Convert std::vector to Rcpp matrix

Question

This is an Rcpp conversion related Q. I\'m looking to convert a long std::vector into a Rcpp matrix object, but want to know if there is an easy conversion format. Naturally

Answer 1

You chose a good approach. In the OpenMP context, you have to stay away from the single-threaded R. So std::vector is good.

And yes, we have constructors from std::vector to Rcpp::NumericVector and Rcpp::NumericMatrix (which is after all just a vector with a dimensions attribute) as well as the Rcoo:Integer* variants, as well as the usual as<> and wrap converters.

There should be plenty of examples in the Rcpp unit tests, the Rcpp examples, and of course on the Rcpp Gallery site.

Edit: Here is an overly simple example:

 R> cppFunction('NumericVector phil(int n) { std::vector<double> x(n);
 +              return wrap(x); }') 
 R> phil(4)  
 [1] 0 0 0 0    
 R>    

Answer 2

To expand on Dirk's answer; R matrices are really just vectors with a dim attribute set

> x <- 1
> y <- as.matrix(x)
> .Internal( inspect(x) )
@7f81a6c80568 14 REALSXP g0c1 [MARK,NAM(2)] (len=1, tl=0) 1 ## data
> .Internal( inspect(y) )
@7f81a3ea86c8 14 REALSXP g0c1 [NAM(2),ATT] (len=1, tl=0) 1 ## data
ATTRIB:
  @7f81a3e68e08 02 LISTSXP g0c0 [] 
    TAG: @7f81a30013f8 01 SYMSXP g1c0 [MARK,LCK,gp=0x4000] "dim" (has value) 
    @7f81a3ea8668 13 INTSXP g0c1 [NAM(2)] (len=2, tl=0) 1,1

Notice how the 'data' component of x and y are just REALSXPs, but you have this extra dim component on the matrix. Using this, we can easily convert a NumericVector to a matrix in Rcpp.

(Note: in the example below, I use SEXP in the attributes just so the conversions between types are explicit):

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
SEXP vec_to_mat(SEXP x_) {
  std::vector<double> x = as< std::vector<double> >(x_);
  /* ... do something with x ... */
  NumericVector output = wrap(x);
  output.attr("dim") = Dimension(x.size(), 1);
  return output;
}

/*** R
m <- c(1, 2, 3)
vec_to_mat(m)
*/

gives me

> m <- c(1, 2, 3)

> vec_to_mat(m)
     [,1]
[1,]    1
[2,]    2
[3,]    3

So, you can use the Dimension class and assign it to the dim attribute of a vector to make a matrix 'by hand' in Rcpp.