std::ranges::find, std::ranges::find_if, std::ranges::find_if_not

< cpp‎ | algorithm‎ | ranges

定义于头文件 `<algorithm>`

调用签名
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 find( I first, S last, const T& value, Proj proj = {} );	(1)	(C++20 起)
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> find( R&& r, const T& value, Proj proj = {} );	(2)	(C++20 起)
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 find_if( I first, S last, Pred pred = {}, Proj proj = {} );	(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 > constexpr ranges::borrowed_iterator_t<R> find_if( R&& r, Pred pred = {}, Proj proj = {} );	(4)	(C++20 起)
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 find_if_not( I first, S last, Pred pred = {}, Proj proj = {} );	(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 > constexpr ranges::borrowed_iterator_t<R> find_if_not( R&& r, Pred pred = {}, Proj proj = {} );	(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：

调用它们中的任何一者时不可指定显式模板实参。
它们无一对实参依赖查找可见。
在通常无限定查找寻找函数调用运算符左侧的名称找到它们之一时，它禁止实参依赖查找。

实际上，它们能以函数对象，或以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
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

ranges::adjacent_find (C++20)	查找首对相邻的相同（或满足给定谓词的）元素 (niebloid)
ranges::find_end (C++20)	在特定范围中寻找最后出现的元素序列 (niebloid)
ranges::find_first_of (C++20)	搜索元素集合中的任一元素 (niebloid)
ranges::mismatch (C++20)	寻找两个范围出现不同的首个位置 (niebloid)
ranges::search (C++20)	搜索一个元素范围 (niebloid)
findfind_iffind_if_not (C++11)	寻找首个满足特定判别标准的元素 (函数模板)

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