rotate.cpp

#include <Rcpp.h>
using namespace Rcpp ;

// [[Rcpp::plugins(cpp11)]]

template <int RTYPE>
Vector<RTYPE> rotate_impl( Vector<RTYPE> x, int pivot ){
  auto n = x.size() ;
  Vector<RTYPE> res = no_init(n) ;

  // similar to wjat head and tail do, if pivot is 
  // negative, interpret it as counting from
  if( pivot < 0 ){
    pivot = n + pivot ;
  }

  // copy the last elements of x into the start of res
  auto it = std::copy(
    x.begin() + pivot ,
    x.end(),
    res.begin()
  ) ;

  // and then the first elements
  std::copy( x.begin(), x.begin() + pivot , it) ;

  return res ;
}


// [[Rcpp::export]]
SEXP rotate( SEXP x, int pivot ){
  RCPP_RETURN_VECTOR(rotate_impl, x, pivot ) ;
}

/*** R
  # move the first 4 letters at the end
  rotate( letters, 4)

  # move the (n-2) first letters at the end
  rotate( letters, -2)
  
  
  friends <- c("Rachel", "Monica", "Phoebe", "Ross", "Chandler", "Joey" )
  tibble( 
    name = sample(friends), 
    partner = rotate(name, 1)
  )
  
*/

> # move the first 4 letters at the end
> rotate( letters, 4)
 [1] "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z" "a" "b" "c" "d"
> # move the (n-2) first letters at the end
> rotate( letters, -2)
 [1] "y" "z" "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x"
> 
> 
> friends <- c("Rachel", "Monica", "Phoebe", "Ross", "Chandler", "Joey" )
> tibble( 
+     name = sample(friends), 
+     partner = rotate(name, 1)
+ )
# A tibble: 6 x 2
      name  partner
     <chr>    <chr>
1     Ross     Joey
2     Joey   Monica
3   Monica   Rachel
4   Rachel Chandler
5 Chandler   Phoebe
6   Phoebe     Ross