std::ranges...copy_result(3)

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

okay bestie, welcome to the C++ Standard Libary manual (yes, that spelling is literally in the header, iykyk), page std::ranges...copy_result(3). This is the group chat for the ranges algorithm that copies a chunk of a source range into a destination range and gives you the first N in sorted-ish order. It is giving very “copy the cool kids into the smaller vector” energy. 🫶

💖 NAME 💖

std::ranges::partial_sort_copy,std::ranges::partial_sort_copy_result — std::ranges::partial_sort_copy, std::ranges::partial_sort_copy_result. That’s the whole name, bestie, both entities are on the invite list. 🎀

🌸 Synopsis 🌸

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)

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🫶 Description 🫶

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 N = min(L₁, L₂), L₁ is equal to ranges::distance(first, last), and L₂ is equal to ranges::distance(result_first, result_last). 💅

The order of equal elements is not guaranteed to be preserved. No cap.

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

💫 Return value 💫

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

🔥 Complexity 🔥

At most L₁ * log(N) comparisons and 2 * L₁ * log(N) projections. That’s the math, bestie, not me being dramatic. 😭

🧸 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

👀 See also 👀

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

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that’s the whole page, bestie. source/version/date: http://cppreference.com · 2024.06.10 · page std::ranges...copy_result(3). mwah 💋🎀