std::ranges:: stable_partition

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定义于头文件 `<algorithm>`
调用签名
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 = { } ) ;	(1)	(C++20 起) (C++26 起为 constexpr)
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 = { } ) ;	(2)	(C++20 起) (C++26 起为 constexpr)

1) 对范围

first

last

中的元素进行重新排序，使得所有满足谓词 pred 返回 true 的元素的投影 proj 位于满足谓词 pred 返回 false 的元素的投影 proj 之前。该算法是稳定的，即元素的相对顺序 保持不变 。

2) 与 (1) 相同，但使用 r 作为范围，如同以 ranges:: begin ( r ) 作为 first ，并以 ranges:: end ( r ) 作为 last 。

本页面描述的函数式实体是 算法函数对象 （非正式称为 niebloids ），即：

在调用其中任何函数时，都不能显式指定模板参数列表。
它们对实参依赖查找不可见。
当通过普通非限定查找在函数调用运算符左侧发现其中任何函数名时，将抑制实参依赖查找。

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>
    constexpr 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>>
    constexpr 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 {};

示例

运行此代码

#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

参见

ranges::partition (C++20)	将元素范围划分为两组 (算法函数对象)
ranges::partition_copy (C++20)	复制范围并将元素划分为两组 (算法函数对象)
ranges::is_partitioned (C++20)	判断范围是否按给定谓词划分 (算法函数对象)
stable_partition	将元素划分为两组并保持其相对顺序 (函数模板)

Compiler support
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Feature test macros (C++20)
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Metaprogramming library (C++11)
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Execution control library (C++26)
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External libraries

first, last	-	定义待重排元素范围的迭代器-哨位对
r	-	待重排的元素范围
pred	-	应用于投影元素的谓词
proj	-	应用于元素的投影

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std::ranges:: stable_partition

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示例

参见