std::ranges::count, std::ranges::count_if
| 定义于头文件 <algorithm>
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| 调用签名 |
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| template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > |
(1) | (C++20 起) |
| template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, |
(2) | (C++20 起) |
| template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(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 > |
(4) | (C++20 起) |
返回范围 [first, last) 中满足特定判别标准的的元素数。
value 的元素数。p 对其返回 true 的元素数。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。
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 |
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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
#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) |
