C++

参考

<algorithm>

function template
<algorithm>

std::count

template <class InputIterator, class T>  typename iterator_traits<InputIterator>::difference_type    count (InputIterator first, InputIterator last, const T& val);
Count appearances of value in range
Returns the number of elements in the range [first,last) that compare equal to val.

该函数使用 operator== 将单个元素与 val 进行比较。

此函数模板的行为等同于
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template <class InputIterator, class T>
  typename iterator_traits<InputIterator>::difference_type
    count (InputIterator first, InputIterator last, const T& val)
{
  typename iterator_traits<InputIterator>::difference_type ret = 0;
  while (first!=last) {
    if (*first == val) ++ret;
    ++first;
  }
  return ret;
}

参数

first, last
Input iterators to the initial and final positions of the sequence of elements. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
val
Value to match.
T shall be a type supporting comparisons with the elements pointed by InputIterator using operator== (with the elements as left-hand side operands, and val as right-hand side).

返回值

The number of elements in the range [first,last) that compare equal to val.
The return type (iterator_traits<InputIterator>::difference_type) is a signed integral type.

示例

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// count algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::count
#include <vector>       // std::vector

int main () {
  // counting elements in array:
  int myints[] = {10,20,30,30,20,10,10,20};   // 8 elements
  int mycount = std::count (myints, myints+8, 10);
  std::cout << "10 appears " << mycount << " times.\n";

  // counting elements in container:
  std::vector<int> myvector (myints, myints+8);
  mycount = std::count (myvector.begin(), myvector.end(), 20);
  std::cout << "20 appears " << mycount  << " times.\n";

  return 0;
}

输出
10 appears 3 times.
20 appears 3 times.


复杂度

Linear in the distance between first and last: Compares once each element.

数据竞争

范围 [first,last) 中的对象将被访问（每个对象恰好访问一次）。

异常

如果元素比较或迭代器操作引发异常，则抛出。
请注意，无效参数会导致未定义行为。

另见

for_each
将函数应用于范围 （函数模板）
count_if
Return number of elements in range satisfying condition (function template)
find
在范围中查找值 （函数模板）
replace
替换范围中的值 （函数模板）