Accessing Named List Elements in Rcpp

Question

I would like to access a named list element by name in Rcpp

In R

> b = list(bgroups=c(1,1,1,1,1,0,0,0,0,0))
> b$bgroups
[1] 1 1 1 1 1 0 0 0 0 0

Then when trying to access this in Rcpp I have tried:

cppFunction(
  "
void f(List & b){
 std::vector<int> c(10) = as<std::vector<int>> b['bgroups'];
}
  "
)

...

NumericVector groupings = b['bgroups'];

...

NumericVector groupings(10) = b(4);

But to no avail.

I've reviewed Dirk's many helpful answers, but have not been able to make the connection https://github.com/eddelbuettel/rcppexamples/blob/master/src/ListExample.cpp

score 4 · Answer 1 · answered Jul 10 '18 at 06:22

4

Not sure if I understand your question completely but have you tired below in your CppFunction.

NumericVector Bgroups = as<NumericVector>(b["bgroups"]);

answered Jul 10 '18 at 06:22

MSW Data

Yep. OP tries to index a `std::vector` which is not a named vector. – Dirk Eddelbuettel Jul 10 '18 at 06:44
Tried NumericVector split(const List & b){ NumericVector c= as(b['bgroups']); return c;}") but that caused my R session to terminate – user2723494 Jul 11 '18 at 15:12

score 0 · Answer 2 · answered Jul 10 '18 at 06:47

0

You already mention the source code for ListExample.cpp, if you also look at the source for the R complement RcppListExample.R it really should become clear.

Your question is also not complete and reproducible so it makes it harder to help you.

answered Jul 10 '18 at 06:47

Dirk Eddelbuettel

Thanks. I tried NumericVector split(const List & b){NumericVector c= as(b['bgroups']); return c;} but that forced my R session to quit. – user2723494 Jul 10 '18 at 15:37
I revised the question for clarity. Could I trouble you to take a look? – user2723494 Jul 10 '18 at 21:31

2 Answers2