[Rcpp-devel] Rcpp-devel Digest, Vol 12, Issue 8

[Christian Gunning]

> Message: 2
> Date: Thu, 7 Oct 2010 21:41:06 -0700
> From: Vinh Nguyen <vinhdizzo at gmail.com>
> Subject: [Rcpp-devel] pass an R list as an argument and accessing
>        elements        of a list

> Dear Rcpp List,
>
> My goal is to write a generic C++ optimizing function (call it "optim"
> in this email) based on the Newton-Raphson algorithm. ?I will be
> passing into the optimizing function 4 arguments:
> 1. ?(pointer to) a function, "f1", to be optimized,
> 2. ?an initial value/vector,
> 3. ?the remaining inputs (other data, say x, y, and z) that argument 1
> needs in order to compute its value, derivative, and 2nd derivative,
> for the optimization, and
> 4. ?the final object to store the value, derivative, and 2nd derivative outputs.

It might be easier for optim to return an SEXP, i.e. a wrapped Rcpp
object, rather than a pointer/reference to place the object in.  The
resulting SEXP can be returned directly to R (which is what you want,
right?).  Packing a Rcpp::List with value, deriv, etc. inside optim
might make sense here.

> As argument 1 will be different from case to case, so too will
> argument 3. ?In other words, the arguments to the function f1 will be
> varying in terms of number and type (could be 2, 3, 4, ... number of
> objects). ?To be as flexible as possible, I'm thinking that argument 3
> should be an R list. ?Questions follow:
>
> I'll pass the R list as an SEXP object and then use Rcpp to access that list:
> extern "C" SEXP MyCppFunc(..., SEXP _list){
> ...
> Rcpp::List list(_list) ;
> ...
> }
>
> Is the above the proper way to pass a list the Rcpp way? ?I've
> used Rcpp::List::create but haven't done passing a list as an
> argument.

Yes, I've used this successfully.  I have run into trouble extracting
elements of length>1 from lists with variable-length elements, e.g.
_mylist = list(a=1, b=1:3), mylist["a"] works but mylist["b"] is
squirrely.

> To access the elements of the list, can I access them as list["x"],
> list["y"], and list["z"], just like the Rcpp::NumericVector case?
> Don't think I saw accessing lists in the vignettes.
>
> When I pass the Rcpp::List into the optim function, is this the
> correct way to pass it:
>
> void optim(..., Rcpp::List list){
> ...
> // do stuff with list, such as list["x"].
> ...
> }

Yes.

hth,
Christian