[Rcpp-devel] Indexing a vector by a vector of indices

[Dirk Eddelbuettel]

On 31 May 2013 at 12:00, Søren Højsgaard wrote:
| Dear all;
| 
| In c++ I want to do:
| x<-c(2,4,6,8,10)
| i<- c(2,3,5)
| x[i]
| 
| I do:
| 
| cat("
| #include <RcppArmadillo.h>
| 
| using namespace Rcpp;
| using namespace arma;
| 
| // [[Rcpp::export]]
| NumericVector select(NumericVector vec_, NumericVector idx_){
|   arma::vec vv  = Rcpp::as<arma::vec>(vec_);
|   arma::uvec ii = Rcpp::as<arma::uvec>(idx_);
|   arma::vec aa  = vv.elem(ii-1);
|   Rcpp::NumericVector bb(as<Rcpp::NumericMatrix>(wrap(aa)));
|   return Rcpp::wrap( bb );
| }
| ", file="select.cpp")
| 
| Rcpp::sourceCpp('./select.cpp')
| select(x, i)
| 
|      [,1]
| [1,]    4
| [2,]    6
| [3,]   10
| 
| Two things: 1) there must be easier ways and 2) why is the result a matrix and not a vector (i.e. why does the result have a dim attribute)?

1) Yes, see the Rcpp Gallery, there are two pieces you want to read,
   particularly
	
      http://gallery.rcpp.org/articles/armadillo-subsetting/

   which uses Armadillo, and maybe also

      http://gallery.rcpp.org/articles/stl-transform-for-subsetting/

   which uses an STL trick I realized.

2) Methinks that is an error on your part where you use 

      Rcpp::NumericVector bb(as<Rcpp::NumericMatrix>(wrap(aa)));

   you probably meant

      Rcpp::NumericVector bb(as<Rcpp::NumericVector>(wrap(aa)));

Best,  Dirk

-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com