How do I create a list of vectors in Rcpp?

Question

Hi all. I'm writing an Rcpp module an would like to return as one element of the RcppResultSet list a list whose elements are vectors. E.g., .Call("myfunc")$foo should be something like:

[[1]]
[1] 1

[[2]]
[1] 1 1

[[3]]
[1] 1 1 1

(the exact numbers are not important here). The issue is that I don't know the right Rcpp way of doing this. I tried passing a vector<vector<int> > but this constructs a matrix by silently taking the length of the first vector as the width (even if the matrix is ragged!). I've tried constructing an RcppList but have a hard time casting various objects (like RcppVector) safely into SEXPs.

Anyone have tips on best practices for dealing with complicated structures such as lists of vectors in Rcpp?

Answer 1

[ Nice to see this here but Romain and I generally recommend the rccp-devel list for question. Please post there going forward as the project is not yet that large it warrants to have questions scattered all over the web. ]

RcppResultSet is part of the older classic API whereas a lot of work has gone into what we call the new API (starting with the 0.7.* releases). Have a look at the current Rcpp page on CRAN and the list of vignettes -- six and counting.

With new API you would return something like

return Rcpp::List::create(Rcpp::Named("vec") = someVector,
                          Rcpp::Named("lst") = someList,
                          Rcpp::Named("vec2") = someOtherVector);

all in one statement (and possibly using explicit Rcpp::wrap() calls), creating what in R would be

list(vec=someVector, lst=someList, vec2=someOtherVector)

And Rcpp::List should also be able to do lists of lists of lists... though I am not sure we have unit tests for this --- but there are numerous examples in the 500+ unit tests.

As it happens, I spent the last few days converting a lot of RQuantLib code from the classic API to the new API. This will probably get released once we get version 0.8.3 of Rcpp out (hopefully in a few days). In the meantime, you can look at the RQuantLib SVN archive

Answer 2

I second Dirk in recommending the mailing list. Dirk is a regular SO poster, but I'm not, and I suppose some other users of the mailing list are not either, so you lower your chances of getting a reply. OTOH, it is good to see Rcpp spreading

I would tend to use a compressed variation of Dirk's solution:

using namespace Rcpp ;
return List::create( 
   _["vec"]  = someVector, 
   _["lst"]  = someList, 
   _["vec2"] = someOtherVector
 ) ;

Also, to come back to the original question, vector< vector<int> > should wrap itself to a list of integer vectors, not a matrix. See:

require( Rcpp )
require( inline )
require( RUnit )

fx <- cxxfunction( , '

    std::vector< std::vector<int> > v ;

    std::vector<int> x1(1) ; v.push_back( x1 );
    std::vector<int> x2(2) ; v.push_back( x2 );
    std::vector<int> x3(3) ; v.push_back( x3 );

    return wrap( v ) ;

', plugin = "Rcpp" ) 

I get :

> fx() 

[[1]]
[1] 0

[[2]]
[1] 0 0

[[3]]
[1] 0 0 0