home | help 
std::ranges::partition(3)      C++ Standard Libary     std::ranges::partition(3)

NAME
     std::ranges::partition - std::ranges::partition

Synopsis
	Defined in header <algorithm>
	Call signature
	template< std::permutable I, std::sentinel_for<I> S, class Proj =
	std::identity,

		  std::indirect_unary_predicate<std::projected<I,  Proj>>    (1)
     (since C++20)
	Pred >
	constexpr ranges::subrange<I>

	    partition( I first, S last, Pred pred, Proj proj = {} );
	template< ranges::forward_range R, class Proj = std::identity,

		  std::indirect_unary_predicate<
		      std::projected<ranges::iterator_t<R>, Proj>> Pred  >   (2)
     (since C++20)
	requires std::permutable<ranges::iterator_t<R>>
	constexpr ranges::borrowed_subrange_t<R>

	    partition( R&& r, Pred pred, Proj proj = {} );

	1)  Reorders  the elements in the range [first, last) in such a way that
     the
	projection proj of all elements for which  the	predicate  pred  returns
     true precede
	the  projection proj of elements for which predicate pred returns false.
     Relative
	order of elements is not preserved.
	2) Same as (1), but uses r as the source range, as if using  ranges::be-
     gin(r) as
	first and ranges::end(r) as last.

	The  function-like  entities  described on this page are niebloids, that
     is:

	  * Explicit template argument lists cannot be	specified  when  calling
     any of them.
	  * None of them are visible to argument-dependent lookup.
	  *  When any of them are found by normal unqualified lookup as the name
     to the left
	    of the function-call operator, argument-dependent lookup  is  inhib-
     ited.

	In  practice,  they may be implemented as function objects, or with spe-
     cial compiler
	extensions.

Parameters
	first, last - the range of elements to reorder
	r	    - the range of elements to reorder
	pred	    - predicate to apply to the projected elements
	proj	    - projection to apply to the elements

Return value
	A subrange starting with an iterator to the first element of the  second
     group and
	finishing  with an iterator equal to last. (2) returns std::ranges::dan-
     gling if r is
	an rvalue of non-borrowed_range type.

Complexity
	Given N = ranges::distance(first, last), exactly N applications  of  the
     predicate and
	projection. At most N / 2 swaps if I models ranges::bidirectional_itera-
     tor, and at
	most N swaps otherwise.

Possible implementation
	struct partition_fn
	{
	    template<std::permutable  I,  std::sentinel_for<I>	S,  class Proj =
     std::identity,
		     std::indirect_unary_predicate<std::projected<I,	  Proj>>
     Pred>
	    constexpr ranges::subrange<I>
		operator()(I first, S last, Pred pred, Proj proj = {}) const
	    {
		first	=   ranges::find_if_not(first,	 last,	 std::ref(pred),
     std::ref(proj));
		if (first == last)
		    return {first, first};

		for (auto i = ranges::next(first); i != last; ++i)
		{
		    if (std::invoke(pred, std::invoke(proj, *i)))
		    {
			ranges::iter_swap(i, first);
			++first;
		    }
		}
		return {std::move(first), std::move(last)};
	    }

	    template<ranges::forward_range R, class Proj = std::identity,
		     std::indirect_unary_predicate<
			 std::projected<ranges::iterator_t<R>, Proj>> Pred>
	    requires std::permutable<ranges::iterator_t<R>>
	    constexpr ranges::borrowed_subrange_t<R>
		operator()(R&& r, Pred pred, Proj proj = {}) const
	    {
		return (*this)(ranges::begin(r), ranges::end(r),
			       std::ref(pred), std::ref(proj));
	    }
	};

	inline constexpr partition_fn partition;

Example
     // Run this code

      #include <algorithm>
      #include <forward_list>
      #include <functional>
      #include <iostream>
      #include <iterator>
      #include <ranges>
      #include <vector>

      namespace ranges = std::ranges;

      template<class I, std::sentinel_for<I> S, class Cmp = ranges::less>
      requires std::sortable<I, Cmp>
      void quicksort(I first, S last, Cmp cmp = Cmp {})
      {
	  using reference = std::iter_reference_t<I>;

	  if (first == last)
	      return;

	  auto size = ranges::distance(first, last);
	  auto pivot = ranges::next(first, size - 1);
	  ranges::iter_swap(pivot, ranges::next(first, size / 2));

	  auto tail = ranges::partition(first, pivot, [=](reference em)
	  {
	      return std::invoke(cmp, em, *pivot); // em < pivot
	  });

	  ranges::iter_swap(pivot, tail.begin());
	  quicksort(first, tail.begin(), std::ref(cmp));
	  quicksort(ranges::next(tail.begin()), last, std::ref(cmp));
      }

      int main()
      {
	  std::ostream_iterator<int> cout {std::cout, " "};

	  std::vector<int> v {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
	  std::cout << "Original vector:  \t";
	  ranges::copy(v, cout);

	  auto tail = ranges::partition(v, [](int i) { return i % 2 == 0; });

	  std::cout << "\nPartitioned vector: \t";
	  ranges::copy(ranges::begin(v), ranges::begin(tail), cout);
	  std::cout << "a ";
	  ranges::copy(tail, cout);

	  std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2,	-5,  64,
     1, 92};
	  std::cout << "\nUnsorted list: \t\t";
	  ranges::copy(fl, cout);

	  quicksort(ranges::begin(fl), ranges::end(fl), ranges::greater {});
	  std::cout << "\nQuick-sorted list: \t";
	  ranges::copy(fl, cout);

	  std::cout << '\n';
      }

Possible output:
      Original vector:	      0 1 2 3 4 5 6 7 8 9
      Partitioned vector:     0 8 2 6 4 a 5 3 7 1 9
      Unsorted list:	      1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92
      Quick-sorted list:      92 64 30 6 5 3 2 1 1 1 -4 -4 -5 -8

See also
	ranges::partition_copy	  copies  a range dividing the elements into two
     groups
	(C++20) 		 (niebloid)
	ranges::is_partitioned	 determines if the range is partitioned  by  the
     given
	(C++20) 		 predicate
				 (niebloid)
	ranges::stable_partition divides elements into two groups while preserv-
     ing their
	(C++20) 		 relative order
				 (niebloid)
	partition		 divides a range of elements into two groups
				 (function template)

http://cppreference.com 	   2024.06.10	       std::ranges::partition(3)
home | help