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 ascending 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)