[Rcpp-devel] Enquire on Rcpp::wrap or equivilant

[Romain Francois]

Le 05/12/10 20:09, Tama Ma a écrit :
> Dear Rcpp developers,
>
> May I kindly enquire on the wrapping of a portion of a container into an
> R numeric array by Rcpp::wrap?
>
> For example, I wish to have:
>
>
> template <class size_type, class element_type>
> Rcpp::NumericVector
> convert
> (element_type*obj
> ,size_type*n
> )
> {
> Rcpp::NumericVectorRvec;
> Rvec.reserve(n);
> std::copy( *obj , *(obj+n), std::back_inserter(Rvec));
>
> return Rvec;
> }
>
>
> May I know how I can do this?
>
> Thank you very much.
>
> Best regards,
> Tama Ma

Hello Tama,

The Vector template already has range based constructors. For example:


require( inline )
require( Rcpp )

morpheus <- cxxfunction( , '
	std::vector<double> x( 10 ) ;
	for( int i=0; i<10; i++) x[i] = sqrt( i ) ;

	NumericVector out( x.begin(), x.end() ) ;
	return out ;

', plugin = "Rcpp" )
morpheus()


Now, if you wanted your template to come up with what Vector to use you 
could do that sort of code by taking advantage of some of the traits we 
have in Rcpp :

trinity <- cxxfunction( , '
     std::vector<double> x( 10 ) ;
	for( int i=0; i<10; i++) x[i] = sqrt( i ) ;
	return convert( &x[0], x.size() )  ;

', includes = '

     template <typename elem_type, typename size_type>
     Vector< traits::r_sexptype_traits<elem_type>::rtype >
     convert( elem_type* start, size_type size ){
         typedef Vector< traits::r_sexptype_traits<elem_type>::rtype > 
VECTOR ;
         return VECTOR( start, start + size ) ;
     }

', plugin = "Rcpp")
trinity()


Or if you know a little bit about the containers you want to convert, 
for example you know they have a nested value_type type, you can use 
something like this:

neo <- cxxfunction( , '

     std::vector<double> x( 10 ) ;
	for( int i=0; i<10; i++) x[i] = sqrt( i ) ;
	return convert(x)  ;

', includes = '

     template <typename T>
     Vector< traits::r_sexptype_traits< typename T::value_type >::rtype >
     convert( const T& x ){
         typedef Vector< traits::r_sexptype_traits< typename 
T::value_type >::rtype > VECTOR ;
         return VECTOR( x.begin(), x.end() ) ;
     }
', plugin = "Rcpp" )
neo()


Romain


-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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