std::ranges::nth_element
Defined in header <algorithm>
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Call signature |
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template< std::random_access_iterator I, std::sentinel_for<I> S, class Comp = ranges::less, class Proj = std::identity > |
(1) | (since C++20) |
template< ranges::random_access_range R, class Comp = ranges::less, class Proj = std::identity > |
(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.
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.
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
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, libstdc++, and libc++: (1) / (2).
[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
(C++20) |
returns the largest element in a range (niebloid) |
(C++20) |
returns the smallest element in a range (niebloid) |
(C++20) |
divides a range of elements into two groups (niebloid) |
(C++20) |
sorts the first N elements of a range (niebloid) |
partially sorts the given range making sure that it is partitioned by the given element (function template) |