std::ranges::adjacent_find() 算法

自 C++20 起

简化

// (1)
constexpr I
    adjacent_find( I first, S last, Pred pred = {}, Proj proj = {} );

// (2)
constexpr ranges::borrowed_iterator_t<R>
    adjacent_find( R&& r, Pred pred = {}, Proj proj = {} );

参数类型是泛型的，并具有以下约束

• I - std::forward_iterator

• S - std::sentinel_for<I>

• Proj - (无)

• Pred:

  • (1) - indirect_binary_predicate<projected<I, Proj>, projected<I, Proj>>
  • (2) - indirect_binary_predicate<projected<ranges::iterator_t<R>, Proj>, projected<ranges::iterator_t<R>, Proj>>

  （为方便阅读，此处省略了 std:: 命名空间）

• (2) - R - std::ranges::input_range

对于所有重载，Proj 模板参数的默认类型为 std::identity。

详细

// (1)
template<
  std::forward_iterator I,
  std::sentinel_for<I> S,
  class Proj = std::identity,
  std::indirect_binary_predicate<
  std::projected<I, Proj>,
  std::projected<I, Proj>> Pred = ranges::equal_to
>
constexpr I adjacent_find( I first, S last, Pred pred = {}, Proj proj = {} );

// (2)
template<
  ranges::forward_range R,
  class Proj = std::identity,
  std::indirect_binary_predicate<
  std::projected<ranges::iterator_t<R>, Proj>,
  std::projected<ranges::iterator_t<R>, Proj>> Pred = ranges::equal_to
>
constexpr ranges::borrowed_iterator_t<R> adjacent_find( R&& r, Pred pred = {}, Proj proj = {} );

在范围 [first; last) 中搜索两个连续相等元素。

• (1) 元素使用 pred 进行比较（在使用投影 proj 进行投影之后）。

• (2) 与 (1) 相同，但使用 r 作为源范围，如同使用 ranges::begin(r) 作为 first 和 ranges::end(r) 作为 last。

本页描述的函数类实体是niebloids。

参数

first last	要检查的元素范围。
r	要检查的元素范围。
pred	应用于投影元素的谓词。
proj	应用于元素的投影。

返回值

指向第一对相同元素的第一个元素的迭代器，即第一个迭代器 `it`，使得 `bool(std::invoke(pred, std::invoke(proj1, *it), std::invoke(proj, *(it + 1))))` 为 `true`。

如果未找到此类元素，则返回等于 last 的迭代器。

复杂度

谓词和投影的精确应用次数为 min((result - first) + 1, (last - first) - 1)，其中 result 是返回值。

异常

(无)

可能的实现

adjacent_find(1) 和 adjacent_find(2)

struct adjacent_find_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_binary_predicate<
                 std::projected<I, Proj>,
                 std::projected<I, Proj>> Pred = ranges::equal_to>
    constexpr I operator()(I first, S last, Pred pred = {}, Proj proj = {}) const
    {
        if (first == last)
            return first;
        auto next = ranges::next(first);
        for (; next != last; ++next, ++first)
            if (std::invoke(pred, std::invoke(proj, *first), std::invoke(proj, *next)))
                return first;
        return next;
    }

    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_binary_predicate<
                 std::projected<ranges::iterator_t<R>, Proj>,
                 std::projected<ranges::iterator_t<R>, Proj>> Pred = ranges::equal_to>
    constexpr ranges::borrowed_iterator_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 adjacent_find_fn adjacent_find;

示例

Main.cpp

#include <algorithm>
#include <functional>
#include <iostream>

int main()
{
    const auto v = {0, 1, 2, 3, 40, 40, 41, 41, 5}; /*
                                ^^          ^^       */
    namespace ranges = std::ranges;

    if (auto it = ranges::adjacent_find(v.begin(), v.end()); it == v.end())
        std::cout << "No matching adjacent elements\n";
    else
        std::cout << "The first adjacent pair of equal elements is at ["
                  << ranges::distance(v.begin(), it) << "] == " << *it << '\n';

    if (auto it = ranges::adjacent_find(v, ranges::greater()); it == v.end())
        std::cout << "The entire vector is sorted in ascending order\n";
    else
        std::cout << "The last element in the non-decreasing subsequence is at ["
                  << ranges::distance(v.begin(), it) << "] == " << *it << '\n';
}

输出

The first adjacent pair of equal elements is at [4] == 40
The last element in the non-decreasing subsequence is at [7] == 41