C++count_if

功能描述：

按条件统计元素个数

函数原型：

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

代码示例：

#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
using namespace std;
//常用查找算法 count_if
class Greater20
{
   
public:
       bool operator()(int val)
       {
   
              return val > 20;
       }
};
//统计内置数据类型
void test01()
{
   
       vector<int>v;
       v.push_back(10);
       v.push_back(40);
       v.push_back(30);
       v.push_back(20);
       v.push_back(40);
       v.push_back(20);
       int num= count_if(v.begin(), v.end(), Greater20());
       cout << "大于20的元素个数为：" <<num<< endl;
}
class Person
{
   
public:
       Person(string name,int age)
       {
   
              this->m_Name = name;
              this->m_Age = age;
       }
       string m_Name;
       int m_Age;
};
class AgeGreater20
{
   
public:
       bool operator()(const Person &p)
       {
   
              return p.m_Age > 20;
       }
};
void test02()
{
   
       vector<Person>v;
       Person p1("刘备", 35);
       Person p2("关羽", 35);
       Person p3("张飞", 35);
       Person p4("赵云", 40);
       Person p5("曹操", 20);
       v.push_back(p1);
       v.push_back(p2);
       v.push_back(p3);
       v.push_back(p4);
       v.push_back(p5);
       //统计 大于
       int num= count_if(v.begin(), v.end(), AgeGreater20());
       cout << "大于20岁的人员个数为：" << num << endl;
}
int main()
{
   
       test01();
       test02();
       system("pause");
       return 0;
}