std::ranges::partial_sort
From cppreference.com
| 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< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_sentinel_for<I> S,
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable<I, Comp, Proj>
I partial_sort( Ep&& policy, I first, I middle, S last,
Comp comp = {}, Proj proj = {} );
|
(3) | (since C++26) |
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
ranges::borrowed_iterator_t<R>
partial_sort( Ep&& policy, R&& r, ranges::iterator_t<R> middle,
Comp comp = {}, Proj proj = {} );
|
(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
1) Places the first
middle - first elements from the target range [first, last) as sorted with respect to the comparator comp and projection proj into the range [first, middle). The rest of the elements are placed in the range [middle, last) in an unspecified order.2) Same as (1), but uses
ranges::begin(r) as first and ranges::end(r) as last.3) Same as (1), but executed according to
policy.4) Same as (3), but uses
ranges::begin(r) as first and ranges::begin(r) + ranges::distance(r) as 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 target range |
| r | - | the target range |
| middle | - | the end of the range of sorted elements |
| comp | - | the comparator to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
| policy | - | the execution policy to use |
Return value
The past-the-end iterator of the target range.
Complexity
Given
- M as
ranges::distance(first, middle), - N as
ranges::distance(first, last):
1,2) Approximately N·log(M) applications of
comp, and twice as many applications of proj.3,4) 𝓞(N·log(M)) applications of
comp, and twice as many applications of proj.Exceptions
3,4) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
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
Run this code
#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
^ ^ ^