std::ranges::find, std::ranges::find_if, std::ranges::find_if_not
来自cppreference.com
| 在标头 <algorithm> 定义
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| 调用签名 |
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| template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > |
(1) | (C++20 起) |
| template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, |
(2) | (C++20 起) |
| template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(3) | (C++20 起) |
| template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred > |
(4) | (C++20 起) |
| template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(5) | (C++20 起) |
| template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred > |
(6) | (C++20 起) |
返回范围 [first, last) 中首个满足特定判别标准的元素：
1)
find 搜索等于 value 的元素。3)
find_if 搜索谓词 pred 对其返回 true 的元素。5)
find_if_not 搜索谓词 pred 对其返回 false 的元素。2,4,6) 同 (1,3,5) ，但以
r 为范围，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 | - | 要检验的范围 |
| r | - | 要检验的范围 |
| value | - | 要与元素比较的值 |
| pred | - | 应用到投影后元素的谓词 |
| proj | - | 应用到元素的投影 |
返回值
指向满足条件的首个元素的迭代器，或若找不到这种元素则为等于 last 的迭代器。
复杂度
至多应用 last - first 次谓词与投影。
可能的实现
| 版本一 |
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struct find_fn { template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>, const T*> constexpr I operator()( I first, S last, const T& value, Proj proj = {} ) const { for (; first != last; ++first) { if (std::invoke(proj, *first) == value) { return first; } } return first; } template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::borrowed_iterator_t<R> operator()( R&& r, const T& value, Proj proj = {} ) const { return (*this)(ranges::begin(r), ranges::end(r), value, std::ref(proj)); } }; inline constexpr find_fn find; |
| 版本二 |
struct find_if_fn { template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr I operator()( I first, S last, Pred pred = {}, Proj proj = {} ) const { for (; first != last; ++first) { if (std::invoke(pred, std::invoke(proj, *first))) { return first; } } return first; } template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred > 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 find_if_fn find_if; |
| 版本三 |
struct find_if_not_fn { template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr I operator()( I first, S last, Pred pred = {}, Proj proj = {} ) const { for (; first != last; ++first) { if (!std::invoke(pred, std::invoke(proj, *first))) { return first; } } return first; } template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred > 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 find_if_not_fn find_if_not; |
示例
以下示例在 int 的 vector 中寻找 int 。
运行此代码
#include <iostream> #include <algorithm> #include <vector> #include <iterator> int main() { int n1 = 3; int n2 = 5; std::vector<int> v{4, 1, 3, 2}; namespace ranges = std::ranges; auto result1 = ranges::find(v, n1); auto result2 = ranges::find(v.begin(), v.end(), n2); if (result1 != v.end()) { std::cout << "v contains: " << n1 << '\n'; } else { std::cout << "v does not contain: " << n1 << '\n'; } if (result2 != v.end()) { std::cout << "v contains: " << n2 << '\n'; } else { std::cout << "v does not contain: " << n2 << '\n'; } auto is_even = [](int x) { return x % 2 == 0; }; auto divides_13 = [](int x) { return x % 13 == 0; }; auto result3 = ranges::find_if(v.begin(), v.end(), is_even); auto result4 = ranges::find_if(v, divides_13); if (result3 != v.end()) { std::cout << "First even element in v: " << *result3 << '\n'; } else { std::cout << "No even elements in v\n"; } if (result4 != v.end()) { std::cout << "First element divisible by 13 in v: " << *result4 << '\n'; } else { std::cout << "No elements in v are divisible by 13\n"; } auto result5 = ranges::find_if_not(v.begin(), v.end(), divides_13); auto result6 = ranges::find_if_not(v, is_even); if (result5 != v.end()) { std::cout << "First element indivisible by 13 in v: " << *result5 << '\n'; } else { std::cout << "No elements in v are divisible by 13\n"; } if (result6 != v.end()) { std::cout << "First odd element in v: " << *result6 << '\n'; } else { std::cout << "No even elements in v\n"; } }
输出：
v contains: 3 v does not contain: 5 First even element in v: 4 No elements in v are divisible by 13 First element indivisible by 13 in v: 4 First odd element in v: 1
参阅
| (C++20) |
查找首对相邻的相同（或满足给定谓词的）元素 (niebloid) |
| (C++20) |
查找特定范围中最后出现的元素序列 (niebloid) |
| (C++20) |
查找元素集合中的任一元素 (niebloid) |
| (C++20) |
寻找两个范围出现不同的首个位置 (niebloid) |
| (C++20) |
搜索一个元素范围 (niebloid) |
| (C++11) |
寻找首个满足特定判别标准的元素 (函数模板) |