std::ranges::nth_element() algorithm

since C++20

Simplified

// (1)constexpr I nth_element( I first, I nth, S last, Comp comp = {}, Proj proj = {} );// (2)constexpr ranges::borrowed_iterator_t<R>    nth_element( R&& r, iterator_t<R> nth, Comp comp = {}, Proj proj = {} );

The type of arguments are generic and have following constraints:

• I - std::random_access_iterator
• S - std::sentinel_for<I>
• R - std::ranges::random_access_range
• Comp - (none)
• Proj - (none)

The Proj and Comp template arguments have, the following default types for all overloads: std::identity, ranges::less.

Additionally, each overload has the following constraints:

• (1) - std::sortable<I, Comp, Proj>
• (2) - std::sortable<ranges::iterator_t<R>, Comp, Proj>

Detailed

// (1)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 = {} );// (2)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 = {} );

Reorders the elements in [first; last) such that:

• 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.

• 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.

• If nth == last then the function has no effect.

• (1) Elements are compared using the given binary comparison function comp and projection proj.

• (2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids.

Parameters

first last	The range to sort.
r	The range to sort.
nth	The iterator defining the sort partition point.
comp	Comparison object to apply to 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.

Complexity

Linear in ranges::distance(first, last) on average.

Exceptions

(none)

Notes

The algorithm used is typically Introselect although other Selection algorithm with suitable average-case complexity are allowed.

Examples

Main.cpp

#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

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