NAME¶

std::ranges::partial_sort - std::ranges::partial_sort

Synopsis¶

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> (1) (since C++20)
constexpr I

partial_sort( I first, I middle, S last, Comp comp = {}, Proj
proj = {} );
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> (2) (since C++20)
partial_sort( R&& r, ranges::iterator_t<R> middle, Comp comp =
{},

Proj proj = {} );

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:

* 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 range to sort
r - the range to sort
middle - the iterator defining the last element to be sorted
comp - comparator to apply to the projected elements
proj - projection to apply to the elements

Return value¶

An iterator equal to last.

Complexity¶

\(\scriptsize \mathcal{O}(N\cdot\log{(M)})\)𝓞(N·log(M)) comparisons and twice as
many projections, where \(\scriptsize N\)N is ranges::distance(first, last),
\(\scriptsize M\)M is ranges::distance(first, middle).

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
^ ^ ^

See also¶

ranges::partial_sort_copy copies and partially sorts a range of elements
(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
(C++20) equal elements
(niebloid)
ranges::nth_element partially sorts the given range making sure that it is
(C++20) partitioned by the given element
(niebloid)
ranges::make_heap creates a max heap out of a range of elements
(C++20) (niebloid)
ranges::pop_heap removes the largest element from a max heap
(C++20) (niebloid)
ranges::push_heap adds an element to a max heap
(C++20) (niebloid)
ranges::sort_heap turns a max heap into a range of elements sorted in
(C++20) ascending order
(niebloid)
partial_sort sorts the first N elements of a range
(function template)