Standard Template Library Basis [2] 函数对象和自定义排序

[1] 一个基本的函数对象的使用 三个例子

#include<iostream>
#include<vector>
using namespace std;
void ShowMessage()
{
    cout<<"Hello, Nice to meet you~"<<endl;
}
class MyShowMessage
{
public:
    void operator()()
    {
        cout<<"Hello, Nice to meet you~"<<endl;
    }
};
int main()
{
    //函数对象与自定义排序
    ShowMessage();
    MyShowMessage m;//m叫做函数对象
    m();//调用函数对象的成员函数
    
    (MyShowMessage())();//以上可以简写为这样
    return 0;
}

另外一个例子（重载括号）

#include<iostream>
#include<vector>
using namespace std;
int AddTen(int n)
{
    return n+10;
}
class MyAddTen
{
public:
    int operator()(int n)
    {
        return n+10;
    }
};
int main()
{
    int a=AddTen(90);
   
    MyAddTen m;
    a=m(990);
   
    a=((MyAddTen()))(9990);//使用临时对象
    
    return 0;
}

再看一个具体的例子：若想要实现和下面这个同样的功能：

#include<iostream>
#include<vector>
using namespace std;
bool GreaterThan(int a, int b)
{
    return a>b;
}
class MyGreaterThan
{
public:
    bool operator()(int a, int b)
    {
        return a>b;
    }
};
int main()
{
    cout<<GreaterThan(1,9)<<endl;
    MyGreaterThan m;
    cout<<m(10,9)<<endl;
    cout<<((MyGreaterThan()))(10,9)<<endl;
    return 0;
}

[2] functional 头文件中已经存在的可以用的函数对象：greater和less

#include<iostream>
#include<vector>
#include<functional>
//很多函数对象已经帮我写好了！greater接收两个数 less也一样
using namespace std;
int main()
{
    greater<int> g;
    cout<<g(10,9)<<endl;
    cout<<((greater<int>()))(10,90)<<endl;
    
    greater<double> g1;
    cout<<g1(10.2,9.4)<<endl;
    cout<<((greater<double>()))(10.2,90.9)<<endl;
    
    return 0;
}

[3] 自定义排序 sort 方法一：直接用函数

#include<iostream>
#include<vector>
#include<cmath>
#include<algorithm>//涉及算法，我们这里直接把sort拿来排序
using namespace std;
struct Point
{
    int x;
    int y;
    int z;
};
bool LessThanPoint(Point a, Point b)
{
    return sqrt(a.x*a.x+a.y*a.y+a.z*a.z)<sqrt(b.x*b.x+b.y*b.y+b.z*b.z);
}

void PrintPoint(Point a)
{
    printf("Point: x=%05d, y=%05d, z=%05d.",a.x,a.y,a.z);
    printf("The distance is:%6.4lf\n",sqrt(a.x*a.x+a.y*a.y+a.z*a.z));
}//方便输出而写的代码
int main()
{
    Point v[]=
    {
        {3,4,5},
        {-3,23,9},
        {1,1,1},
        {2,2,2},
        {3,3,3},
        {3,4,5},
        {5,5,5},
        {6,6,6},
        {7,7,7},
        {8,8,8},
    };
    sort(v,v+10,LessThanPoint);//按照自定义的规则，排序
    
    return 0;
}

[4] 自定义排序 sort 方法二：使用函数对象的自定义排序 [代码有误..]

#include<iostream>
#include<vector>
#include<cmath>
#include<algorithm>//涉及算法，我们这里直接把sort拿来排序
using namespace std;
struct Point
{
    int x;
    int y;
    int z;
};
class MyLessThanPoint
{
public:
    bool operator()(Point a, Point b)
    {
        return sqrt(a.x*a.x+a.y*a.y+a.z*a.z)<sqrt(b.x*b.x+b.y*b.y+b.z*b.z);
    }
};


void PrintPoint(Point a)
{
    printf("Point: x=%05d, y=%05d, z=%05d.",a.x,a.y,a.z);
    printf("The distance is:%6.4lf\n",sqrt(a.x*a.x+a.y*a.y+a.z*a.z));
}//方便输出而写的代码
int main()
{
    Point v[]=
    {
        {3,4,5},
        {-3,23,9},
        {1,1,1},
        {2,2,2},
        {3,3,3},
        {3,4,5},
        {5,5,5},
        {6,6,6},
        {7,7,7},
        {8,8,8},
    };
    /*MyLessThanPoint m;
    sort(v,v+10,m);*/
    //按照函数对象的规则，排序,可以简写成下面一行语句:
    sort(v,v+10,MyLessThanPoint());
    for(int i=0;i<10;i++)
    {
        PrintPoint(v[i]);
    }
    return 0;
}

[5] 自定义排序 sort 方法三：直接重载小于号！对于vector也适用 [代码有误..]

#include<iostream>
#include<vector>
#include<cmath>
#include<algorithm>//涉及算法，我们这里直接把sort拿来排序
using namespace std;
struct Point
{
    int x;
    int y;
    int z;
    bool operator <(const Point &a)
    {
        return sqrt(this->x*this->x+this->y*this->y+this->z*this->z)<sqrt(a.x*a.x+a.y*a.y+a.z*a.z);
    }
};
void PrintPoint(Point a)
{
    printf("Point: x=%05d, y=%05d, z=%05d.",a.x,a.y,a.z);
    printf("The distance is:%6.4lf\n",sqrt(a.x*a.x+a.y*a.y+a.z*a.z));
}//方便输出而写的代码
int main()
{
    Point v[]=
    {
        {3,4,5},
        {-3,23,9},
        {1,1,1},
        {2,2,2},
        {3,3,3},
        {3,4,5},
        {5,5,5},
        {6,6,6},
        {7,7,7},
        {8,8,8},
    }; 
    sort(v,v+10);//已经有了小于关系，所以就不用再进行排序了
    for(int i=0;i<10;i++)
    {
        PrintPoint(v[i]);
    }
    return 0;
}

[6] 简单应用&总结：从大到小的排序 自定义或者greater（记得头文件functional）

#include<iostream>
#include<vector>
#include<cmath>
#include<algorithm>//涉及算法，我们这里直接把sort拿来排序
#include<functional>
using namespace std;
bool GreaterThan(int a, int b)
{
    return a>b;
}
int main()
{
    int v[]={23,13,43,564,-4,5,-23,44,55,13,-33};
    sort(v,v+11,greater<int>());
    for(int i=0;i<11;i++)
    {
        cout<<v[i]<<" ";
    }
    return 0;
}

[7] 考题运用：将学生的成绩从高到低排序，成绩相同按某科成绩排序，成绩均相同，按照ID排序

#include<iostream>
#include<vector>
#include<cmath>
#include<algorithm>//涉及算法，我们这里直接把sort拿来排序
#include<functional>
using namespace std;
struct Student
{
    int ID;
    int Math;
    int English;
};
bool MyCmp(Student x, Student y)
{
    if((x.Math+x.English)!=(y.Math+y.English))
    {
        return (x.Math+x.English)>(y.Math+y.English);
    }
    else if(x.Math!=y.Math)
    {
        return (x.Math)>(y.Math);
    }
    else
    {
        return x.ID<y.ID;
    }
}
void PrintStudent(Student s)
{
    printf("Student:Total %7d Math %3d ID %2d\n",s.English+s.Math,s.Math,s.ID);
}
int main()
{
    Student v[]=
    {{90,45,21},{89,55,88},{99,10,33},{21,23,97},{4,20,44},{8,50,14},{70,45,21},{9,80,87},{33,20,90}
    };
    sort(v,v+9,MyCmp);
    for(int i=0;i<9;i++)
    {
        PrintStudent(v[i]);
    }
    
    return 0;
}