Tal,
You were close. The error you got indicated that some of (our) wrapping
around (our) class String was missing somehow. String is pretty new; Romain
just added it a few month ago under funding by Hadley -- and I am still
pretty unfamiliar with it.
Which is why I always go back to std::string. So I converted your code back,
which then built find but ran into one run-time error: you didn't test for
the attribute before extracting it. That is corrected too. So your
list-walker is below, with some extra verbose stdout prints.
Hope this helps, it is a nice example and always was a very good question.
Dirk
// Code first
#include <Rcpp.h>
using namespace Rcpp;
bool is_list(RObject x){
return TYPEOF(x) == VECSXP ;
}
bool is_string(RObject x){
return TYPEOF(x) == STRSXP && Rf_length(x) == 1 ;
}
bool is_logical(RObject x){
return TYPEOF(x) == LGLSXP && Rf_length(x) == 1 ;
}
bool is_leaf(RObject x){
if( TYPEOF(x) != REALSXP ) return false ;
if( !is_logical( x.attr("leaf") ) ) return false ;
bool leaf = x.attr( "leaf" ) ;
return leaf; // either TRUE or FALSE. But often, if it exists - it is TRUE.
}
std::string get_label(RObject x){
std::string label = "<empty>";
if (x.hasAttribute("label")) {
label = as<std::string>(x.attr( "label" )) ;
}
return label; // either TRUE or FALSE. But often, if it exists - it is TRUE.
}
void process( List data, std::vector<std::string>& results){
Rcout << "List with " << data.size() << " elements\n";
for( int i=0; i<data.size(); i++){
if( is_list( data[i] ) ){
// recurse
Rcout << "Recursing into list\n";
process( data[i], results ) ;
} else if( is_leaf( data[i] ) ){
Rcout << "Looking at leaf\n";
// we want to collect them. we can use the NumericVector class
// wince we know this is a numeric vector.
std::string x_label = get_label(data[i]);
results.push_back(x_label);
} // else do nothing
}
}
// [[Rcpp::export]]
std::vector<std::string> extract_fun(List x){
std::vector<std::string> results ;
process(x, results) ;
return(results) ;
}
/*** R
x <- list(a = 1, b = 2, c = list(ca = 3, cb = 4, 5), 6)
attr(x[[1]], "leaf") = TRUE
attr(x[[1]], "label") = "leaf 1"
attr(x[[3]][[1]], "leaf") = TRUE
attr(x[[2]], "leaf") = TRUE
attr(x[[2]], "label") = "leaf 2"
str(x)
extract_fun(x)
*/
// Output below:
extract_fun(x)
List with 4 elements
Looking at leaf
Looking at leaf
Recursing into list
List with 3 elements
Looking at leaf
[1] "leaf 1" "leaf 2" "<empty>"