std::ranges::count_if() 算法

自 C++20 起

简化

// (1)
constexpr std::iter_difference_t<I>
    count_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
constexpr ranges::range_difference_t<R>
    count_if( R&& r, Pred pred, Proj proj = {} );

参数类型是泛型的，并具有以下约束

• I - std::input_iterator
• S - std::sentinel_for<I>
• R - std::ranges::input_range
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::projected<ranges::iterator_t<R>, Proj>>
• P - (无)

对于所有重载，Proj 模板参数的默认类型为 std::identity。

详细

// (1)
template<
	std::input_iterator I,
	std::sentinel_for<I> S,
	std::indirect_unary_predicate<std::projected<I, Proj>> Pred
	class Proj = std::identity
>
constexpr std::iter_difference_t<I>
    count_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
template<
	ranges::input_range R,
	std::indirect_unary_predicate<std::projected<I, Proj>> Pred
	class Proj = std::identity
>
constexpr ranges::range_difference_t<R>
    count_if( R&& r, Pred pred, Proj proj = {} );

返回给定范围内满足特定条件的元素数量。

• (1) 计算谓词 p 返回 true 的元素。
• (2) 与 (1) 相同，但使用 r 作为源范围，如同使用 ranges::begin(r) 作为 first 和 ranges::end(r) 作为 last。

本页描述的函数类实体是niebloids。

参数

first last	要检查的元素范围。
r	要检查的元素范围。
pred	应用于投影元素的谓词。
proj	应用于元素的投影。

返回值

• (1 - 2) 谓词 p 为 true 的元素数量。

复杂度

给定 N 为 ranges::distance(first, last)

• (1 - 2) p 和投影的调用次数恰好为 N。

异常

(无)

可能的实现

ranges::count_if

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;

备注

如果您想获取范围 [first; last) 或 r 中元素的数量，且没有任何额外条件，请使用 ranges::distance。

示例

Main.cpp

#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;

    // determine how many integers in a std::vector match a target value.
    int target1 = 3;
    int target2 = 5;
    int num_items1 = ranges::count_if(v.begin(), v.end(), target1);
    int num_items2 = ranges::count_if(v, target2);
    std::cout << "number: " << target1 << " count_if: " << num_items1 << '\n';
    std::cout << "number: " << target2 << " count_if: " << num_items2 << '\n';

    // use a lambda expression to count_if elements divisible by 3.
    int num_items3 = ranges::count_if_if(v.begin(), v.end(), [](int i) {return i % 3 == 0;});
    std::cout << "number divisible by three: " << num_items3 << '\n';

    // use a lambda expression to count_if elements divisible by 11.
    int num_items11 = ranges::count_if_if(v, [](int i) {return i % 11 == 0;});
    std::cout << "number divisible by eleven: " << num_items11 << '\n';
}

输出

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