std::ranges::nth_element - cppreference.com (original) (raw)

Defined in header
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.

Elements are compared using the given binary comparison function object comp and projection object proj.
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.

[edit] Parameters

first, last	-	the iterator-sentinel pair defining 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

An iterator equal to last.

[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, libstdc++, and libc++: (1) / (2).

[edit] Example

#include #include #include #include #include #include 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()); 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