How to get column names even if it is NULL in Rcpp?

Question

I want to get column names of a matrix to set another one, but if matrix does not have column names (or is set to NULL), the following code crashes my R session.

CharacterVector cn = colnames(x);

The following code is the way how I get column names of a matrix even if it does not have.

#include <Rcpp.h>
using namespace Rcpp;

// Get column names or empty
// [[Rcpp::export]]
CharacterVector get_colnames(const NumericMatrix &x) {
   CharacterVector cn;

   SEXP cnm = colnames(x);
   if (!Rf_isNull(cnm)) cn = cnm;

   return(cn);
}

Is there a more elegant way?

Answer 1

I had started this and then got distracted. @coatless covered it, this is simply shorter.

Code

#include <Rcpp.h>

// [[Rcpp::plugins(cpp11)]]
using namespace Rcpp;

// [[Rcpp::export]]
CharacterVector getColnames(const NumericMatrix &x) {
  size_t nc = x.cols();
  SEXP s = x.attr("dimnames");  // could be nil or list
  if (Rf_isNull(s)) {           // no dimnames, need to construct names
    CharacterVector res(nc);
    for (size_t i=0; i<nc; i++) {
      res[i] = std::string("V") + std::to_string(i);
    }
    return(res);
  } else {                      // have names, return colnames part
    List dn(s);
    return(dn[1]);
  }

}

/*** R
m <- matrix(1:9,3,3)
getColnames(m)
colnames(m) <- c("tic", "tac", "toe")
getColnames(m)
*/

Output

R> Rcpp::sourceCpp("~/git/stackoverflow/55850510/answer.cpp")

R> m <- matrix(1:9,3,3)

R> getColnames(m)
[1] "V0" "V1" "V2"

R> colnames(m) <- c("tic", "tac", "toe")

R> getColnames(m)
[1] "tic" "tac" "toe"
R>