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

< cpp‎ | algorithm‎ | ranges
 
 
算法库
有制约算法及范围上的算法 (C++20)
有制约算法: 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
未初始化存储上的操作
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
C 库
 
有制约算法
不修改序列的操作
ranges::findranges::find_ifranges::find_if_not
修改序列的操作
未初始化存储上的操作
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
 
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)(1,3,5) ,但以 r 为范围,如同以 ranges::begin(r)first 并以 ranges::end(r)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 。

#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

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