18.2.5【STL常用查找算法count_if】

#include<iostream>
#include<cstdlib>
using namespace std;
#include<vector>
#include<algorithm>
#include<string>


/*
     5.2.6 count_if
        按条件统计元素个数
        count_if(iterator beg, iterator end, _Pred);
        // 按条件统计元素出现次数
        // beg 开始迭代器
        // end 结束迭代器
        // _Pred 谓词
*/


class Greater3
{
public:
    bool operator()(int val)
    {
        return val > 3;
    }
};


void test526() //内置数据类型
{
    vector<int> v;
    v.push_back(1);
    v.push_back(2);
    v.push_back(4);
    v.push_back(5);
    v.push_back(2);
    v.push_back(4);
    v.push_back(4);
    v.push_back(3);

    int num = count_if(v.begin(), v.end(), Greater3());
    cout << "v中大于3的元素个数：" << num << endl;
}


class Person
{
public:
    string name;
    int age;

public:
    Person(string _name, int _age)
    {
        this->name = _name;
        this->age = _age;
    }
};


class Greater20
{
public:
    bool operator()(const Person & p)
    {
        return p.age > 20;
    }
};


void test526_1() //自定义数据类型
{
    vector<Person> vv;
    Person p1("aaa", 10);
    Person p2("bbb", 20);
    Person p3("ccc", 30);
    Person p4("ddd", 30);
    Person p5("eee", 40);
    Person p6("fff", 30);
    vv.push_back(p1);
    vv.push_back(p2);
    vv.push_back(p3);
    vv.push_back(p4);
    vv.push_back(p5);
    vv.push_back(p6);

    //统计vv中年龄大于20的有多少人
    int num = count_if(vv.begin(), vv.end(), Greater20());
    cout << "num:" << num << endl;
}


int main()
{
    test526();
    test526_1();

    system("pause");
    return 0;
}