std::ranges::nth_element

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> constexpr I nth_element( I first, I nth, S last, Comp comp = {}, Proj proj = {} );	(1)	(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 = {} );	(2)	(since C++20)

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 may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

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 and libstdc++.

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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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