How do I create a list of vectors in Rcpp?

Question

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 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