[Rcpp-devel] Extract a function from a list and create a call

[Douglas Bates]

By the way, I'm sorry for the misleading subject line in my original
post.  When I started the message I was trying to extract one of those
functions in the "family" list and create a call.


On Fri, Mar 19, 2010 at 3:20 PM, Romain Francois
<romain.francois at dbmail.com> wrote:
> Hello,
>
> The following works for me :
>
> require( Rcpp )
> require( inline )
>
> fx <- cfunction( signature( data_ = "list" ),
> '
> List data(data_) ;
> std::string family = data["family"] ;
> std::string link   = data["link"] ;
>
> std::cout << "family : " << family << std::endl ;
> std::cout << "link   : " << link   << std::endl ;
>
> return R_NilValue ;
> ', Rcpp = TRUE, includes = "using namespace Rcpp;" )
>
>> fx( )
> family : poisson
> link   : log
> NULL
>
>
>
> But we have an issue with the proxy class we use in CharacterVector. It
> seems if you have a CharacterVector :
>
> CharacterVector family = data["family"] ;
>
> you can grab one element as a C string (const char*)
>
> const char* fam = family[0] ;
>
> but not as a std::string for some reason. some investigation needed.

I think the underlying question for me is why the Rcpp::Vector
template does not take both the element type and the Rtype as
arguments.  Having the () and [] operators in the template return a
Proxy type may be necessary but I don't know enough about templates to
decide why.  If I have instantiated the NumericVector class from the
template as wrapping a REALSXP then the only thing I can imagine the
() and [] operators returning is a double.

Part of the problem for me is what to do with the result of () or []
from an Rcpp::List.  I can't manage to convince the compiler that it's
some kind of Rcpp::RObject, which I would say it has to be.

>
> Romain
>
>
> Le 19/03/10 20:21, Douglas Bates a écrit :
>>
>> I must be missing something horribly obvious but I have now spent
>> several hours trying to find a way to extract a character string from
>> a list.  I am more-or-less certain that I am copying a construction in
>> the unitTests/runit.CharacterVector.R but the compiler just keeps
>> spitting out error messages at me.
>>
>> The background is that a glm "family" object in R is a named list (or
>> should be, it has an S3 class but that doesn't mean anything about the
>> structure).  The standard ones look like
>>
>>> str(poisson())
>>
>> List of 12
>>  $ family    : chr "poisson"
>>  $ link      : chr "log"
>>  $ linkfun   :function (mu)
>>  $ linkinv   :function (eta)
>>  $ variance  :function (mu)
>>  $ dev.resids:function (y, mu, wt)
>>  $ aic       :function (y, n, mu, wt, dev)
>>  $ mu.eta    :function (eta)
>>  $ initialize:  expression({     if (any(y<  0))
>> stop("negative values not allowed for the Poisson family")     n<-
>> rep.int(1, nobs)     mustart<- y + 0.1 })
>>  $ validmu   :function (mu)
>>  $ valideta  :function (eta)
>>  $ simulate  :function (object, nsim)
>>  - attr(*, "class")= chr "family"
>>
>> All I want to do is to take such a list and extract the "family" and
>> "link" strings as std::string.  If the name of the Rcpp::List object
>> in the C++ code is lst, I can get as far as
>>
>> StringVector fam = lst["family"];
>>
>> but after that it all turns to custard.  In the class I am defining
>> the family member is declared as a std::string.  In the constructor
>> for the class from a List object I try
>>
>> family = Rcpp::as<std::string>(fam[0]);
>>
>> or even, mimicking a couple of the tests in
>> unitTests/runit.CharacterVector.R,
>>
>> family += fam[0];
>>
>> but I have not been able to construct anything that my compiler will
>> accept.
>>
>> So, how do I get the value of one of the elements of a CharacterVector
>> (or StringVector, I think they are synonyms) as a std::string, short
>> of using std::string(CHAR(STRING_ELT(fam, 0))
>
>
> --
> Romain Francois
> Professional R Enthusiast
> +33(0) 6 28 91 30 30
> http://romainfrancois.blog.free.fr
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>