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std::partition_point(3)	      C++ Standard Libary      std::partition_point(3)

NAME
       std::partition_point - std::partition_point

Synopsis
	  Defined in header <algorithm>
	  template<	class	  ForwardIt,	 class	   UnaryPredicate    >
       (since C++11)
	  ForwardIt  partition_point(	ForwardIt   first,   ForwardIt	 last,
       (until C++20)
	  UnaryPredicate p );
	  template< class ForwardIt, class UnaryPredicate >
	  constexpr  ForwardIt	partition_point(  ForwardIt  first,  ForwardIt
       last,  (since C++20)
	  UnaryPredicate p );

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

Parameters
	  first, last -	the partitioned	range of elements to examine
			unary  predicate  which	 returns true for the elements
       found in	the
			beginning of the range.

			The expression p(v) must be convertible	 to  bool  for
       every argument v
	  p	       -  of  type  (possibly const) VT, where VT is the value
       type of ForwardIt,
			regardless of value category, and must not  modify  v.
       Thus, a parameter
			type of	VT&is not allowed
			,  nor	is  VT unless for VT a move is equivalent to a
       copy
			(since C++11).

Type requirements
	  -
	  ForwardIt must meet the requirements of LegacyForwardIterator.
	  -
	  UnaryPredicate must meet the requirements of Predicate.

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

Complexity
	  Given	N = std::distance(first, last),	performs O(log N) applications
       of the
	  predicate p.

	  However, for non-LegacyRandomAccessIterators,	the number of iterator
       increments is
	  O(N).

Notes
	  This algorithm is a more general form	of std::lower_bound, which can
       be expressed in
	  terms	 of std::partition_point with the predicate [&](auto const& e)
       { return	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::partition(v.begin(), v.end(), is_even);
	    print_seq("After partitioning, v: ", v.cbegin(), v.cend());

	    const   auto   pp	=  std::partition_point(v.cbegin(),  v.cend(),
       is_even);
	    const auto i = std::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: 8 2 6 4 5 3 7 1 9
	Partition point	is at 4; v[4] =	5
	First partition	(all even elements): 8 2 6 4
	Second partition (all odd elements): 5 3 7 1 9

See also
	  find
	  find_if		  finds	the first element satisfying  specific
       criteria
	  find_if_not		  (function template)
	  (C++11)
	  is_sorted		   checks  whether  a range is sorted into as-
       cending order
	  (C++11)		  (function template)
				  returns an iterator to the first element not
       less than the
	  lower_bound		  given	value
				  (function template)
	  ranges::partition_point locates the partition	point of a partitioned
       range
	  (C++20)		  (niebloid)

http://cppreference.com		  2022.07.31	       std::partition_point(3)

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