std::partition_point(3)

You're on the std::partition_point(3) page from the C++ Standard Libary manual — yes, "Libary" is the header's spelling, I'm just the messenger 💅.

🌸 NAME 🌸

std::partition_point - std::partition_point. That's the whole name entry, bestie; minimal, iconic, and we love that for it ✨.

✨ Synopsis ✨

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

It examines the partitioned 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.

🚨 If the elements elem of [first, last) are not partitioned with respect to the expression bool(p(elem)), the behavior is undefined. No cap, that's the scary part.

🎀 Parameters 🎀

• first, last — the partitioned range of elements to examine.
• p — a 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 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.
• UnaryPred 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 as std::distance(first, last), performs O(log(N)) applications of the predicate p.

💅 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 [&](const auto& e) { return e < value; });.

👑 Possible implementation 👑

template<class ForwardIt, class UnaryPred>
constexpr //< since C++20
ForwardIt partition_point(ForwardIt first, ForwardIt last, UnaryPred p)
{
    for (auto length = std::distance(first, last); 0 < length; )
    {
        auto half = length / 2;
        auto middle = std::next(first, half);
        if (p(*middle))
        {
            first = std::next(middle);
            length -= (half + 1);
        }
        else
            length = half;
    }

    return first;
}

🌈 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)
• lower_bound — returns an iterator to the first element not less than the given value (function template)
• ranges::partition_point — locates the partition point of a partitioned range (C++20) (niebloid)

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