std::ranges::nth_element(3)

NAME
std::ranges::nth_element - std::ranges::nth_element

Synopsis
   Defined in header <algorithm>
   Call signature
   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> (1) (since C++20)
   constexpr I
       nth_element( I first, I nth, S last, Comp comp = {}, Proj proj
   = {} );
   template< ranges::random_access_range R,
             class Comp = ranges::less, class Proj = std::identity >
   requires std::sortable<iterator_t<R>, Comp, Proj> (2) (since C++20)
   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 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 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.

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.

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

Notes
   The algorithm used is typically introselect although other selection algorithms with
   suitable average-case complexity are allowed.

Possible implementation
   See also the implementation in msvc stl, libstdc++, and libc++: (1) / (2).

Example
// Run this code
 #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: 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)
                        partially sorts the given range making sure that it is
   nth_element partitioned by the given element
                        (function template)