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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 >
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 = {} );
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(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 = {} );
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(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 = {} );
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(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 = {} );
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(4) |
(C++20 起) |
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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，或者某些特殊编译器扩展实现。
参数
| first, last
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-
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要检验的元素范围
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| r
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-
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要检验的元素范围
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| value
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-
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要搜索的值
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| pred
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-
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应用到被投影元素的谓词
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| proj
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-
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应用到元素的投影
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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;
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| 版本二
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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;
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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
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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) |
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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) |
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返回满足指定判别标准的元素数
(函数模板) |