std::partial_sort

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Defined in header <algorithm>
template< class RandomIt >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last );
 (1) (constexpr since C++20)
template< class ExecutionPolicy, class RandomIt >

void partial_sort( ExecutionPolicy&& policy,

                   RandomIt first, RandomIt middle, RandomIt last );
 (2) (since C++17)
template< class RandomIt, class Compare >

void partial_sort( RandomIt first, RandomIt middle, RandomIt last,

                   Compare comp );
 (3) (constexpr since C++20)
template< class ExecutionPolicy, class RandomIt, class Compare >

void partial_sort( ExecutionPolicy&& policy,
                   RandomIt first, RandomIt middle, RandomIt last,

                   Compare comp );
 (4) (since C++17)

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.

1) Elements are sorted with respect to operator<(until C++20)std::less{}(since C++20).
3) Elements are sorted with respect to comp.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

If any of the following conditions is satisfied, the behavior is undefined:

  • [first, middle) or [middle, last) is not a valid range.
(until C++11)
(since C++11)

Parameters

first, last - random access iterators defining the range
middle - random access iterator defining the one-past-the-end iterator of the range to be sorted
policy - the execution policy to use. See execution policy for details.
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1& a, const Type2& b);

While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that an object of type RandomIt can be dereferenced and then implicitly converted to both of them. ​

Type requirements
-
RandomIt must meet the requirements of LegacyRandomAccessIterator.
-
Compare must meet the requirements of Compare.

Complexity

Given M as middle - first, N as last - first:

1,2) Approximately N·log(M) comparisons using operator<(until C++20)std::less{}(since C++20).
3,4) Approximately N·log(M) applications of the comparator comp.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

See also the implementations in libstdc++ and libc++.

partial_sort (1) 
template<typename RandomIt>
constexpr //< since C++20
void partial_sort(RandomIt first, RandomIt middle, RandomIt last)
{
    typedef typename std::iterator_traits<RandomIt>::value_type VT;
    std::partial_sort(first, middle, last, std::less<VT>());
}
partial_sort (3) 
namespace impl
{
    template<typename RandomIt, typename Compare>
    constexpr //< since C++20
    void sift_down(RandomIt first, RandomIt last, const Compare& comp)
    {
        // sift down element at “first”
        const auto length = static_cast<std::size_t>(last - first);
        std::size_t current = 0;
        std::size_t next = 2;
        while (next < length)
        {
            if (comp(*(first + next), *(first + (next - 1))))
                --next;
            if (!comp(*(first + current), *(first + next)))
                return;
            std::iter_swap(first + current, first + next);
            current = next;
            next = 2 * current + 2;
        }
        --next;
        if (next < length && comp(*(first + current), *(first + next)))
            std::iter_swap(first + current, first + next);
    }
 
    template<typename RandomIt, typename Compare>
    constexpr //< since C++20
    void heap_select(RandomIt first, RandomIt middle, RandomIt last, const Compare& comp)
    {
        std::make_heap(first, middle, comp);
        for (auto i = middle; i != last; ++i)
        {
            if (comp(*i, *first))
            {
                std::iter_swap(first, i);
                sift_down(first, middle, comp);
            }
        }
    }
} // namespace impl
 
template<typename RandomIt, typename Compare>
constexpr //< since C++20
void partial_sort(RandomIt first, RandomIt middle, RandomIt last, Compare comp)
{
    impl::heap_select(first, middle, last, comp);
    std::sort_heap(first, middle, comp);
}

Notes

Algorithm

The algorithm used is typically heap select to select the smallest elements, and heap sort to sort the selected elements in the heap in ascending order.

To select elements, a heap is used (see heap). For example, for operator< as comparison function, max-heap is used to select middle − first smallest elements.

Heap sort is used after selection to sort [first, middle) selected elements (see std::sort_heap).

Intended use

std::partial_sort algorithms are intended to be used for small constant numbers of [first, middle) selected elements.

Example

Run this code
#include <algorithm>
#include <array>
#include <functional>
#include <iostream>
 
void print(const auto& s, int middle)
{
    for (int a : s)
        std::cout << a << ' ';
    std::cout << '\n';
    if (middle > 0)
    {
        while (middle-- > 0)
            std::cout << "--";
        std::cout << '^';
    }
    else if (middle < 0)
    {
        for (auto i = s.size() + middle; --i; std::cout << "  ")
        {}
 
        for (std::cout << '^'; middle++ < 0; std::cout << "--")
        {}
    }
    std::cout << '\n';
};
 
int main()
{
    std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
    print(s, 0);
    std::partial_sort(s.begin(), s.begin() + 3, s.end());
    print(s, 3);
    std::partial_sort(s.rbegin(), s.rbegin() + 4, s.rend());
    print(s, -4);
    std::partial_sort(s.rbegin(), s.rbegin() + 5, s.rend(), std::greater{});
    print(s, -5);
}

Possible output: 

5 7 4 2 8 6 1 9 0 3
 
0 1 2 7 8 6 5 9 4 3
------^
4 5 6 7 8 9 3 2 1 0
          ^--------
4 3 2 1 0 5 6 7 8 9
        ^----------

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P0896R4 C++98 [first, middle) and [middle, last)
were not required to be valid ranges 		the behavior is undefined
if any of them is invalid

See also

 partially sorts the given range making sure that it is partitioned by the given element
(function template)
copies and partially sorts a range of elements
(function template)
sorts a range of elements while preserving order between equal elements
(function template)
sorts a range into ascending order
(function template)
(C++20)
 sorts the first N elements of a range
(niebloid)
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