[Rcpp-devel] Returning a list of matrices: Interesting difference: Sometimes need wrap(); sometimes not

[Søren Højsgaard]

Dear list,

Given a matrix X, I want to return a list containing X and its inverse: This works:

library(inline)
code <- '
arma::mat X    = as<arma::mat>(X_);
arma::mat Xinv = inv( X );
return Rcpp::List::create(Rcpp::Named("X", X),Rcpp::Named("Xinv", Xinv));'
sol <- cxxfunction(signature(X_ = "matrix"), code, plugin = "RcppArmadillo", verbose=F)
M <- matrix(c(1,2,3,4),nr=2)

> sol(M)
$X
     [,1] [,2]
[1,]    1    3
[2,]    2    4

$Xinv
     [,1] [,2]
[1,]   -2  1.5
[2,]    1 -0.5

Now I create a corresponding function (same content) and compile:

#include <Rcpp.h>
#include <RcppArmadillo.h>

RcppExport SEXP sol1 ( SEXP X_ ){
  using namespace Rcpp;
  arma::mat X    = as<arma::mat>(X_);
  arma::mat Xinv = inv( X );
  return Rcpp::List::create(Rcpp::Named("X", X),Rcpp::Named("Xinv", Xinv));
}

> dyn.load("sol.dll")
> .Call("sol1", M)
$X
[1] 1 2 3 4

$Xinv
[1] -2.0  1.0  1.5 -0.5

> dyn.unload("sol.dll")

So the dim attributes are lost. Interesting difference to the inline approach.

On the other hand, if I use wrap() I get what I want:

RcppExport SEXP sol2 ( SEXP X_ ){
  using namespace Rcpp;
  arma::mat X    = as<arma::mat>(X_);
  arma::mat Xinv = inv( X );
  return Rcpp::List::create(Rcpp::Named("X", wrap(X)),Rcpp::Named("Xinv", wrap(Xinv)));
}

> dyn.load("sol.dll")
> .Call("sol2", M)
$X
     [,1] [,2]
[1,]    1    3
[2,]    2    4

$Xinv
     [,1] [,2]
[1,]   -2  1.5
[2,]    1 -0.5

> dyn.unload("sol.dll")


Can anyone point me to why there are these differences whether I use inline or not. I must overlook something trivial.
Regards
Søren