[Rcpp-devel] Wrapping uBlas vectors into Rcpp Vectors.

Le 01/06/11 14:28, Cedric Ginestet a ?crit :

Dear Romain,

Thank you very much for your help. I tried what you suggested by
including the following templated function in templatedFunction.h, as
follows:
template <typename T>
Rcpp::Vector< Rcpp::traits::r_sexptype_traits<T>::rtype >
ublas2rcpp( const vector<T>& x ){
return Rcpp::Vector< r_sexptype_traits<T>::rtype >(
x.begin(), x.end()
) ;
}
In addition, I have tested the function using in subgraph.cpp:
Rcpp::Vector<int> xY = ublas2rcpp(Y);

And I got the following error messages:
templatedFunction.h: In function
?Rcpp::Vector<Rcpp::traits::r_sexptype_traits<T>::rtype>
ublas2rcpp(const boost::numeric::ublas::vector<T>&)?:
templatedFunction.h:50:26: error: ?r_sexptype_traits? was not declared
in this scope
templatedFunction.h:50:45: error: template argument 1 is invalid
subgraph.cpp: In function ?SEXPREC* cxx_Mask2Graph(SEXPREC*, SEXPREC*,
SEXPREC*, SEXPREC*)?:
subgraph.cpp:32:19: error: type/value mismatch at argument 1 in template
parameter list for ?template<int RTYPE> class Rcpp::Vector?
subgraph.cpp:32:19: error: expected a constant of type ?int?, got ?int?
subgraph.cpp:32:24: error: invalid type in declaration before ?=? token
subgraph.cpp:32:38: error: invalid conversion from ?SEXPREC*? to ?int?
subgraph.cpp:34:8: error: invalid conversion from ?int? to ?SEXPREC*?
...

Sure. This was a typo/thinko: go with something like this :

template <typename T>
Rcpp::Vector< Rcpp::traits::r_sexptype_traits<T>::rtype >
ublas2rcpp( const vector<T>& x ){
    return Rcpp::Vector< Rcpp::traits::r_sexptype_traits<T>::rtype >(
        x.begin(), x.end()
    )  ;
}

and Rcpp::Vector<int> makes no sense, you probably want IntegerVector, 
or (the same class):

Rcpp::Vector< r_sexptype_traits<int>::rtype >

Rcpp::Vector is templated by the SEXP type.

Also, as an aside, I was wondering what I should use instead of
push_back for Rcpp Vectors. Do I necessarily have to specify the size of
the vector before I assign its elements to specific values?

That is much better yes. ublas probably gives a way to access the size 
of the vector.

Thanks a lot,
Cedric


On 01/06/11 11:44, Romain Francois wrote:

Hi,

I've not used uBlas, but what you are trying to do is quite similar to
what we do in RcppArmadillo.

You can probably manage to guess the output type from the input type,
so you only have to parameterise your template on the input type.
something like (untested) :

template <typename T>
Rcpp::Vector< Rcpp::traits::r_sexptype_traits<T>::rtype >
ublas2rcpp( const vector<T>& x ){
return Rcpp::Vector< r_sexptype_traits<T>::rtype >(
x.begin(), x.end()
) ;
}

This way you don't have to specify template parameter when you call
ublas2rcpp because the compiler is smart enough.

Nicer than this would be to implement wrap and as for ublas vectors,
the way to go is documented in the Rcpp-extended vignettes, with
examples implementations in RcppArmadillo and RcppGSL.

As a side note, you don't want to use push_back on Rcpp types, because
it creates a new vector each time, so this is HUGE memory waste.

Now, this could get much smarter as ublas has vector expressions,
similar to armadillo, so I suppose someone could write something like
RcppUBlas with nice goodies. This is not me, at least not now ;-)

Romain


Le 01/06/11 12:24, Cedric Ginestet a ?crit :

Dear Rcpp experts,

I have started to use the uBlas library, and I am trying to ensure 
that
I can pass from uBlas vectors to Rcpp vectors relatively easily. So 
far,
I have tried the following templated function:

/////////////////////////////////////////////////////////////////////// 

using namespace Rcpp;
using namespace boost::numeric::ublas;

template <class T1, class T2>
T1 ublas2rcpp(T2& uVec){
T1 rcppVec;
for(int i=0; i<uVec.size(); ++i) rcppVec.push_back(uVec(i));
return rcppVec;
}//ublas2rcpp.

////////
SEXP foo(vector<int> &y){
...
IntegerVector rcppY=ublas2rcpp<IntegerVector,vector<int> >(y);
....
return rcppY;
}
/////////////////////////////////////////////////////////////////////// 


I have got two questions:
a. This templated function doesn't work. It compiles fine, but hangs
when I try to run it in R. What do you think is faulty in the codes
above?
b. Is there a better way to wrap uBlas vectors into Rcpp ones? Is that
something that you are planning to implement (or have already
implemented) within the Rcpp suite?

Thank you very much for your help,

-- 
Cedric Ginestet, PhD
Centre for Neuroimaging Sciences (L3.04)
NIHR Biomedical Research Centre
Department of Neuroimaging
Institute of Psychiatry, Box P089
King's College London
De Crespigny Park
London
SE5 8AF
http://arxiv.org/find/q-bio/1/au:+Ginestet_C/0/1/0/all/0/1