[Rcpp-devel] add new components to list without specifying list size initially

On 12 August 2011 at 16:26, Walrus Foolhill wrote:
| Thanks for your advice, I now understand how to manipulate one-level
lists:
|
| fn <- cxxfunction(signature(l_in="list"),
|                   body='
| using namespace Rcpp;
| List l(l_in);
| IntegerVector lf = l["foo"];
| CharacterVector lb = l["bar"];
| for(int i=0; i<lf.size(); ++i)
|   Rprintf("l[%s][%i] %i\\n", "foo", i, lf[i]);
| for(int i=0; i<lb.size(); ++i)
|   Rprintf("l[%s][%i] %s\\n", "bar", i, std::string(lb[i]).c_str());
| ', plugin="Rcpp", verbose=TRUE)
| z <- fn(list(foo=c(1,2,3,4),bar=c("bar1","bar2")))
|
| But what about 2-level lists? Why the following code doesn't compile?
|
| fn <- cxxfunction(signature(l_in="list"),
|                   body='
| using namespace Rcpp;
| List l(l_in);
| List lf(l["foo"]);
| ', plugin="Rcpp", verbose=TRUE)
| z <- fn(list(foo=list(bar=1)))
|
| And what the following message mean? "error: call of overloaded ?Vector
| (Rcpp::internal::generic_name_proxy<19>)? is ambiguous"
|
| I had a look at "runit.Vector.R" on r-forge, but couldn't find any test
| involving 2-level (or more) lists, although on SO in June 2010 (http://
|
stackoverflow.com/questions/3088650/how-do-i-create-a-list-of-vectors-in-rcpp/
| 3088744#3088744), you said that it should work.
|
| I checked that I can create a 2-level list, but the code below doesn't
compile
| if I uncomment the last Rprintf line:

There can be times when the C++ templating gets in the way, so if this
doesn't work in a single statement, decompose it into two (one to assign to
a
temp, another to print them temp) and move on.

I have done two-level lists in the past; one key is that a list ... is just
another SEXP, or can be wrap()'ed to a SEXP, and you can hence assign a
list
to be a component of another. And then another and so on...

Dirk