Rcpp How to change a value directly inside a R List

Question

I know how to get values from a List objet, and how to create one within a C function.

But I would like to change a value in a List which is given as a parameter and that the modification would be effective when exiting the function.

Some thing like:

void myfunc(SEXP *lst) '
   List mylist (lst)
// make some modification

}

I need to modify a list within a recursive loop.

Thanks in advance Daniel

Answer 1

There is no magic. Just assign elements from a List object, and modify:

R> Rcpp::cppFunction("List foo(List L) { List L2 = L[0]; L2[1] = 42; return L2;} ")
R> L <- list(list(0,1,2), 2:4, 3.3)
R> foo(L)
[[1]]
[1] 0

[[2]]
[1] 42

[[3]]
[1] 2

R> 

We pick the first element (and know it is a List itself; there are predicates for testing). In that list, we set the second element. You can do the same returning the original but now modified list:

R> Rcpp::cppFunction("List bar(List L) { List L2 = L[0]; L2[1] = 42; return L;} ")
R> L <- list(list(0,1,2), 2:4, 3.3)
R> bar(L)
[[1]]
[[1]][[1]]
[1] 0

[[1]][[2]]
[1] 42

[[1]][[3]]
[1] 2


[[2]]
[1] 2 3 4

[[3]]
[1] 3.3

R>