[Rcpp-devel] Rcpp::List lost attributes after updating elements

[Dirk Eddelbuettel]

On 9 December 2012 at 14:07, Jiqiang Guo wrote:
| Note in the above, the printout for the second does not print any attribute as
| for the first function. 

Hm. That it not desirable so maybe we can do better, but you should also note
that you are using the list type in a suboptimal way by growing piece by
piece which leads to __copies__ being made at which point we may cast to
vector and drop the attributes. 

Here is a ever-so-slightlt modified example that works:

-----------------------------------------------------------------------------
#include <Rcpp.h>

// [[Rcpp::export]]
SEXP crtlist3() {
    Rcpp::List lst(2);   // reserve known size
    lst[0] = 10;
    lst.attr("att1") = 1;
    lst[1] = 20;
    return lst;
}
-----------------------------------------------------------------------------

R> sourceCpp("/tmp/jiqiang.cpp")
R> crtlist3()
[[1]]
[1] 10

[[2]]
[1] 20

attr(,"att1")
[1] 1
R> 


Essentially I do two things differently:  reserve the needed size, and index
by position.  The attr then remains.

Dirk

-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com