Convert individual Rcpp::IntegerVector element to a character - r

Question

I have to convert individual elements of Rcpp::IntegerVector into their string form so I can add another string to them. My code looks like this:
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
Rcpp::String int_to_char_single_fun(int x){
// Obtain environment containing function
Rcpp::Environment base("package:base");
// Make function callable from C++
Rcpp::Function int_to_string = base["as.character"];
// Call the function and receive its list output
Rcpp::String res = int_to_string(Rcpp::_["x"] = x); // example of original param
// Return test object in list structure
return (res);
}
//[[Rcpp::export]]
Rcpp::CharacterVector add_chars_to_int(Rcpp::IntegerVector x){
int n = x.size();
Rcpp::CharacterVector BASEL_SEG(n);
for(int i = 0; i < n; i++){
BASEL_SEG[i] = "B0" + int_to_char_single_fun(x[i]);
}
return BASEL_SEG;
}
/*** R
int_vec <- as.integer(c(1,2,3,4,5))
BASEL_SEG_char <- add_chars_to_int(int_vec)
*/
I get the following error:
no match for 'operator+'(operand types are 'const char[3]' and 'Rcpp::String')
I cannot import any C++ libraries like Boost to do this and can only use Rcpp functionality to do this. How do I add string to integer here in Rcpp?

Answer 1

We basically covered this over at the Rcpp Gallery when we covered Boost in an example for lexical_cast (though that one went the other way). So rewriting it quickly yields this:
Code
// We can now use the BH package
// [[Rcpp::depends(BH)]]
#include <Rcpp.h>
#include <boost/lexical_cast.hpp>
using namespace Rcpp;
using boost::lexical_cast;
using boost::bad_lexical_cast;
// [[Rcpp::export]]
std::vector<std::string> lexicalCast(std::vector<int> v) {
std::vector<std::string> res(v.size());
for (unsigned int i=0; i<v.size(); i++) {
try {
res[i] = lexical_cast<std::string>(v[i]);
} catch(bad_lexical_cast &) {
res[i] = "(failed)";
}
}
return res;
}
/*** R
lexicalCast(c(42L, 101L))
*/
Output
R> Rcpp::sourceCpp("/tmp/lexcast.cpp")
R> lexicalCast(c(42L, 101L))
[1] "42" "101"
R>
Alternatives
Because converting numbers to strings is as old as computing itself you could also use:
itoa()
snprintf()
streams
and probably a few more I keep forgetting.

Answer 2

As others have pointed out, there are several ways to do this. Here are two very straightforward approaches.
1. std::to_string
Rcpp::CharacterVector add_chars_to_int1(Rcpp::IntegerVector x){
int n = x.size();
Rcpp::CharacterVector BASEL_SEG(n);
for(int i = 0; i < n; i++){
BASEL_SEG[i] = "B0" + std::to_string(x[i]);
}
return BASEL_SEG;
}
2. Creating a new Rcpp::CharacterVector
Rcpp::CharacterVector add_chars_to_int2(Rcpp::IntegerVector x){
int n = x.size();
Rcpp::CharacterVector BASEL_SEG(n);
Rcpp::CharacterVector myIntToStr(x.begin(), x.end());
for(int i = 0; i < n; i++){
BASEL_SEG[i] = "B0" + myIntToStr[i];
}
return BASEL_SEG;
}
Calling them:
add_chars_to_int1(int_vec) ## using std::to_string
[1] "B01" "B02" "B03" "B04" "B05"
add_chars_to_int2(int_vec) ## converting to CharacterVector
[1] "B01" "B02" "B03" "B04" "B05"