\"");
       if (!lnks.count("identity")) initMaps();
   }