非修改性序列算法之count和count_if

count主要用来统计容器内元素个数，其格式有两种：

第一种，第三个参数为元素值

template<class _InIt,    class _Ty> inline    typename iterator_traits<_InIt>::difference_type        count(_InIt _First, _InIt _Last, const _Ty& _Val)

第二种，第三个参数为条件函数：

template<class _InIt,    class _Pr> inline    typename iterator_traits<_InIt>::difference_type        count_if(_InIt _First, _InIt _Last, _Pr _Pred)

程序实例：

#include<iostream>#include<functional>#include<algorithm>#include<vector>template<typename T>int PushNum(T &vec, int first, int last) {    int ret = 0;    if (first > last) {        ret = -1;        cout << "function PushNum first > last error:" << ret << endl;    }    while (first <= last) {        vec.push_back(first);        first++;    }    return ret;}template<typename T>void print(T &ele) {    cout << ele << " ";}bool isEven(int &ele) {    return ele % 2 == 0;}using namespace std;int main(){    vector<int> vec;    PushNum(vec, 1, 10);    for_each(vec.begin(), vec.end(), print<int>);    cout << endl;    int count1 = count(vec.begin(), vec.end(), 5);    int count2 = count_if(vec.begin(), vec.end(), isEven);    int count3 = count_if(vec.begin(), vec.end(), bind2nd(greater<int>(), 2));    cout << count1 << endl;    cout << count2 << endl;    cout << count3 << endl;    system("pause");    return 0;}

输出结果：

第一个count是求元素值等于5的个数，第二个count是求满足偶数的个数，第三个count是求大于2的个数。