std::partial_sort
| Defined in header <algorithm>
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||
template< class RandomIt >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last );
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(1) | (constexpr since C++20) |
template< class RandomIt, class Compare >
void partial_sort( RandomIt first, RandomIt middle, RandomIt last,
Compare comp );
|
(2) | (constexpr since C++20) |
template< class ExecutionPolicy, class RandomIt >
void partial_sort( ExecutionPolicy&& policy,
RandomIt first, RandomIt middle, RandomIt last );
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(3) | (since C++17) |
template< class ExecutionPolicy, class RandomIt, class Compare >
void partial_sort( ExecutionPolicy&& policy,
RandomIt first, RandomIt middle, RandomIt last,
Compare comp );
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(4) | (since C++17) |
Places the first middle - first elements from the target range [first, last) as sorted into the range [first, middle). The rest of the elements are placed in the range [middle, last) in an unspecified order.
comp.policy.true:
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(until C++20) |
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(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.
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(until C++11) |
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(since C++11) |
Parameters
| first, last | - | the pair of iterators defining the target range |
| middle | - | the end of the range of sorted elements |
| 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:
While the signature does not need to have |
| policy | - | the execution policy to use |
| Type requirements | ||
-RandomIt must meet the requirements of LegacyRandomAccessIterator.
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-Compare must meet the requirements of Compare.
| ||
Complexity
Given
- M as
middle - first, - N as
last - first:
operator<(until C++20)std::less{}(since C++20).comp.operator<(until C++20)std::less{}(since C++20).comp.Exceptions
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
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 (2) |
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);
}
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Notes
std::partial_sort algorithms are intended to be used for small constant numbers of [first, middle) selected elements.
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).
Example
#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 |