std::ranges::partial_sort_copy, std::ranges::partial_sort_copy_result

Defined in header `<algorithm>`
Call signature
template< std::input_iterator I1, std::sentinel_for<I1> S1, std::random_access_iterator I2, std::sentinel_for<I2> S2, class Comp = ranges::less, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_copyable<I1, I2> && std::sortable<I2, Comp, Proj2> && std::indirect_strict_weak_order<Comp, std::projected<I1, Proj1>, std::projected<I2, Proj2>> constexpr partial_sort_copy_result<I1, I2> partial_sort_copy( I1 first, S1 last, I2 result_first, S2 result_last, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );	(1)	(since C++20)
template< ranges::input_range R1, ranges::random_access_range R2, class Comp = ranges::less, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_copyable<ranges::iterator_t<R1>, ranges::iterator_t<R2>> && std::sortable<ranges::iterator_t<R2>, Comp, Proj2> && std::indirect_strict_weak_order<Comp, std::projected<ranges::iterator_t<R1>, Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>> constexpr partial_sort_copy_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>> partial_sort_copy( R1&& r, R2&& result_r, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );	(2)	(since C++20)
Helper types
template< class I, class O > using partial_sort_copy_result = ranges::in_out_result<I, O>;	(3)	(since C++20)

Copies the first N elements from the source range [first, last), as if it was partially sorted with respect to comp and proj1, into the destination range [result_first, result_first + N), where \(\scriptsize N = \min{(L_1, L_2)}\)N = min(L₁, L₂), \(\scriptsize L_1\)L₁ is equal to ranges::distance(first, last), and \(\scriptsize L_2\)L₂ is equal to ranges::distance(result_first, result_last).

The order of equal elements is not guaranteed to be preserved.

1) The source range elements are projected using the function object proj1, and the destination elements are projected using the function object proj2.

2) Same as (1), but uses r as the source range and result_r as the destination range, as if using ranges::begin(r) as first, ranges::end(r) as last, ranges::begin(result_r) as result_first, and ranges::end(result_r) as result_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.

Parameters

first, last	-	the iterator-sentinel pair defining the source range of elements to copy from
r	-	the source range to copy from
result_first, result_last	-	the iterator-sentinel pair defining the destination range of elements
result_r	-	the destination range
comp	-	comparison to apply to the projected elements
proj1	-	projection to apply to the elements of source range
proj2	-	projection to apply to the elements of destination range

Return value

An object equal to {last, result_first + N}.

Complexity

At most \(\scriptsize L_1 \cdot \log{(N)}\)L₁•log(N) comparisons and \(\scriptsize 2 \cdot L_1 \cdot \log{(N)}\)2•L₁•log(N) projections.

Possible implementation

struct partial_sort_copy_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::random_access_iterator I2, std::sentinel_for<I2> S2,
             class Comp = ranges::less, class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_copyable<I1, I2> && std::sortable<I2, Comp, Proj2> &&
             std::indirect_strict_weak_order<Comp, std::projected<I1, Proj1>,
             std::projected<I2, Proj2>>
    constexpr ranges::partial_sort_copy_result<I1, I2>
        operator()(I1 first, S1 last, I2 result_first, S2 result_last,
                   Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        if (result_first == result_last)
            return {std::move(ranges::next(std::move(first), std::move(last))),
                    std::move(result_first)};

        auto out_last{result_first};
        // copy first N elements
        for (; !(first == last or out_last == result_last); ++out_last, ++first)
            *out_last = *first;

        // convert N copied elements into a max-heap
        ranges::make_heap(result_first, out_last, comp, proj2);

        // process the rest of the input range (if any), preserving the heap property
        for (; first != last; ++first)
        {
            if (std::invoke(comp, std::invoke(proj1, *first),
                                  std::invoke(proj2, *result_first)))
            {
                // pop out the biggest item and push in a newly found smaller one
                ranges::pop_heap(result_first, out_last, comp, proj2);
                *(out_last - 1) = *first;
                ranges::push_heap(result_first, out_last, comp, proj2);
            }
        }

        // first N elements in the output range is still
        // a heap - convert it into a sorted range
        ranges::sort_heap(result_first, out_last, comp, proj2);

        return {std::move(first), std::move(out_last)};
    }

    template<ranges::input_range R1, ranges::random_access_range R2,
             class Comp = ranges::less, class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_copyable<ranges::iterator_t<R1>, ranges::iterator_t<R2>> &&
             std::sortable<ranges::iterator_t<R2>, Comp, Proj2> &&
             std::indirect_strict_weak_order<Comp, std::projected<ranges::iterator_t<R1>,
             Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>>
    constexpr ranges::partial_sort_copy_result<ranges::borrowed_iterator_t<R1>,
              ranges::borrowed_iterator_t<R2>>
        operator()(R1&& r, R2&& result_r, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
                       ranges::begin(result_r), ranges::end(result_r),
                       std::move(comp), std::move(proj1), std::move(proj2));
    }
};

inline constexpr partial_sort_copy_fn partial_sort_copy {};

Example

#include <algorithm>
#include <forward_list>
#include <functional>
#include <iostream>
#include <ranges>
#include <string_view>
#include <vector>

void print(std::string_view rem, std::ranges::input_range auto const& v)
{
    for (std::cout << rem; const auto& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}

int main()
{
    const std::forward_list source{4, 2, 5, 1, 3};

    print("Write to the smaller vector in ascending order: ", "");

    std::vector dest1{10, 11, 12};
    print("const source list: ", source);
    print("destination range: ", dest1);
    std::ranges::partial_sort_copy(source, dest1);
    print("partial_sort_copy: ", dest1);

    print("Write to the larger vector in descending order:", "");

    std::vector dest2{10, 11, 12, 13, 14, 15, 16};
    print("const source list: ", source);
    print("destination range: ", dest2);
    std::ranges::partial_sort_copy(source, dest2, std::greater{});
    print("partial_sort_copy: ", dest2);
}

Output:

Write to the smaller vector in ascending order:
const source list: 4 2 5 1 3
destination range: 10 11 12
partial_sort_copy: 1 2 3
Write to the larger vector in descending order:
const source list: 4 2 5 1 3
destination range: 10 11 12 13 14 15 16
partial_sort_copy: 5 4 3 2 1 15 16