table.cpp

IntegerVector table1(const CharacterVector x) {
  std::map<char*, int> counts;

  int n = x.length();
  for (int i = 0; i < n; i++) {
    counts[x[i]]++;
  }

  // Loop through each element of map and output into named vector
  IntegerVector out(counts.size());
  CharacterVector names(counts.size());

  std::map<char*, int>::iterator it;
  int i;
  for (i = 0, it = counts.begin(); it != counts.end(); i++, it++) {
    names[i] = it->first;
    out[i] = it->second;
  }
  out.attr("names") = names;
  return(out);
}

table1.r

library(Rcpp)
cppFunction('
  IntegerVector table1(const CharacterVector x) {
    std::map<std::string, int> counts;
    std::vector<std::string> vec = as<std::vector<std::string> >(x);

    int n = x.length();
    for (int i = 0; i < n; i++) {
      counts[vec[i]]++;
    }

    // Loop through each element of map and output into named vector
    IntegerVector out(counts.size());
    CharacterVector names(counts.size());

    std::map<std::string, int>::iterator it;
    int i;
    for (i = 0, it = counts.begin(); it != counts.end(); i++, it++) {
      names[i] = it->first;
      out[i] = it->second;
    }
    out.attr("names") = names;
    return(out);
  }
')

library(microbenchmark)
x <- sample(letters, 1e4, rep = T)
microbenchmark(
  table(x),
  table1(x)
)
# About 3x faster