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std::partial_sort(3) C++ Standard Libary std::partial_sort(3) NAME std::partial_sort - std::partial_sort Synopsis Defined in header <algorithm> template< class RandomIt > void partial_sort( RandomIt first, RandomIt middle, (1) (const- expr since C++20) RandomIt last ); template< class ExecutionPolicy, class RandomIt > void partial_sort( ExecutionPolicy&& policy, (2) (since C++17) RandomIt first, RandomIt middle, RandomIt last ); template< class RandomIt, class Compare > void partial_sort( RandomIt first, RandomIt middle, (3) (const- expr since C++20) RandomIt last, Compare comp ); template< class ExecutionPolicy, class RandomIt, class Compare > void partial_sort( ExecutionPolicy&& policy, (4) (since C++17) RandomIt first, RandomIt middle, RandomIt last, Compare comp ); 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 un- defined: * [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. (since C++11) * The type of *first is not MoveAssignable. 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. comparison function object (i.e. an object that satis- fies 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); comp - 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 re- gardless 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 \(\scriptsize N \cdot \log(M)\)Nlog(M) compar- isons using operator< (until C++20) std::less{} (since C++20). 3,4) Approximately \(\scriptsize N \cdot \log(M)\)Nlog(M) applica- tions 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::termi- nate 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, Com- pare 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 or- der. To select elements, a heap is used (see heap). For example, for op- erator< as comparison function, max-heap is used to select middle first small- est 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 con- stant 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior P0896R4 C++98 [first, middle) and [middle, last) the behavior is undefined were not required to be valid ranges if any of them is invalid See also partially sorts the given range making sure that it is nth_element partitioned by the given element (function template) partial_sort_copy copies and partially sorts a range of elements (function template) sorts a range of elements while preserving or- der between equal stable_sort elements (function template) sort sorts a range into ascending order (function template) ranges::partial_sort sorts the first N elements of a range (C++20) (niebloid) http://cppreference.com 2024.06.10 std::partial_sort(3)
NAME | Synopsis | Parameters | Type requirements | Complexity | Exceptions | Possible implementation | Notes | Algorithm | Example | Possible output: | See also
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