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std::ranges::partial_sort

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: 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
Operations on uninitialized storage
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
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
ranges::partial_sort
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
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

ranges::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>

ranges::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:

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

Contents

[edit] 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 - comparison to apply to the projected elements
proj - projection to apply to the elements.

[edit] Return value

An iterator equal to last.

[edit] Complexity

𝓞(N·log(M)) comparisons and twice as many projections, where
N = ranges::distance(first, last), M = ranges::distance(first, middle).

[edit] 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{};

[edit] 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);
    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
^ ^ ^

[edit] See also

copies and partially sorts a range of elements
(niebloid) [edit]
sorts a range into ascending order
(niebloid) [edit]
sorts a range of elements while preserving order between equal elements
(niebloid) [edit]
partially sorts the given range making sure that it is partitioned by the given element
(niebloid) [edit]
creates a max heap out of a range of elements
(niebloid) [edit]
removes the largest element from a max heap
(niebloid) [edit]
adds an element to a max heap
(niebloid) [edit]
turns a max heap into a range of elements sorted in ascending order
(niebloid) [edit]
sorts the first N elements of a range
(function template) [edit]