std::count

template <class InputIterator, class T>
  typename iterator_traits<InputIterator>::difference_type
    count (InputIterator first, InputIterator last, const T& val);

Returns the number of elements in the range [first,last) that compare equal to val.

返回first,last之间和元素val相等的元素的个数。

例子：

#include <iostream>
#include <algorithm>
#include <vector>
#include <array>
using namespace std;
void count1(){
    vector<int> vi{1,5,7,8,9,9,8,5,9};
    array<int,7> ai{888,666,555,222,111,555,777};
    cout<<"vi=";
    for(int &i:vi)
        cout<<i<<" ";
    cout<<endl;

    int cv=count(vi.begin(),vi.end(),9);
    cout<<"9 在vi中出现了"<<cv<<"次"<<endl;

    cout<<"ai=";
    for(int &i:ai)
        cout<<i<<" ";
    cout<<endl;
    int ca=count(ai.begin(),ai.end(),555);
    cout<<"555 在ai中出现了"<<ca<<"次"<<endl;




}

运行结果：

The function uses operator== to compare the individual elements to val.

该函数使用operator==来比较各个元素和val是否相等。

The behavior of this function template is equivalent to:

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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;
}

Parameters

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.
比较的范围。

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).
要比较的值。

Return value

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.

匹配的元素的个数。

Example

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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;
}

Output:

10 appears 3 times.
20 appears 3 times.

Complexity

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

Data races

The objects in the range [first,last) are accessed (each object is accessed exactly once).

Exceptions

Throws if either an element comparison or an operation on an iterator throws.

Note that invalid arguments cause undefined behavior.

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转载请注明出处：http://blog.csdn.net/qq844352155

author:天下无双

Email:coderguang@gmail.com

2014-9-11

于GDUT

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