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

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< cpp‎ | algorithm‎ | ranges
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
(C++17)  
Non-modifying sequence operations
(C++11)(C++11)(C++11)
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(C++11)
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(C++11)
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Permutations
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Operations on uninitialized storage
(C++17)
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(C++17)
(C++20)
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
ranges::partial_sort_copy
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
Constrained numeric operations
(C++23)
Fold operations
Operations on uninitialized storage
Return types
 
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 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.

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:

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 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 L₁•log(N) comparisons and 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

Run this code
#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

(C++20)
 sorts the first N elements of a range
(niebloid)
(C++20)
 sorts a range into ascending order
(niebloid)
(C++20)
 sorts a range of elements while preserving order between equal elements
(niebloid)
(C++20)
 turns a max heap into a range of elements sorted in ascending order
(niebloid)
(C++20)
 creates a max heap out of a range of elements
(niebloid)
(C++20)
 adds an element to a max heap
(niebloid)
(C++20)
 removes the largest element from a max heap
(niebloid)
copies and partially sorts a range of elements
(function template)
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