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std::ranges::count, std::ranges::count_if

          class T, class Proj = std::identity >
requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
                                        const T*>
constexpr std::iter_difference_t<I>

  count( 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::range_difference_t<R> count( 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 std::iter_difference_t<I>

  count_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::range_difference_t<R>

  count_if( R&& r, Pred pred = {}, Proj proj = {} );
(4) (C++20 起)

返回范围 [first, last) 中满足特定判别标准的的元素数。

1) 计数等于 value 的元素数。
3) 计数谓词 p 对其返回 true 的元素数。
2,4) 同 (1,3) ，但以 r 为源范围，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，或以某些特殊编译器扩展实现。

目录

参数

first, last - 要检验的元素范围
r - 要检验的元素范围
value - 要搜索的值
pred - 应用到被投影元素的谓词
proj - 应用到元素的投影

返回值

范围中满足条件的元素数。

复杂度

准确比较和应用投影 last - first 次。

注解

对于无任何判别标准的范围中的元素数，见 std::ranges::distance 。

可能的实现

版本一 
struct count_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 std::iter_difference_t<I>
    operator()( I first, S last, const T& value, Proj proj = {} ) const
  {
    std::iter_difference_t<I> counter = 0;
    for (; first != last; ++first) {
      if (std::invoke(proj, *first) == value)
      {
        ++counter;
      }
    }
 
    return counter;
  }
 
  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::range_difference_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 count_fn count;
版本二 
struct count_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 std::iter_difference_t<I>
    operator()( I first, S last, Pred pred = {}, Proj proj = {} ) const
  {
    std::iter_difference_t<I> counter = 0;
    for (; first != last; ++first) {
      if (std::invoke(pred, std::invoke(proj, *first)))
      {
        ++counter;
      }
    }
 
    return counter;
  }
 
  template< ranges::input_range R, class Proj = std::identity,
            std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred >
  constexpr ranges::range_difference_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 count_if_fn count_if;

示例

运行此代码
#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v{ 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
 
    namespace ranges = std::ranges;
 
    // 确定 std::vector 中有多少整数匹配目标值。
    int target1 = 3;
    int target2 = 5;
    int num_items1 = ranges::count(v.begin(), v.end(), target1);
    int num_items2 = ranges::count(v, target2);
    std::cout << "number: " << target1 << " count: " << num_items1 << '\n';
    std::cout << "number: " << target2 << " count: " << num_items2 << '\n';
 
    // 用 lambda 表达式计数被 3 整除的元素。
    int num_items3 = ranges::count_if(v.begin(), v.end(), [](int i){return i % 3 == 0;});
    std::cout << "number divisible by three: " << num_items3 << '\n';
 
    // 用 lambda 表达式计数被 11 整除的元素。
    int num_items11 = ranges::count_if(v, [](int i){return i % 11 == 0;});
    std::cout << "number divisible by eleven: " << num_items11 << '\n';
}

输出： 

number: 3 count: 2
number: 5 count: 0
number divisible by three: 3
number divisible by eleven: 0

参阅

(C++20)
 返回迭代器与哨位间的距离，或范围起始与结尾间的距离
(niebloid)
由另一 view 的到首个谓词返回 false 为止的起始元素组成的 view
(类模板) (范围适配器对象)
返回满足指定判别标准的元素数
(函数模板)