Rcpp map/dictionary/list

Question

How do I pass in a map/dictionary/list from R as a parameter to a c++ function?

For example, I want to do something like the following:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
int test(List map) {
    int val = map["test"];
    return(val);
}

/*** R
map <- list(test = 200, hello = "a")
test(map)
*/

where the output should be 200.

Answer 1

I have a similar problem on Mac OS X. Running your snippet seems to always return 1. However, if I modify the code in the following way it works:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
int test(List map) {
    int val = as<int>( map["test"] );
    return(val);
}

/*** R
map <- list(test = 200, hello = "a")
test(map)
*/

It seems like something is going wrong with the type inference -- the compiler should "know" that, since we're assigning map["test"] to an int-declared variable that it should be converted as int, but this does not seem to be the case. So, to be safe -- be sure to as anything that's coming out of an R list.

Also, it's worth stating: in R 200 is a double; if you want to explicitly pass an int you should write 200L.

FWIW, I'm compiling with clang++:

> clang++ -v
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.4.0
Thread model: posix

and

> sessionInfo()
R version 3.0.0 (2013-04-03)
Platform: x86_64-apple-darwin10.8.0 (64-bit)

locale:
[1] en_CA.UTF-8/en_CA.UTF-8/en_CA.UTF-8/C/en_CA.UTF-8/en_CA.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] Rcpp_0.10.4