[Rcpp-devel] A clean way to handle character/strings between R and Rcpp

[prezzemolo]

Hey.

I am wondering if there's some general guidance on what the 'right' options are to cleanly handle character arguments passed from R to Rcpp.

It is all rather simple: let's say I have a human-readable argument vector c("tree", "herb", "shrub") [It could also be a factor in R], which I can send to an Rcpp function along with other arguments for evaluation.
What would be the cleanest way to define those arguments (plant types) in R to then have as little trouble sending them over to Rcpp for use in cases like if() statements?


The use of enums comes to mind:

// beginningoffile
enum class var_type { flower, tree };

RCPP_EXPOSED_ENUM_NODECL(var_type)

// [[Rcpp::export]]
int charHandle1(var_type text_arg = var_type::flower) {
 if(text_arg == var_type::flower) {
  Rcout << static_cast<int>(text_arg) << " says '0 (flower)'.\n";
 } else if(text_arg == var_type::tree) {
  Rcout << static_cast<int>(text_arg) << " says '1 (tree)'.\n";
 }
 return 0;
}
// endoffile

This however, doesn't seem to work. I understand R would have to know the var_type for it to work.
Then I can do it simply by comparing the const char * argument. This will work, but the strcmp() comparison isn't very straightforward for someone who doesn't know C++ (and perhaps for some other reasons).

// [[Rcpp::export]]
int charHandle2(const char* text_arg = "flower") {
 if(strcmp(text_arg, "flower") == 0) {
  Rcout << text_arg << " says 'flower'.\n";
 } else if(strcmp(text_arg, "tree") == 0) {
  Rcout << text_arg << " says 'tree'.\n";
 }
 return 0;
}

I am looking for some good practice guidance on how to handle this safely and legibly to avoid sending people to function definitions. An argument could be made, for instance, that a list of plant type used as function arguments could be stored as factor in R and then sent and used in some way in Rcpp. What other options are there? To my best knowledge, support for enums is limited - I glanced over the vignettes and couldn't find any significant mention of enums (or factors, really), so I guess some other way of handling such cases should be taken. Has anyone dealt with such cases and has recommendations?

Regards,
Angelo (Greetings from Genoa)