std::ranges::partial_sort

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 partial_sort( I first, I middle, 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<ranges::iterator_t<R>, Comp, Proj> constexpr ranges::borrowed_iterator_t<R> partial_sort( R&& r, ranges::iterator_t<R> middle, Comp comp = {}, Proj proj = {} );	(2)	(since C++20)

1) Rearranges elements such that the range [first, middle) contains the sorted middle - first smallest elements in the range [first, last).

The order of equal elements is not guaranteed to be preserved. The order of the remaining elements in the range [middle, last) is unspecified.

The elements are compared using the given binary comparison function comp and projected using proj function object.

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	-	iterator-sentinel defining the range to sort
r	-	the range to sort
middle	-	the iterator defining the last element to be sorted
comp	-	comparator to apply to the projected elements
proj	-	projection to apply to the elements

Return value

An iterator equal to last.

Complexity

$𝓞(N\cdotlog(M))$ comparisons and twice as many projections, where $\scriptsize N$ $N$ is ranges::distance(first, last), $\scriptsize M$ $M$ is ranges::distance(first, middle).

Possible implementation

struct partial_sort_fn {
    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
    operator()( I first, I middle, S last, Comp comp = {}, Proj proj = {} ) const {
        if (first == middle) return ranges::next(first, last);
        ranges::make_heap(first, middle, comp, proj);
        auto it {middle};
        for (; it != last; ++it) {
            if (std::invoke(comp, std::invoke(proj, *it), std::invoke(proj, *first))) {
                ranges::pop_heap(first, middle, comp, proj);
                ranges::iter_swap(middle - 1, it);
                ranges::push_heap(first, middle, comp, proj);
            }
        }
        ranges::sort_heap(first, middle, comp, proj);
        return it;
    }
 
    template <ranges::random_access_range R, class Comp = ranges::less,
             class Proj = std::identity>
    requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
    constexpr ranges::borrowed_iterator_t<R>
    operator()( R&& r, ranges::iterator_t<R> middle, Comp comp = {}, Proj proj = {} ) const {
        return (*this)(ranges::begin(r), std::move(middle), ranges::end(r),
                       std::move(comp), std::move(proj));
    }
};
 
inline constexpr partial_sort_fn partial_sort{};

Example

Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <string>
#include <vector>
 
void print(const auto& v) {
    for (const char e : v) { std::cout << e << ' '; }
    std::cout << '\n';
}
 
void underscore(int n) {
    while (n-- > 0) { std::cout << "^ "; }
    std::cout << '\n';
}
 
int main()
{
    static_assert('A' < 'a');
    std::vector<char> v{'x', 'P', 'y', 'C', 'z', 'w', 'P', 'o'};
    print(v);
    std::ranges::partial_sort(v, v.begin() + 3);
    print(v); underscore(3);
 
    static_assert('1' < 'a');
    std::string s{"3a1b41c5"};
    print(s);
    std::ranges::partial_sort(s.begin(), s.begin() + 3, s.end(), std::greater{});
    print(s); underscore(3);
}

Output:

x P y C z w P o
C P P y z x w o
^ ^ ^
3 a 1 b 4 1 c 5
c b a 1 3 1 4 5
^ ^ ^

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::partial_sort_copy (C++20)	copies and partially sorts a range of elements (niebloid)
ranges::sort (C++20)	sorts a range into ascending order (niebloid)
ranges::stable_sort (C++20)	sorts a range of elements while preserving order between equal elements (niebloid)
ranges::nth_element (C++20)	partially sorts the given range making sure that it is partitioned by the given element (niebloid)
ranges::make_heap (C++20)	creates a max heap out of a range of elements (niebloid)
ranges::pop_heap (C++20)	removes the largest element from a max heap (niebloid)
ranges::push_heap (C++20)	adds an element to a max heap (niebloid)
ranges::sort_heap (C++20)	turns a max heap into a range of elements sorted in ascending order (niebloid)
partial_sort	sorts the first N elements of a range (function template)