std::ranges::find_first_of

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< cpp‎ | algorithm‎ | ranges
 
 
算法库
受约束算法及范围上的算法 (C++20)
受约束算法: std::ranges::copy, std::ranges::sort, ...
执行策略 (C++17)
不修改序列的操作
(C++11)(C++11)(C++11)
(C++17)
修改序列的操作
Partitioning operations
划分操作
排序操作
(C++11)
二分搜索操作
集合操作(在已排序范围上)
堆操作
(C++11)
最小/最大操作
(C++11)
(C++17)

排列
数值运算
未初始化存储上的操作
(C++17)
(C++17)
(C++17)
C 库
 
受约束算法
不修改序列的操作
修改序列的操作
划分操作
排序操作
二分搜索操作
集合操作(在已排序范围上)
堆操作
最小/最大操作
排列
未初始化存储上的操作
返回类型
 
在标头 <algorithm> 定义
调用签名
template< std::input_iterator I1, std::sentinel_for<I1> S1,

          std::forward_iterator I2, std::sentinel_for<I2> S2,
          class Pred = ranges::equal_to,
          class Proj1 = std::identity,
          class Proj2 = std::identity >
requires  std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr I1 find_first_of( I1 first1, S1 last1, I2 first2, S2 last2,

                            Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
(1) (C++20 起)
template< ranges::input_range R1, ranges::forward_range R2,

          class Pred = ranges::equal_to,
          class Proj1 = std::identity,
          class Proj2 = std::identity >
requires  std::indirectly_comparable<ranges::iterator_t<R1>,
                                     ranges::iterator_t<R2>,
                                     Pred, Proj1, Proj2>
constexpr ranges::borrowed_iterator_t<R1>
  find_first_of( R1&& r1, R2&& r2, Pred pred = {},

                 Proj1 proj1 = {}, Proj2 proj2 = {} );
(2) (C++20 起)
1) 在范围 [first1, last1) 中搜索任何范围 [first2, last2) 中的元素,分别以 proj1proj2 投影范围。用二元谓词 pred 比较投影后的元素。
2)(1) ,但以 r1 为第一源范围并以 r2 为第二源范围,如同以 ranges::begin(r1)first1 ,以 ranges::end(r1)last1 ,以 ranges::begin(r2)first2 ,并以 ranges::end(r2)last2

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

first1, last1 - 要检验的元素范围(又称草堆
first2, last2 - 要搜索的元素范围(又称
r1 - 要检验的元素范围(又称草堆
r2 - 要搜索的元素范围(又称
pred - 比较元素的二元谓词
proj1 - 应用到第一范围中元素的投影
proj2 - 应用到第二范围中元素的投影

返回值

指向范围 [first1, last1) 中首个在投影后等于来自范围 [first2, last2) 中元素的迭代器。若找不到这种元素,则返回等于 last1 的迭代器。

复杂度

至多应用 (S*N) 次比较和各自的投影,其中
(1) S = ranges::distance(first2, last2)N = ranges::distance(first1, last1)
(2) S = ranges::size(r2)N = ranges::size(r1)

可能的实现

struct find_first_of_fn {
 
  template<std::input_iterator I1, std::sentinel_for<I1> S1,
           std::forward_iterator I2, std::sentinel_for<I2> S2,
           class Pred = ranges::equal_to,
           class Proj1 = std::identity,
           class Proj2 = std::identity>
  requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
  constexpr I1 operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
                          Proj1 proj1 = {}, Proj2 proj2 = {}) const {
    for (; first1 != last1; ++first1)
      for (auto i = first2; i != last2; ++i)
        if (std::invoke(pred, std::invoke(proj1, *first1), std::invoke(proj2, *i)))
          return first1;
    return first1;
  }
 
  template<ranges::input_range R1, ranges::forward_range R2,
           class Pred = ranges::equal_to,
           class Proj1 = std::identity,
           class Proj2 = std::identity>
    requires std::indirectly_comparable<ranges::iterator_t<R1>,
                                        ranges::iterator_t<R2>,
                                        Pred, Proj1, Proj2>
    constexpr ranges::borrowed_iterator_t<R1>
      operator()(R1&& r1, R2&& r2, Pred pred = {},
                 Proj1 proj1 = {}, Proj2 proj2 = {}) const {
      return (*this)(ranges::begin(r1), ranges::end(r1),
                     ranges::begin(r2), ranges::end(r2),
                     std::move(pred), std::move(proj1), std::move(proj2));
    }
};
 
inline constexpr find_first_of_fn find_first_of{};

示例

#include <algorithm>
#include <iostream>
#include <iterator>
 
int main()
{
    namespace rng = std::ranges;
 
    constexpr static auto haystack = {1, 2, 3, 4};
    constexpr static auto needles  = {0, 3, 4, 3};
 
    constexpr auto found1 = rng::find_first_of(haystack.begin(), haystack.end(),
                                               needles.begin(), needles.end());
    static_assert(std::distance(haystack.begin(), found1) == 2);
 
    constexpr auto found2 = rng::find_first_of(haystack, needles);
    static_assert(std::distance(haystack.begin(), found2) == 2);
 
 
    constexpr static auto negatives = {-6, -3, -4, -3};
    constexpr auto not_found = rng::find_first_of(haystack, negatives);
    static_assert(not_found == haystack.end());
 
    constexpr auto found3 = rng::find_first_of(haystack, negatives,
        [](int x, int y) { return x == -y; }); // 使用二元比较器
    static_assert(std::distance(haystack.begin(), found3) == 2);
 
 
    struct P { int x, y; };
    constexpr static auto p1 = { P{1, -1}, P{2, -2}, P{3, -3}, P{4, -4} };
    constexpr static auto p2 = { P{5, -5}, P{6, -3}, P{7, -5}, P{8, -3} };
 
    // 仅比较 P::y 数据成员,通过投影它们:
    const auto found4 = rng::find_first_of(p1, p2, {}, &P::y, &P::y);
    std::cout << "First equivalent element {" << found4->x << ", " << found4->y
              << "} was found at position " << std::distance(p1.begin(), found4)
              << ".\n";
}

输出:

First equavalent element {3, -3} was found at position 2.

参阅

搜索元素集合中的任意元素
(函数模板)
查找首对相邻的相同(或满足给定谓词的)元素
(niebloid)
查找满足特定条件的的第一个元素
(niebloid)
查找特定范围中最后出现的元素序列
(niebloid)
搜索一个元素范围
(niebloid)
在范围中搜索一定量的某个元素的连续副本
(niebloid)