C++

參考

<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.

該函式使用 operator== 將單個元素與 val 進行比較。

此函式模板的行為等同於
1
2
3
4
5
6
7
8
9
10
11
 
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;
}

引數

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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
 
// 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;
}

輸出
10 appears 3 times.
20 appears 3 times.


複雜度

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

資料競爭

範圍 [first,last) 中的物件將被訪問（每個物件恰好訪問一次）。

異常

如果元素比較或迭代器操作引發異常，則丟擲。
請注意，無效引數會導致未定義行為。

另見

for_each
將函式應用於範圍 （函式模板）
count_if
Return number of elements in range satisfying condition (function template)
find
在範圍中查詢值 （函式模板）
replace
替換範圍中的值 （函式模板）