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

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定义于头文件 `<algorithm>`
调用签名
	(1)
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 > count ( I first, S last, const T & value, Proj proj = { } ) ;			(C++20 起) (C++26 前)
template < std:: input_iterator I, std:: sentinel_for < I > S, class Proj = std:: identity , class T = std :: projected_value_t < I, Proj > > 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 = { } ) ;			(C++26 起)
		(2)
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 = { } ) ;				(C++20 起) (C++26 前)
template < ranges:: input_range R, class Proj = std:: identity , class T = std :: projected_value_t < ranges:: iterator_t < R > , Proj > > 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 = { } ) ;				(C++26 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

返回范围内 [ first , last ) 满足特定条件的元素数量。

1) 统计等于 value 的元素数量。

3) 统计谓词 p 返回 true 的元素数量。

2,4) 与 (1,3) 相同，但使用 r 作为源范围，如同使用 ranges:: begin ( r ) 作为 first ，以及 ranges:: end ( r ) 作为 last 。

本页面描述的函数式实体是 算法函数对象 （非正式称为 niebloids ），即：

在调用其中任何一个时，都不能指定显式模板实参列表。
它们对实参依赖查找均不可见。
当通过普通非限定查找将其中任何一个作为函数调用运算符左侧的名称找到时，会抑制实参依赖查找。

struct count_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity, class T = std::projected_value_t<I, Proj>>
    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 Proj = std::identity
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
    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;

count_if (3)

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 <cassert>
#include <complex>
#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';
    // 使用 lambda 表达式统计能被 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';
    // 使用 lambda 表达式统计能被 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';
    std::vector<std::complex<double>> nums{{4, 2}, {1, 3}, {4, 2}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto c = ranges::count(nums, {4, 2});
    #else
        auto c = ranges::count(nums, std::complex<double>{4, 2});
    #endif
    assert(c == 2);
}

输出：

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

参见

ranges::distance (C++20)	返回迭代器与哨位之间的距离，或范围的起始与末尾之间的距离 (算法函数对象)
views::counted (C++20)	通过迭代器和计数创建子范围 (定制点对象)
ranges::filter_view views::filter (C++20)	由满足谓词的 `range` 元素组成的 `view` (类模板) (范围适配器对象)
count count_if	返回满足特定条件的元素数量 (函数模板)

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first, last	-	定义待检查元素范围的迭代器-哨位对
r	-	待检查元素的范围
value	-	要搜索的值
pred	-	应用于投影元素的谓词
proj	-	应用于元素的投影

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

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