std::ranges...ition_point(3)

🌸 NAME 🌸

std::ranges::partition_point — literally std::ranges::partition_point, bestie 💅. We're on the std::ranges::partition_point(3) page from the C++ Standard Libary manual ✨ — manual name folded in near the top, no header-line copy-paste energy.

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✨ 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 >
constexpr I
    partition_point( I first, S last, Pred pred, Proj proj = {} );

(1) (since C++20)

template< ranges::forward_range R,
          class Proj = std::identity,
          std::indirect_unary_predicate<
              std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr ranges::borrowed_iterator_t<R>
    partition_point( R&& r, Pred pred, Proj proj = {} );

(2) (since C++20)

okay so what does it actually do? It examines the partitioned (as if by ranges::partition) range [first, last) or r, and locates the end of the first partition. That's 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, bestie, which means:

• 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.

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🎀 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

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💖 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 ✨.

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🔥 Complexity 🔥

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

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

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🦋 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); });.

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🌈 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());
}

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⭐ 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

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👑 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 value; (C++20); (niebloid)
• partition_point — locates the partition point of a partitioned range; (C++11); (function template)

okay that's the whole page, bestie — source/version: http://cppreference.com, date: 2024.06.10 🌈💅✨