I mainly use R, but eventually would like to use Rcpp to interface with some C++ functions that take in and return 2d numeric arrays. So to start out playing around with C++ and Rcpp, I thought I'd just make a little function that converts my R list of variable-length numeric vectors to the C++ equivalent and back again.
require(inline)
require(Rcpp)
test1 = cxxfunction(signature(x='List'), body =
'
using namespace std;
List xlist(x);
int xlen = xlist.size();
vector< vector<int> > xx;
for(int i=0; i<xlen; i++) {
vector<int> test = as<vector<int> > (xlist[i]);
xx.push_back(test);
}
return(wrap(xx));
'
, plugin='Rcpp')
This works like I expect:
> test1(list(1:2, 4:6))
[[1]]
[1] 1 2
[[2]]
[1] 4 5 6
Admittedly I am only part way through the very thorough documentation, but is there a nicer (i.e. more Rcpp-like) way to do the R -> C++ conversion than with the for loop? I am thinking possibly not, since the documentation mentions that (at least with the built-in methods) as
"offers less flexibility and currently handles conversion of R objects into primitive types", but I wanted to check because I'm very much a novice in this area.
I will give you bonus points for a reproducible example, and of course for using Rcpp :) And then I will take those away for not asking on the rcpp-devel list...
As for converting STL types: you don't have to, but when you decide to do it, the as<>()
idiom is correct. The only 'better way' I can think of is to do name lookup as you would in R itself:
require(inline)
require(Rcpp)
set.seed(42)
xl <- list(U=runif(4), N=rnorm(4), T2df=rt(4,2))
fun <- cxxfunction(signature(x="list"), plugin="Rcpp", body = '
Rcpp::List xl(x);
std::vector<double> u = Rcpp::as<std::vector<double> >(xl["U"]);
std::vector<double> n = Rcpp::as<std::vector<double> >(xl["N"]);
std::vector<double> t2 = Rcpp::as<std::vector<double> >(xl["T2df"]);
// do something clever here
return(R_NilValue);
')
Hope that helps. Otherwise, the list is always open...
PS As for the two-dim array, that is trickier as there is no native C++ two-dim array. If you actually want to do linear algebra, look at RcppArmadillo and RcppEigen.
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