C++

参考

<unordered_set>

unordered_set

public member function
<unordered_set>

std::unordered_set::count

size_type count ( const key_type& k ) const;
计算具有特定键的元素
Searches the container for elements with a value of k and returns the number of elements found. Because unordered_set containers do not allow for duplicate values, this means that the function actually returns1if an element with that value exists in the container, and zero otherwise.

参数

k
Value of the elements to be counted.
成员类型key_type是容器中元素的类型。在 unordered_set 容器中，它与value_type相同，定义为类模板参数的别名（键).

返回值

1if an element with a value equivalent to k is found, or zero otherwise.

成员类型size_type是一种无符号整型类型。

示例

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// unordered_set::count
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_set<std::string> myset = { "hat", "umbrella", "suit" };

  for (auto& x: {"hat","sunglasses","suit","t-shirt"}) {
    if (myset.count(x)>0)
      std::cout << "myset has " << x << std::endl;
    else
      std::cout << "myset has no " << x << std::endl;
  }

  return 0;
}

输出
myset has hat
myset has no sunglasses
myset has suit
myset has no t-shirt


复杂度

平均情况：常量。
最坏情况：与 容器大小成线性关系。

迭代器有效性

没有变化。

另见

unordered_set::find
获取指向元素的迭代器 (公共成员函数)
unordered_set::size
返回容器大小 (公共成员函数)