[Rcpp-devel] Turning a vector into a matrix

Søren Højsgaard sorenh at math.aau.dk
Fri May 31 11:08:45 CEST 2013

Dear all,

Dirk: Thanks for the reply :). 

I shall dare to ask a "bonus question": I've played around with templating so that I get the right output type. I know (thanks to an earlier reply by Romain) how to do so using SEXP's but if I am constructing a utility function to be used on c++ objects, I can't that to work. What I am after should mimic the following such that the output type is determined by the input type:

NumericMatrix vec2matrix(NumericVector x, int nrow, int ncol) {
  return NumericMatrix(nrow, ncol, x.begin()); 

IntegerMatrix vec2matrix(IntegerVector x, int nrow, int ncol) {
  return IntegerMatrix(nrow, ncol, x.begin()); 

Any help would be more than welcome.

Best regards

-----Original Message-----
From: Dirk Eddelbuettel [mailto:edd at debian.org] 
Sent: 31. maj 2013 03:01
To: Søren Højsgaard
Cc: rcpp-devel at r-forge.wu-wien.ac.at
Subject: Re: [Rcpp-devel] Turning a vector into a matrix


You missed the most appropriate constructor:  

    template <typename Iterator>
    Matrix( const int& nrows_, const int& ncols, Iterator start ) ;

With that we just do this:

  R> Rcpp::sourceCpp('/tmp/soren.cpp')
  R> soren(1:4,2,2)
       [,1] [,2]
  [1,]    1    3
  [2,]    2    4

and the file soren.cpp is below.


#include <Rcpp.h>

using namespace Rcpp;

// [[Rcpp::export]]
NumericMatrix soren(NumericVector x, int n, int k) {
  return NumericMatrix(n, k, x.begin()); }

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

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