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

 

C++

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独立和宿主式

语言

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标准库头文件

具名要求

特性测试宏 (C++20)

语言支持库

概念库 (C++20)

诊断库

内存管理库

元编程库 (C++11)

通用工具库

容器库

迭代器库

范围库 (C++20)

算法库

字符串库

文本处理库

数值库

日期和时间库

输入/输出库

文件系统库 (C++17)

并发支持库 (C++11)

执行控制库 (C++26)

技术规范

符号索引

外部库

 

算法库

受约束算法和范围上的算法 (C++20)
受约束算法，例如 ranges::copy, ranges::sort, ...
执行策略 (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
非修改序列操作 批量操作 for_each for_each_n (C++17) 搜索操作 all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n 修改序列操作 复制操作 copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) 交换操作 swap iter_swap swap_ranges 转换操作 replacereplace_if transform replace_copyreplace_copy_if 生成操作 fill fill_n generate generate_n 移除操作 removeremove_if unique remove_copyremove_copy_if unique_copy 顺序更改操作 reverse reverse_copy rotate rotate_copy random_shuffleshuffle (直到 C++17)(C++11) shift_leftshift_right (C++20)(C++20) 采样操作 sample (C++17)	排序和相关操作 分区操作 partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) 排序操作 sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element 二分搜索操作 (在已分区范围上) lower_bound upper_bound equal_range binary_search 集合操作 (在已排序范围上) includes set_union set_intersection set_difference set_symmetric_difference 归并操作 (在已排序范围上) merge inplace_merge 堆操作 push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) 最小/最大操作 max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) 字典序比较操作 lexicographical_compare lexicographical_compare_three_way (C++20) 排列操作 next_permutation prev_permutation is_permutation (C++11) C 库 qsort bsearch
数值操作
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
未初始化内存上的操作
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

受约束算法

此菜单中的所有名称都属于命名空间 std::ranges

非修改序列操作

all_ofany_ofnone_of for_each for_each_n    countcount_if adjacent_find mismatch equal lexicographical_compare    containscontains_subrange (C++23)(C++23) findfind_iffind_if_not find_lastfind_last_iffind_last_if_not (C++23)(C++23)(C++23)   find_end find_first_of search search_n starts_with (C++23) ends_with (C++23)

修改序列操作

copycopy_if copy_n copy_backward    move move_backward shift_leftshift_right (C++23)(C++23)   fill fill_n generate generate_n    transform swap_ranges    shuffle sample removeremove_if replacereplace_if reverse rotate remove_copyremove_copy_if    unique replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy

分区操作

is_partitioned    partition_point    partition    partition_copy    stable_partition

排序操作

is_sorted is_sorted_until     sort stable_sort         partial_sort       nth_element partial_sort_copy

二分搜索操作 (在已排序范围上)

lower_bound         upper_bound         binary_search      equal_range

集合操作 (在已排序范围上)

merge inplace_merge       set_difference set_intersection    set_union          includes set_symmetric_difference

堆操作

is_heap is_heap_until       make_heap           sort_heap push_heap pop_heap

最小/最大操作

max max_element         min min_element         minmax minmax_element     clamp

排列操作

is_permutation      next_permutation    prev_permutation

折叠操作

fold_left (C++23) fold_left_first (C++23)   fold_right (C++23) fold_right_last (C++23)   fold_left_with_iter (C++23) fold_left_first_with_iter (C++23)

数值操作 iota (C++23)             随机数生成 generate_random (C++26)

未初始化存储上的操作

construct_at    destroy destroy_n destroy_at uninitialized_copy    uninitialized_move uninitialized_fill uninitialized_copy_n    uninitialized_move_n uninitialized_fill_n uninitialized_default_construct uninitialized_default_construct_n uninitialized_value_construct uninitialized_value_construct_n

返回类型

in_fun_result in_in_result in_found_result    in_out_result in_in_out_result    in_out_out_result    min_max_result out_value_result (C++23) in_value_result (C++23)

 

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 = {} );			(自 C++20 起) (直到 C++26)
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 = {} );			(自 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 = {} );				(自 C++20 起) (直到 C++26)
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 = {} );				(自 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)	(自 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)	(自 C++20 起)

返回范围 [first, last) 中满足特定条件的元素数量。

1) 计数等于 value 的元素。

3) 计数谓词 p 返回 true 的元素。

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 和 ranges::end(r) 作为 last。

在此页面上描述的类似函数的实体是算法函数对象 (非正式地称为 niebloids)，即

• 当调用它们中的任何一个时，不能指定显式模板参数列表。
• 它们中没有一个对实参依赖查找可见。
• 当通过正常非限定查找找到它们中的任何一个作为函数调用运算符左侧的名称时，实参依赖查找被禁止。

参数

first, last	-	定义要检查元素范围的迭代器-哨兵对
r	-	要检查元素的范围
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

返回值

满足条件的元素数量。

复杂度

恰好 last - first 次比较和投影。

注释

对于范围中没有任何附加条件的元素数量，请参见 std::ranges::distance。

特性测试 宏	值	Std	特性
__cpp_lib_algorithm_default_value_type	202403	(C++26)	算法的列表初始化 (1,2)

可能的实现

count (1)
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;
 
    // determine how many integers in a std::vector match a target value.
    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';
 
    // use a lambda expression to count elements divisible by 3.
    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';
 
    // use a lambda expression to count elements divisible by 11.
    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)	返回迭代器和哨兵之间，或范围的开始和结束之间的距离 (算法函数对象)
views::counted (C++20)	从迭代器和计数创建子范围 (自定义点对象)
ranges::filter_viewviews::filter (C++20)	由满足谓词的range的元素组成的view (类模板) (范围适配器对象)
countcount_if	返回满足特定条件的元素数量 (函数模板)