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>> Pred > (1) (since C++20)
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 may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

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