Ordering Permutation in Rcpp i.e. base::order()

Question

I have a ton of code using the base::order() command and I am really too lazy to code around that in rcpp. Since Rcpp only supports sort, but not order, I spent 2 m

Answer 1

Here's a simple version leveraging Rcpp sugar to implement an order function. We put in a check for duplicates so that we guarantee that things work 'as expected'. (There is also a bug with Rcpp's sort method when there are NAs, so that may want to be checked as well -- this will be fixed by the next release).

#include 
using namespace Rcpp;

// [[Rcpp::export]]
IntegerVector order_(NumericVector x) {
  if (is_true(any(duplicated(x)))) {
    Rf_warning("There are duplicates in 'x'; order not guaranteed to match that of R's base::order");
  }
  NumericVector sorted = clone(x).sort();
  return match(sorted, x);
}

/*** R
library(microbenchmark)
x <- runif(1E5)
identical( order(x), order_(x) )
microbenchmark(
  order(x),
  order_(x)
)
*/

gives me

> Rcpp::sourceCpp('~/test-order.cpp')

> set.seed(456)

> library(microbenchmark)

> x <- runif(1E5)

> identical( order(x), order_(x) )
[1] TRUE

> microbenchmark(
+   order(x),
+   order_(x)
+ )
Unit: milliseconds
      expr      min       lq   median       uq      max neval
  order(x) 15.48007 15.69709 15.86823 16.21142 17.22293   100
 order_(x) 10.81169 11.07167 11.40678 11.87135 48.66372   100
> 

Of course, if you're comfortable with the output not matching R, you can remove the duplicated check -- x[order_(x)] will still be properly sorted; more specifically, all(x[order(x)] == x[order_(x)]) should return TRUE.