How create an Rcpp NumericVector with more than 20 entries? - r

Question

Creating a NumericVector with more than 20 elements leads to error messages.
This is in agreement with this document (at the very bottom): http://statr.me/rcpp-note/api/Vector_funs.html
Currently, I expose a class (using RCPP_MODULE) of which one of its methods returns the desired NumericVector. How can I return more than 20 elements?
#include <Rcpp.h>
class nvt {
public:
nvt(int x, double y) {...}
NumericVector run(void) {
....
return NumericVector::create(_["a"]=1,_["b"]=2, .....,_["c"]=21);
}
};
RCPP_MODULE(nvt_module){
class_<nvt>("nvt")
.constructor<int,double>("some description")
.method("run", &nvt::run,"some description")
;
}

Answer 1

Create the vector with the size you need then assign the values & names. This is an Rcpp "inline" function (easier for folks to try it out) but it'll work in your context:
library(Rcpp)
library(inline)
big_vec <- rcpp(body="
NumericVector run(26);
CharacterVector run_names(26);
# make up some data
for (int i=0; i<26; i++) { run[i] = i+1; };
# make up some names
for (int i=0; i<26; i++) { run_names[i] = std::string(1, (char)('A'+i)); };
run.names() = run_names;
return(run);
")
big_vec()
## A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
## 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Answer 2

Bob already showed you that a) you mistakenly took the constraint on the macro-defined create() helper to be binding, and b) how to do this via the inline package and loops.
Here is an alternate solution using Rcpp Attribute. Copy the following to a file, say, /tmp/named.cpp:
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector makevec(CharacterVector nm) {
NumericVector v(nm.size());
v = Range(1, nm.size());
v.attr("names") = nm;
return v;
}
/*** R
makevec(LETTERS)
makevec(letters[1:10])
*/
Simply calling sourceCpp("/tmp/named.cpp") will compile, link, load and also execute the R illustration at the bottom:
R> sourceCpp("/tmp/named.cpp")
R> makevec(LETTERS)
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
R> makevec(letters[1:10])
a b c d e f g h i j
1 2 3 4 5 6 7 8 9 10
R>