List iteration crashes R

Question

I'd like to do a list iteration in Rcpp, but this code crashes R:

Rcpp::cppFunction('List foo(List bc) {

              for (List::iterator i = bc.begin(); i != bc.end(); ++i) i[0] = i[1];

              return(bc);

            }'
)

If we take the following foo(list(a = c(1, 2, 3, 4), b = c(4, 3, 2, 1))), R will crash. The example above is just a dummy one - replace first element with second in every sublist (e.g. we should get c(2, 2, 3, 4) for a and for b c(3, 3, 2, 1)).

Could anyone help? I'm really new to both R and Rcpp and just going through the literature but have no idea about why the iterator doesn't work.

Answer 1

The problem is with i[0] and i[1]. Iterators are kinda-sorta-like pointers, you need to instantiate them first. Here is a variant of your code that works:

Code

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::List foo(Rcpp::List bc) {
  for (Rcpp::List::iterator i = bc.begin(); i != bc.end(); ++i) {
    SEXP a = *i;
    Rcpp::print(a);
  }

  return(bc);
}

/*** R
ll <- list(a = c(1, 2, 3, 4), b = c(4, 3, 2, 1))
foo(ll)
*/

Output

edd@rob:~/git/stackoverflow/60291024(master)$ Rscript -e 'Rcpp::sourceCpp("question.cpp")'

R> ll <- list(a = c(1, 2, 3, 4), b = c(4, 3, 2, 1))

R> foo(ll)
[1] 1 2 3 4
[1] 4 3 2 1
$a
[1] 1 2 3 4

$b
[1] 4 3 2 1

edd@rob:~/git/stackoverflow/60291024(master)$