std::partial_sort

 

C++

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Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

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Iterators library

Ranges library (C++20)

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Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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.

The type of *first is not Swappable.	(until C++11)
RandomIt is not ValueSwappable. The type of *first is not MoveConstructible. The type of *first is not MoveAssignable.	(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 \(\scriptsize M\)M as middle - first, \(\scriptsize N\)N as last - first:

1,2) Approximately \(\scriptsize N \cdot \log(M)\)N·log(M) comparisons using operator<(until C++20)std::less{}(since C++20).

3,4) Approximately \(\scriptsize N \cdot \log(M)\)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

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