Rcpp sort descending

Question

I cannot sort in descending order using Rcpp

Sort in ascending order:

NumericVector sortIt(NumericVector v){
    std::sort(v.begin(), v.end());
    return v;
}

Attempt to sort in descending order:

NumericVector sortIt(NumericVector v){
    std::sort(v.begin(), v.end(), std::greater<int>()); // does not work returns ascending
    return v;
}

and

NumericVector sortIt(NumericVector v){
    std::sort(numbers.rbegin(), numbers.rend()); // errors
    return v;
}

Answer 1

This functionality has since been added (Rcpp version >= 0.12.7) to the Vector member function sort. This is necessary for sorting CharacterVector objects in particular (either ascending or descending) because the underlying element type requires special handling, and is incompatible with std::sort + std::greater (as well as certain other STL algorithms).

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::CharacterVector char_sort(Rcpp::CharacterVector x) {
    Rcpp::CharacterVector res = Rcpp::clone(x);
    res.sort(true);
    return res;
}

// [[Rcpp::export]]
Rcpp::NumericVector dbl_sort(Rcpp::NumericVector x) {
    Rcpp::NumericVector res = Rcpp::clone(x);
    res.sort(true);
    return res;
}

Note the use of clone to avoid modifying the input vector.

---

char_sort(c("a", "c", "b", "d"))
# [1] "d" "c" "b" "a"

dbl_sort(rnorm(5))
# [1]  0.8822381  0.7735230  0.3879146 -0.1125308 -0.1929413