原文地址：http://www.cplusplus.com/reference/algorithm/count_if/

std::count_if

template <class InputIterator, class Predicate>
  typename iterator_traits<InputIterator>::difference_type
    count_if (InputIterator first, InputIterator last, UnaryPredicate pred);

Returns the number of elements in the range [first,last) for which pred is true.
返回范围内对每个元素调用pred返回true的次数。例子：

#include <iostream>
#include <algorithm>
#include <vector>
#include <array>
using namespace std;
bool ismod2(int i){
    return i%2==0;
}
void countif(){
    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_if(vi.begin(),vi.end(),ismod2);
    cout<<"能被2整除的一共有"<<cv<<"个"<<endl;

    cout<<"ai=";
    for(int &i:ai)
        cout<<i<<" ";
    cout<<endl;
    int ca=count_if(ai.begin(),ai.end(),[](int i){return i>500;});
    cout<<"大于500的一共有"<<ca<<"个"<<endl;




}

运行截图：

The behavior of this function template is equivalent to:

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template <class InputIterator, class UnaryPredicate>
  typename iterator_traits<InputIterator>::difference_type
    count_if (InputIterator first, InputIterator last, UnaryPredicate pred)
{
  typename iterator_traits<InputIterator>::difference_type ret = 0;
  while (first!=last) {
    if (pred(*first)) ++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.
元素的范围。

pred

Unary function that accepts an element in the range as argument, and returns a value convertible to bool. The value returned indicates whether the element is counted by this function.
The function shall not modify its argument.
This can either be a function pointer or a function object.
接受一个元素作为参数兵返回一个bool值的一元函数。

Return value

The number of elements in the range [first,last) for which pred does not return false.
The return type (iterator_traits<InputIterator>::difference_type) is a signed integral type.
返回范围内对每个元素调用pred返回true的次数。

Example

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

bool IsOdd (int i) { return ((i%2)==1); }

int main () {
  std::vector<int> myvector;
  for (int i=1; i<10; i++) myvector.push_back(i); // myvector: 1 2 3 4 5 6 7 8 9

  int mycount = count_if (myvector.begin(), myvector.end(), IsOdd);
  std::cout << "myvector contains " << mycount  << " odd values.\n";

  return 0;
}

Output:

myvector contains 5 odd values.

Complexity

Linear in the distance between first and last: Calls pred once for each element.

Data races

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

Exceptions

Throws if pred trhows or if any of the operations on iterators 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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