how can I handle vectors without knowing the type in Rcpp

Question

I want to replicate the following R function in Rcpp:

fR = function(x) x[1:2]

fR(c(1,2,3))
#[1] 1 2
fR(c('a','b','c'))
#[1] "a" "b"

I can do it for a fixed output type like so:

library(inline)
library(Rcpp)

fint = cxxfunction(signature(x = "SEXP"), '
          List xin(x);
          IntegerVector xout;

          for (int i = 0; i < 2; ++i) xout.push_back(xin[i]);

          return xout;', plugin = "Rcpp")

But this will only work for integers, and if I try replacing the xout type with List (or GenericVector, which are the same) - it works with any input type, but I get back a list instead of a vector.

What's the correct Rcpp way of doing this?

Answer 1

Don't use push_back on Rcpp types. The way Rcpp vectors are currently implemented this requires copying all of the data each time. This is a very expensive operation.

We have RCPP_RETURN_VECTOR for dispatching, this requires that you write a template function taking a Vector as input.

#include <Rcpp.h>
using namespace Rcpp ;

template <int RTYPE>
Vector<RTYPE> first_two_impl( Vector<RTYPE> xin){
    Vector<RTYPE> xout(2) ;
    for( int i=0; i<2; i++ ){
        xout[i] = xin[i] ;    
    }
    return xout ;
}

// [[Rcpp::export]]
SEXP first_two( SEXP xin ){
  RCPP_RETURN_VECTOR(first_two_impl, xin) ;
}

/*** R
    first_two( 1:3 )
    first_two( letters )
*/

Just sourceCpp this file, this will also run the R code which calls the two functions. Actually, the template could be simpler, this would work too:

template <typename T>
T first_two_impl( T xin){
    T xout(2) ;
    for( int i=0; i<2; i++ ){
        xout[i] = xin[i] ;    
    }
    return xout ;
}

The template parameter T only needs:

• A constructor taking an int
• An operator[](int)

Alternatively, this might be a job for dplyr vector visitors.

#include <dplyr.h>
// [[Rcpp::depends(dplyr,BH)]]

using namespace dplyr ;
using namespace Rcpp ;

// [[Rcpp::export]]
SEXP first_two( SEXP data ){
    VectorVisitor* v = visitor(data) ;
    IntegerVector idx = seq( 0, 1 ) ;
    Shield<SEXP> out( v->subset(idx) ) ;
    delete v ;
    return out ;
}

visitors let you do a set of things on a vector regardless of the type of data it holds.

> first_two(letters)
[1] "a" "b"

> first_two(1:10)
[1] 1 2

> first_two(rnorm(10))
[1] 0.4647190 0.9790888

Answer 2

You need to pick a type (ie do not use signature="SEXP" [ oh and you should look into Attributes anyway ]).

Or you keep the SEXP type, and dispatch internally. See for example this post on the Rcpp Gallery.

Edit: And C is of course statically typed. These very switches depending on the type are all over the R sources too. No free lunch here.