std::ranges::partition
来自cppreference.com
| 在标头 <algorithm> 定义
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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 > |
(1) | (C++20 起) |
| template<ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate< |
(2) | (C++20 起) |
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 | - | 要重排的范围 |
| r | - | 要重排的范围 |
| pred | - | 应用到投影后元素的谓词 |
| proj | - | 应用到谓词的投影 |
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; |
示例
运行此代码
#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
| (C++20) |
判断范围是否已按给定的谓词划分 (niebloid) |
| (C++20) |
将元素分成二组，同时保持其相对顺序 (niebloid) |
| 将范围中的元素分为两组 (函数模板) |