template< class RandomIt >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last );
void partial_sort( ExecutionPolicy&& policy,
void partial_sort( RandomIt first, RandomIt middle, RandomIt last,
void partial_sort( ExecutionPolicy&& policy,
RandomIt first, RandomIt middle, RandomIt last,
Rearranges elements such that the range [
first,
middle)
contains the sorted middle â 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.
Elements are
sortedwith 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 all following conditions are satisfied:
If any of the following conditions is satisfied, the behavior is undefined:
[
first,
middle)
or [
middle,
last)
is not a valid range.[
first,
middle)
will contain sorted elements policy - the execution policy to use comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns âtrue 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. â
RandomIt
must meet the requirements of LegacyRandomAccessIterator. -Compare
must meet the requirements of Compare. [edit] 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.
[edit] ExceptionsThe overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the standard policies, std::terminate is called. For any other ExecutionPolicy
, the behavior is implementation-defined.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 âfirstâ 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); }[edit] Notes [edit] 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 â first smallest elements.
Heap sort is used after selection to sort [
first,
middle)
selected elements (see std::sort_heap).
std::partial_sort
algorithms are intended to be used for small constant numbers of [
first,
middle)
selected elements.
#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 ^----------[edit] 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)
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