std::ranges::nth_element(3)

💅 std::ranges::nth_element(3) — C++ Standard Library

okay so bestie, buckle up, because we're talking about std::ranges::nth_element from the C++ Standard Library, and honestly? it's giving niche but iconic 🎀

✨ NAME ✨

std::ranges::nth_element — yes that's literally it, that's the name, no notes 💅

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🌸 SYNOPSIS 🌸

Miss thing lives in the header <algorithm>, and she has two call signatures. Let me lay them out for you, exactly, no gatekeeping 🫶

Signature (1) — since C++20:

template< std::random_access_iterator I, std::sentinel_for<I> S,
          class Comp = ranges::less, class Proj = std::identity >
    requires std::sortable<I, Comp, Proj>
constexpr I
    nth_element( I first, I nth, S last, Comp comp = {}, Proj proj = {} );

Signature (2) — also since C++20:

template< ranges::random_access_range R,
          class Comp = ranges::less, class Proj = std::identity >
    requires std::sortable<iterator_t<R>, Comp, Proj>
constexpr ranges::borrowed_iterator_t<R>
    nth_element( R&& r, iterator_t<R> nth, Comp comp = {}, Proj proj = {} );

🦋 DESCRIPTION 🦋

So what does she do? She reorders the elements in [first, last) so that all of the following is true, periodt:

• the element pointed at by nth is changed to whatever element would occur in that position if [first, last) were sorted with respect to comp and proj. like, she's manifesting the position ✨
• all of the elements before this new nth element are less than or equal to the elements after the new nth element. that is, for every iterator i, j in the ranges [first, nth), [nth, last) respectively, the expression std::invoke(comp, std::invoke(proj, *j), std::invoke(proj, *i)) evaluates to false. yes that's the whole vibe check 💀
• if nth == last then the function has no effect. she does absolutely nothing, and honestly? valid 🙈

Then the two flavors:

1. elements are compared using the given binary comparison function object comp and projection object proj.
2. same as (1), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

🧚 the niebloid lore (iykyk)

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. no cap, that's the tea ☕

💜 Parameters 💜

• first, last — the range of elements to reorder
• r — the range of elements to reorder
• nth — the iterator defining the partition point
• comp — comparator used to compare the projected elements
• proj — projection to apply to the elements

👑 Return value 👑

1. an iterator equal to last.
2. same as (1) if r is an lvalue or of a borrowed_range type. otherwise returns std::ranges::dangling. bestie watch out for the dangling one, that's the plot twist 😭

🔥 Complexity 🔥

linear in ranges::distance(first, last) on average. she's efficient, she's lowkey humble about it 💅

⭐ Notes ⭐

the algorithm used is typically introselect, although other selection algorithms with suitable average-case complexity are allowed. so like, the vibe is introselect but she's not gonna fight you over it 🌈

🎀 Possible implementation 🎀

see also the implementation in msvc stl, libstdc++, and libc++: (1) / (2). the girls are all doing it, respectfully 🫶

🍓 Example 🍓

#include <algorithm>
#include <array>
#include <functional>
#include <iostream>
#include <ranges>
#include <string_view>

void print(std::string_view rem, std::ranges::input_range auto const& a)
{
    for (std::cout << rem; const auto e : a)
        std::cout << e << ' ';
    std::cout << '\n';
}

int main()
{
    std::array v{5, 6, 4, 3, 2, 6, 7, 9, 3};
    print("Before nth_element: ", v);

    std::ranges::nth_element(v, v.begin() + v.size() / 2);
    print("After nth_element:  ", v);
    std::cout << "The median is: " << v[v.size() / 2] << '\n';

    std::ranges::nth_element(v, v.begin() + 1, std::greater<int>());
    print("After nth_element:  ", v);
    std::cout << "The second largest element is: " << v[1] << '\n';
    std::cout << "The largest element is: " << v[0] << "\n\n";

    using namespace std::literals;
    std::array names
    {
        "Diva"sv, "Cornelius"sv, "Munro"sv, "Rhod"sv,
        "Zorg"sv, "Korben"sv, "Bender"sv, "Leeloo"sv,
    };
    print("Before nth_element: ", names);
    auto fifth_element{std::ranges::next(names.begin(), 4)};
    std::ranges::nth_element(names, fifth_element);
    print("After nth_element:  ", names);
    std::cout << "The 5th element is: " << *fifth_element << '\n';
}

Output — she ate and left zero crumbs 🔥

Before nth_element: 5 6 4 3 2 6 7 9 3
After nth_element:  2 3 3 4 5 6 6 7 9
The median is: 5
After nth_element:  9 7 6 6 5 4 3 3 2
The second largest element is: 7
The largest element is: 9

Before nth_element: Diva Cornelius Munro Rhod Zorg Korben Bender Leeloo
After nth_element:  Diva Cornelius Bender Korben Leeloo Rhod Munro Zorg
The 5th element is: Leeloo

🌈 See also 🌈

• ranges::max_element — returns the largest element in a range (C++20) (niebloid)
• ranges::min_element — returns the smallest element in a range (C++20) (niebloid)
• ranges::partition — divides a range of elements into two groups (C++20) (niebloid)
• ranges::partial_sort — sorts the first N elements of a range (C++20) (niebloid)
• nth_element — partially sorts the given range making sure that it is partitioned by the given element (function template)

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ok that's the whole page bestie, go forth and partition 💖

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