# std::ranges::nth_element

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< cpp‎ | algorithm‎ | ranges

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 algorithm function objects (informally known as 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.

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

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