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std::ranges::partial_sort

Defined in header <algorithm>

Call signature

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)

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 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.

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

\(\scriptsize \mathcal{O}(N\cdot\log{(M)})\)𝓞(N·log(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

#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);
    const int m {3};
    std::ranges::partial_sort(v, v.begin() + m);
    print(v), underscore(m);
 
    static_assert('1' < 'a');
    std::string s {"3a1b41c5"};
    print(s);
    std::ranges::partial_sort(s.begin(), s.begin() + m, s.end(), std::greater {});
    print(s), underscore(m);
}

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
^ ^ ^