subset a vector and sort it

Question

I'm looking into using some C++ for simple parts of my R package using the Rcpp package. I'm a C++ novice (but keen to learn!). I've implemented a few simple cpp programs using the excellent Rcpp - in fact that package has motivated me to learn C++...

Anyway, I've got stuck with a simple problem, which if I can fix would help lots. I have a NumericVector I want to subset and then sort. The code below sorts the whole vector (and would also deal with NAs, which is what I need).

My question is, say I want to extract a part of this vector, sort and have it available for other processing - how can I do that? For example, for a vector of length 10, how do I extract and sort the elements 5:10?

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
RcppExport SEXP rollP(SEXP x) {
  NumericVector A(x); // the data  
  A = sort_unique(A);  
  return A;
}

which I call from R:

sourceCpp( "rollP.cpp")
rollP(10:1)
# [1]  1  2  3  4  5  6  7  8  9 10

Answer 1

Here are 3 variants:

include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector rollP(NumericVector A, int start, int end) {
  NumericVector B(end-start+1) ;
  std::copy( A.begin() + start-1, A.begin() + end, B.begin() ) ;
  return B.sort() ;
}

// [[Rcpp::export]]
NumericVector rollP2(NumericVector A, int start, int end) {
  NumericVector B( A.begin() + start-1, A.begin() + end ) ;
  return B.sort() ;
}

// [[Rcpp::export]]
NumericVector rollP3(NumericVector A, int start, int end) {
  NumericVector B = A[seq(start-1, end-1)] ;
  return B.sort() ;
}

start and end are meant as 1-based indices, as if you were passing A[start:end] from R.

Answer 2

You need to look into C++ indexing, iterators and the whole bit. At a minimum, you need to change your interface (vector, fromInd, toInd) and figure out what you want to return.

One interpretation of your question would be to copy the subset from [fromInd, toInd) into a new vector, sort it and return it. All that is standard C++ fare, and a good text like the excellent (and free!!) C++ Annotations will be of help. It has a pretty strong STL section too.

Answer 3

You can use std::slice on a std::valarray. But if you want to use std::vector specifically then you can use std::copy to extract a portion of the vector and then use std::sort to sort the extracted slice of the vector.

Answer 4

You can do this quite easily by using the std::sort implementation that receives two iterators:

#include <vector>
#include <cinttypes>
#include <algorithm>

template <typename SeqContainer>
SeqContainer slicesort(SeqContainer const& sq, size_t begin, size_t end) {
  auto const b = std::begin(sq)+begin;
  auto const e = std::begin(sq)+end;
  if (b <= std::end(sq) && e <= std::end(sq)) {
    SeqContainer copy(b,e);
    std::sort(copy.begin(),copy.end());
    return copy;
  }
  return SeqContainer();
}

Which can be invoked like

  std::vector<int> v = {3,1,7,3,6,-2,-8,-7,-1,-4,2,3,9};
  std::vector<int> v2 = slicesort(v,5,10);