Keep leading zero in rccp int

Question

I have a function via Rcpp that reverses an integer

#include <Rcpp.h>
using namespace Rcpp;

//' Reverse an integer
//'
//' @param x A single integer.
//' @export
// [[Rcpp::export]]
int reverse_integer(int x) {
  int reverse = 0;
  while(x != 0){
    int remainder = x%10;
    reverse = reverse*10 + remainder;
    x/=10;
  }
  return reverse;
}

This works fine for numbers without leading or trailing zeroes

reverse_integer(123) == 321

However, when there are zeroes, they will get stripped, i.e.,

reverse_integer(100) == 1

How can I best modify this function so I can still operate on it as an integer, but without losing zeroes?

Answer 1

You need to return a string. 001 is not a different integer than 1 in any programming language. Here's how:

#include <Rcpp.h>
using namespace Rcpp;

//' Reverse an integer
//'
//' @param x A single integer.
//' @export
// [[Rcpp::export]]
std::string reverse_integer(int x) {
  int reverse = 0;
  int ndigits = 0;
  while(x != 0){
    int remainder = x%10;
    reverse = reverse*10 + remainder;
    x/=10;
    ndigits++;
  }
  std::string output(ndigits, '\0');
  char * outputp = &output[0];
  std::string fmt_string = "%0" + std::to_string(ndigits) + "d";
  sprintf(outputp, fmt_string.c_str(), reverse);
  return output;
}

Example:

reverse_integer(103400)
[1] "004301"