Setting a `NULL` default for a parameter in Rcpp function

Question

I want to set the default value of a parameter to NULL in an Rcpp function and do some calculation based on the parameter if the parameter is not NULL. An example of such a code would be

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
int test_null(Nullable<DataFrame> p1 = R_NilValue){

  if(p1.isNull()){
    NumericMatrix T(2,2);
    Rcout << T << std::endl;
  }
    else{
    NumericMatrix T(p1.nrow());
    Rcout << T << std::endl;
  }
  return (42);
}

However, I am not able to compile this function and am getting an error message

error: no member named 'nrow' in 'Rcpp::Nullable<Rcpp::DataFrame_Impl<PreserveStorage> >'

which tells me that there is no nrow defined for a Nullable DataFrame. Is there any other way to do implement a default NULL value for a parameter (i.e., a DataFrame) in Rcpp so that I can calculate other properties (no. or rows, columns etc.) of the DataFrame when it is not NULL.

Any help would be greatly appreciated!

Thanks you!

SN248

Answer 1

You were very close. You missed one instantiation: a Nullable<> is not yet the same as its template type---we need to create an object first.

So here is your code with corrected whitespace ;-) and the missing line plus a test invocation of each case:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
int test_null(Nullable<DataFrame> p1 = R_NilValue){

  if (p1.isNull()) {
    NumericMatrix T(2,2);
    Rcout << T << std::endl;
  } else {
    DataFrame S(p1);
    NumericMatrix T(S.nrow());
    Rcout << T << std::endl;
  }
  return (42);
}

/*** R
test_null(NULL)
test_null(data.frame(a=1:3, b=letters[1:3]))
*/

For which I get the expected result:

R> Rcpp::sourceCpp("~/git/stackoverflow/61701367/answer.cpp")

R> test_null(NULL)
0.00000 0.00000
0.00000 0.00000

[1] 42

R> test_null(data.frame(a=1:3, b=letters[1:3]))
0.00000 0.00000 0.00000
0.00000 0.00000 0.00000
0.00000 0.00000 0.00000

[1] 42
R>