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调用签名
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template<std::bidirectional_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I,
Proj>> Pred>
requires std::permutable<I>
ranges::subrange<I>
stable_partition( I first, S last, Pred pred, Proj
proj = {} );
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(1) |
(C++20 起) |
template<ranges::bidirectional_range R, class
Proj = std::identity,
std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred>
requires std::permutable<ranges::iterator_t<R>>
ranges::borrowed_subrange_t<R>
stable_partition( R&& r, Pred pred, Proj proj = {} );
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(2) |
(C++20 起) |
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1) 重排序范围 [first, last) 中的元素，使得所有谓词
pred 对 proj 投影后返回 true 的元素在所有谓词
pred 对 proj 投影后返回 false
的元素之前。算法是稳定的，即保持元素的相对顺序。
2) 同 (1) ，但以 r
为范围，如同以 ranges::begin(r) 为 first 并以 ranges::end(r) 为 last 。
此页面上描述的仿函数实体是 niebloid，即：
实际上，它们能以函数对象，或者某些特殊编译器扩展实现。
参数
| first, last
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要重排序的元素范围
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| r
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要重排序的元素范围
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| pred
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应用到投影后元素的谓词
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| proj
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应用到元素的投影
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返回值
1) 等于 {pivot, last} 的对象，其中 pivot 是指向第二群首元素的迭代器。
复杂度
给定 N =
ranges::distance(first, last) ，复杂度最坏为
\(\scriptsize N\cdot\log{(N)}\)N·log(N) 次交换，而在使用额外内存时仅
\(\scriptsize \mathcal{O}(N)\)𝓞(N) 次交换。准确应用 \(\scriptsize N\)N 次谓词 pred
和投影 proj 。
注解
此函数尝试分配临时缓冲区。若分配失败，则选择效率较低的算法。
可能的实现
此实现不使用额外内存缓冲区而这样可能较为低效。参阅 MSVC
STL 与 libstdc++
中的实现。
struct stable_partition_fn {
template<std::bidirectional_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
requires std::permutable<I>
ranges::subrange<I>
operator()( I first, S last, Pred pred, Proj proj = {} ) const {
first = ranges::find_if_not(first, last, pred, proj);
I mid = first;
while (mid != last) {
mid = ranges::find_if(mid, last, pred, proj);
if (mid == last) break;
I last2 = ranges::find_if_not(mid, last, pred, proj);
ranges::rotate(first, mid, last2);
first = ranges::next(first, ranges::distance(mid, last2));
mid = last2;
}
return {std::move(first), std::move(mid)};
}
template<ranges::bidirectional_range R, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred>
requires std::permutable<ranges::iterator_t<R>>
ranges::borrowed_subrange_t<R>
operator()( R&& r, Pred pred, Proj proj = {} ) const {
return (*this)(ranges::begin(r), ranges::end(r), std::move(pred), std::move(proj));
}
};
inline constexpr stable_partition_fn stable_partition{};
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示例
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
namespace rng = std::ranges;
template <std::permutable I, std::sentinel_for<I> S>
constexpr void stable_sort(I first, S last)
{
if (first == last)
return;
auto pivot = *rng::next(first, rng::distance(first, last) / 2, last);
auto left = [pivot](const auto& em) { return em < pivot; };
auto tail1 = rng::stable_partition(first, last, left);
auto right = [pivot](const auto& em) { return !(pivot < em); };
auto tail2 = rng::stable_partition(tail1, right);
stable_sort(first, tail1.begin());
stable_sort(tail2.begin(), tail2.end());
}
void print(const auto rem, auto first, auto last, bool end = true) {
std::cout << rem;
for (; first != last; ++first) { std::cout << *first << ' '; }
std::cout << (end ? "\n" : "");
}
int main()
{
const auto original = { 9, 6, 5, 2, 3, 1, 7, 8 };
std::vector<int> vi;
auto even = [](int x) { return 0 == (x % 2); };
print("Original vector:\t", original.begin(), original.end(), "\n");
vi = original;
const auto ret1 = rng::stable_partition(vi, even);
print("Stable partitioned:\t", vi.begin(), ret1.begin(), 0);
print("│ ", ret1.begin(), ret1.end());
vi = original;
const auto ret2 = rng::partition(vi, even);
print("Partitioned:\t\t", vi.begin(), ret2.begin(), 0);
print("│ ", ret2.begin(), ret2.end());
vi = {16, 30, 44, 30, 15, 24, 10, 18, 12, 35};
print("Unsorted vector: ", vi.begin(), vi.end());
stable_sort(rng::begin(vi), rng::end(vi));
print("Sorted vector: ", vi.begin(), vi.end());
}
可能的输出：
Original vector: 9 6 5 2 3 1 7 8
Stable partitioned: 6 2 8 │ 9 5 3 1 7
Partitioned: 8 6 2 │ 5 3 1 7 9
Unsorted vector: 16 30 44 30 15 24 10 18 12 35
Sorted vector: 10 12 15 16 18 24 30 30 35 44
参阅
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将范围中的元素分为二组
(niebloid) |
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复制一个范围，将各元素分为二组
(niebloid) |
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判断范围是否已按给定的谓词划分
(niebloid) |
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将元素分为两组，同时保留其相对顺序
(函数模板) |