std::ranges::partition_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 inhibited.
   In practice, they may be implemented as function objects, or with special 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 order
   (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)