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std::ranges...ition_point(3)   C++ Standard Libary  std::ranges...ition_point(3)

NAME
     std::ranges::partition_point - std::ranges::partition_point

Synopsis
	Defined in header <algorithm>
	Call signature
	template< std::forward_iterator I, std::sentinel_for<I> S,

		  class Proj = std::identity,
		  std::indirect_unary_predicate<std::projected<I,  Proj>>    (1)
     (since C++20)
	Pred >
	constexpr I

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

		  class Proj = std::identity,
		  std::indirect_unary_predicate<			     (2)
     (since C++20)
		      std::projected<ranges::iterator_t<R>, Proj>> Pred >
	constexpr ranges::borrowed_iterator_t<R>

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

	Examines  the  partitioned  (as  if  by ranges::partition) range [first,
     last) or r and
	locates the end of the first partition, that is, the  projected  element
     that does not
	satisfy pred or last if all projected elements satisfy pred.

	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 - iterator-sentinel defining the partially-ordered range  to
     examine
	r	    - the partially-ordered range to examine
	pred	    - predicate to apply to the projected elements
	proj	    - projection to apply to the elements

Return value
	The iterator past the end of the first partition within [first, last) or
     the
	iterator equal to last if all projected elements satisfy pred.

Complexity
	Given  N = ranges::distance(first, last), performs O(log N) applications
     of the
	predicate pred and projection proj.

	However, if sentinels don't model std::sized_sentinel_for<I>, the number
     of iterator
	increments is O(N).

Notes
	This algorithm is a more general form of ranges::lower_bound, which  can
     be expressed
	in  terms  of ranges::partition_point with the predicate [&](auto const&
     e) { return
	std::invoke(pred, e, value); });.

Example
     // Run this code

      #include <algorithm>
      #include <array>
      #include <iostream>
      #include <iterator>

      auto print_seq = [](auto rem, auto first, auto last)
      {
	  for (std::cout << rem; first != last; std::cout << *first++ << ' ') {}
	  std::cout << '\n';
      };

      int main()
      {
	  std::array v {1, 2, 3, 4, 5, 6, 7, 8, 9};

	  auto is_even = [](int i) { return i % 2 == 0; };

	  std::ranges::partition(v, is_even);
	  print_seq("After partitioning, v: ", v.cbegin(), v.cend());

	  const auto pp = std::ranges::partition_point(v, is_even);
	  const auto i = std::ranges::distance(v.cbegin(), pp);
	  std::cout << "Partition point is at " << i << "; v[" << i << "] = " <<
     *pp << '\n';

	  print_seq("First partition (all even elements): ", v.cbegin(), pp);
	  print_seq("Second partition (all odd elements): ", pp, v.cend());
      }

Possible output:
      After partitioning, v: 2 4 6 8 5 3 7 1 9
      Partition point is at 4; v[4] = 5
      First partition (all even elements): 2 4 6 8
      Second partition (all odd elements): 5 3 7 1 9

See also
	ranges::is_sorted   checks whether a range is sorted into ascending  or-
     der
	(C++20) 	    (niebloid)
	ranges::lower_bound  returns  an  iterator to the first element not less
     than the given
	(C++20) 	    value
			    (niebloid)
	partition_point     locates the partition point of a partitioned range
	(C++11) 	    (function template)

http://cppreference.com 	   2024.06.10	    std::ranges...ition_point(3)
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