How to extract multiple values from a NumericVector in C++

Question

I'm quite a newbie to Rcpp and its functionalities, not to mention C++ per se, so this might probably seem trivial to the experts among you. However, there's no such thing as a stupid question, so anyway:

I was wondering if there was a method to address multiple elements of a NumericVector in C++ at once using indexing. To make the whole thing more clear, here's the R equivalent of what I'm trying to do:

# Initial vector
x <- 1:10

# Extract the 2nd, 5th and 8th element of the vector
x[c(2, 5, 8)]
[1] 2 5 8

This is what I got so far in the C++ function that I'm executing in R using sourceCpp. It works, but it seems quite inconvenient to me. Is there any easier way to achieve my goals?

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector subsetNumVec(NumericVector x, IntegerVector index) {  
  // Length of the index vector
  int n = index.size();
  // Initialize output vector
  NumericVector out(n);

  // Subtract 1 from index as C++ starts to count at 0
  index = index - 1; 
  // Loop through index vector and extract values of x at the given positions
  for (int i = 0; i < n; i++) {
    out[i] = x[index[i]];
  }

  // Return output
  return out;
}

/*** R
  subsetNumVec(1:10, c(2, 5, 8))
*/
>   subsetNumVec(1:10, c(2, 5, 8))
[1] 2 5 8

Answer 1

You can do this if you use Armadillo vectors, rather than Rcpp vectors.

The Rcpp Gallery has a post with a complete example: see in particular the second example. Your indexing entries have to be in a (unsigned) uvec or umat.