std::ranges:: partition

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

1) 对范围

first

last

中的元素进行重排，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。不保留元素的相对顺序。

2) 与 (1) 相同，但使用 r 作为源范围，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：

在调用其中任何一个函数时，都不能显式指定模板参数列表。
它们对实参依赖查找均不可见。
当通过常规非限定查找将其中任何一个函数作为函数调用运算符左侧的名称找到时，会抑制实参依赖查找。

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};
        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(first), std::move(last)};
    }
    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 <forward_list>
#include <functional>
#include <iostream>
#include <iterator>
#include <ranges>
#include <vector>
namespace ranges = std::ranges;
template<class I, std::sentinel_for<I> S, class Cmp = ranges::less>
requires std::sortable<I, Cmp>
void quicksort(I first, S last, Cmp cmp = Cmp {})
{
    using reference = std::iter_reference_t<I>;
    if (first == last)
        return;
    auto size = ranges::distance(first, last);
    auto pivot = ranges::next(first, size - 1);
    ranges::iter_swap(pivot, ranges::next(first, size / 2));
    auto tail = ranges::partition(first, pivot, [=](reference em)
    {
        return std::invoke(cmp, em, *pivot); // em < pivot
    });
    ranges::iter_swap(pivot, tail.begin());
    quicksort(first, tail.begin(), std::ref(cmp));
    quicksort(ranges::next(tail.begin()), last, std::ref(cmp));
}
int main()
{
    std::ostream_iterator<int> cout {std::cout, " "};
    std::vector<int> v {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::cout << "Original vector:  \t";
    ranges::copy(v, cout);
    auto tail = ranges::partition(v, [](int i) { return i % 2 == 0; });
    std::cout << "\nPartitioned vector: \t";
    ranges::copy(ranges::begin(v), ranges::begin(tail), cout);
    std::cout << "│ ";
    ranges::copy(tail, cout);
    std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\nUnsorted list: \t\t";
    ranges::copy(fl, cout);
    quicksort(ranges::begin(fl), ranges::end(fl), ranges::greater {});
    std::cout << "\nQuick-sorted list: \t";
    ranges::copy(fl, cout);
    std::cout << '\n';
}

可能的输出：

Original vector:        0 1 2 3 4 5 6 7 8 9
Partitioned vector:     0 8 2 6 4 │ 5 3 7 1 9
Unsorted list:          1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92
Quick-sorted list:      92 64 30 6 5 3 2 1 1 1 -4 -4 -5 -8

参见

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)
ranges::stable_partition (C++20)	将元素划分为两组并保持其相对顺序 (算法函数对象)
partition	将元素范围划分为两组 (函数模板)

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first, last	-	定义待重排元素范围的区间的迭代器-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:: partition

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