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调用签名
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template< std::permutable I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I,
Proj>> Pred >
constexpr ranges::subrange<I>
partition( I first, S last, Pred pred, Proj proj
= {} );
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(1) |
(C++20 起) |
template<ranges::forward_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>>
constexpr ranges::borrowed_subrange_t<R>
partition(R&& r,
Pred pred, Proj proj = {});
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(2) |
(C++20 起) |
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1) 重排范围 [first, last) 中的元素，使谓词
pred 对其投影 proj 返回 true 的所有元素在谓词
pred 对其投影 proj 返回 false
的所有元素之前。不保持等价元素的相对顺序。
2) 同 (1) ，但以 r
为源范围，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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-
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要重排的范围
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| r
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-
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要重排的范围
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| pred
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-
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应用到投影后元素的谓词
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| proj
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-
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应用到谓词的投影
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返回值
始于指向第二组首元素的迭代器，终于等于 last 的迭代器的 subrange 。若 r 是非 borrowed_range 类型的右值则 (2) 返回 std::ranges::dangling 。
复杂度
给定 N = ranges::distance(first,last) 。
准确应用 N 次谓词与投影，若 I 实现 ranges::bidirectional_iterator 则至多交换 N/2 次，否则至多交换
N 次。
可能的实现
struct partition_fn {
template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
constexpr ranges::subrange<I>
operator()(I first, S last, Pred pred, Proj proj = {}) const
{
first = ranges::find_if_not(first, last, std::ref(pred), std::ref(proj));
if (first == last) {
return {first, first};
}
auto begin = first;
for (auto i = ranges::next(first); i != last; ++i) {
if (std::invoke(pred, std::invoke(proj, *i))) {
ranges::iter_swap(i, first);
++first;
}
}
return {std::move(begin), std::move(first)};
}
template<ranges::forward_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>>
constexpr ranges::borrowed_subrange_t<R>
operator()(R&& r, Pred pred, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r),
std::ref(pred), std::ref(proj));
}
};
inline constexpr partition_fn partition;
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示例
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
#include <forward_list>
namespace ranges = std::ranges;
template <std::permutable I, std::sentinel_for<I> S>
void quicksort(I first, S last)
{
if(first == last) {
return;
}
auto pivot = *ranges::next(first, ranges::distance(first,last)/2, last);
auto middle1 = ranges::partition(first, last, [pivot](const auto& em){
return em < pivot;
});
auto middle2 = ranges::partition(middle1, last, [pivot](const auto& em){
return !(pivot < em);
});
quicksort(first, middle1);
quicksort(middle2, last);
}
int main()
{
std::vector<int> v = {0,1,2,3,4,5,6,7,8,9};
std::cout << "Original vector:\n ";
for(int elem : v) {
std::cout << elem << ' ';
}
auto it = ranges::partition(v, [](int i){return i % 2 == 0;});
std::cout << "\nPartitioned vector:\n ";
ranges::copy(ranges::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
std::cout << " * ";
ranges::copy(it, ranges::end(v), std::ostream_iterator<int>(std::cout, " "));
std::forward_list<int> fl = {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
std::cout << "\nUnsorted list:\n ";
for(int n : fl) {
std::cout << n << ' ';
}
std::cout << '\n';
quicksort(ranges::begin(fl), ranges::end(fl));
std::cout << "Sorted using quicksort:\n ";
for(int fi : fl) {
std::cout << fi << ' ';
}
std::cout << '\n';
}
输出：
Original vector:
0 1 2 3 4 5 6 7 8 9
Partitioned vector:
0 * 2 4 6 8
Unsorted list:
1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92
Sorted using quicksort:
-4 -4 -8 -5 1 1 3 2 5 1 30 64 6 92
参阅
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判断范围是否已按给定的谓词划分
(niebloid) |
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将元素分成二组，同时保持其相对顺序
(niebloid) |
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将范围中的元素分为两组
(函数模板) |