Rcpp function for subsetting strings

I was wondering if there was an Rcpp function which takes an Rcpp::String data type as input and returns a given character (by index) of the string. For example, extracting the character at index 0 of the string. This would be equivalent to the string::at method from the string header in c++. I have written this:

#include <vector>
#include <string>
#include <Rcpp.h>

using namespace Rcpp;

typedef std::vector<std::string> stringList;

int SplitGenotypesA(std::string s) {
    char a = s.at(0);
    int b = a - '0';
    return b;
}

But would prefer not to have to convert between Rcpp::String and std::string types.

Solution

You can feed an R vector of strings directly to C++ using Rcpp::StringVector. This will obviously handle single elements too.

Getting the nth character of the ith element of your vector is as simple as vector[i][n].

So, without using std::string you can do this:

#include<Rcpp.h>

// [[Rcpp::export]]
Rcpp::NumericVector SplitGenotypesA(Rcpp::StringVector R_character_vector)
{
  int number_of_strings = R_character_vector.size();
  Rcpp::NumericVector result(number_of_strings);
  for(int i = 0; i < number_of_strings; ++i)
  {
    char a = R_character_vector[i][0];
    result[i] = a - '0';
  }
  return result;
}

Now in R you can do:

SplitGenotypesA("9C")
# [1] 9

or better yet,

SplitGenotypesA(c("1A", "2B", "9C"))
# [1] 1 2 9

Which is even a little faster than the native R method of doing the same thing:

microbenchmark::microbenchmark(
  R_method    = as.numeric(substr(c("1A", "2B", "9C"), 1, 1)), 
  Rcpp_method = SplitGenotypesA(c("1A", "2B", "9C")),
  times = 1000)

# Unit: microseconds
#         expr   min    lq     mean median    uq    max neval
#     R_method 3.422 3.765 4.076722  4.107 4.108 46.881  1000
#  Rcpp_method 3.080 3.423 3.718779  3.765 3.765 32.509  1000