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std::ranges::nth_element

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
ranges::nth_element

Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
Operations on uninitialized storage
Return types
 
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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] 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

[edit] 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.

[edit] Complexity

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

[edit] Notes

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

[edit] Possible implementation

See also the implementation in MSVC STL and libstdc++.

[edit] 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:

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

[edit] See also

returns the largest element in a range
(niebloid) [edit]
returns the smallest element in a range
(niebloid) [edit]
divides a range of elements into two groups
(niebloid) [edit]
sorts the first N elements of a range
(niebloid) [edit]
partially sorts the given range making sure that it is partitioned by the given element
(function template) [edit]