Rcpp sample sugar function, how to use

Question

I am trying to permute the order of elements in a CharacterVector. In R I would simply use:

sample(charvec)

I am trying the same thing in Rcpp using the sample sugar function, but it keeps throwing 'error: no matching function for call to 'sample(Rcpp::CharacterVector&)'. Other sugar functions I have tried, like intersect or sort_unique work fine with CharacterVector, but sample refuses to work. This is the minimal example I have been experimenting with:

cppFunction('CharacterVector samplefunc() {
  CharacterVector v = {"Cat", "Dog", "Fox", "Fish", "Lion"} ;
  CharacterVector v2 = sample(v) ;
  return v2 ;
}')

What I am doing wrong when trying to use the sample sugar function?

Answer 1

You are just missing the size parameter, which is mandatory for Rcpp::sample:

set.seed(42)

Rcpp::cppFunction('CharacterVector samplefunc() {
  CharacterVector v = {"Cat", "Dog", "Fox", "Fish", "Lion"} ;
  CharacterVector v2 = sample(v, v.size()) ;
  return v2 ;
}')

samplefunc()
#> [1] "Lion" "Fish" "Cat"  "Dog"  "Fox"

---

UPDATE (about debugging this kind of errors): Admittedly, the error you see when you do not provide the size argument is kind of obscure (at least with gcc), but you can see:

file1294a34f4734f.cpp: In function ‘Rcpp::CharacterVector samplefunc()’:
file1294a34f4734f.cpp:8:30: error: no matching function for call to ‘sample(Rcpp::CharacterVector&)’
    8 |   CharacterVector v2 = sample(v) ;
      |                        ~~~~~~^~~

This is the error: no matching function. And then,

In file included from /***/Rcpp/include/Rcpp/sugar/functions/functions.h:89,
                 from /***/Rcpp/include/Rcpp/sugar/sugar.h:31,
                 from /***/Rcpp/include/Rcpp.h:78,
                 from file1294a34f4734f.cpp:1:
/***/Rcpp/include/Rcpp/sugar/functions/sample.h:437:1: note: candidate: ‘template<int RTYPE> Rcpp::Vector<RTYPE, Rcpp::PreserveStorage> Rcpp::sample(const Rcpp::Vector<RTYPE, Rcpp::PreserveStorage>&, int, bool, Rcpp::sugar::probs_t)’
  437 | sample(const Vector<RTYPE>& x, int size, bool replace = false, sugar::probs_t probs = R_NilValue)
      | ^~~~~~

where gcc is showing you a candidate, and you can see that this function accepts a constant Vector of any RTYPE (numeric, character...), and then it needs a size argument, because there is no default. The others (replace, probs) do have a default. R functions may have missing arguments, C++ functions cannot.