如何自定义map模板排序

Question

我先定义了一个Stu类：

class Stu {
private:
    string name;
    int id;
    string sex;
    int type;
    float mathscore;
    float engscore;
    float cppscore;
public:
    Stu()=default;
    Stu(string n,int i,string s,float math,float eng,float cpp):name(n),id(i),sex(s),mathscore(math),engscore(eng),cppscore(cpp){};
    ~Stu();

主程序中定义了map<int,Stu>Stu_map;
默认是按id排序，如何实现按mathscore（数学成绩）排序并输出？

Answer 1

map默认是使用key排序的，因为mathScore是value中的值，需要借助vector和sort函数来实现对map的排序。
调用add函数，在map中添加数据及排序后的运行结果：

代码如下：

#include <iostream>
#include <string>
#include <map>
#include <vector>
#include <algorithm>
using namespace std;
class Stu 
{
private:
    string name;
    int id;
    string sex;
    int type;
    float mathscore;
    float engscore;
    float cppscore;
public:
    Stu(){}
    Stu(string n,int i,string s,float math,float eng,float cpp):name(n),id(i),sex(s),mathscore(math),engscore(eng),cppscore(cpp){}
    ~Stu(){}
    //暂时写这几个
    string getname(){return name;}
    int get_id(){return id;};
    float getmathscore() const {return mathscore;}  //不要漏了这里的const 否则cmp函数会报错


    int get_type(){return type;}
    void set_type(int t){type = t;}

    void change_id(int d){id = d;}
    void change_name(char* nn){name = nn;}
    void change_sex(char* sx){sex = sx;}

    void set_mathscore(float f){mathscore = f;}
    void set_engscore(float f){engscore = f;}
    void set_cppscore(float f){cppscore = f;}
};

map<int,Stu>Stu_map;  //声明全局变量，因为add函数中没有参数，所以把这个作为全局变量


bool cmp(const pair<int,Stu> p1,const pair<int,Stu> p2)//要用常数，不然编译错误 
{
    return p1.second.getmathscore() > p2.second.getmathscore();
}

istream &operator>>(istream &is,Stu &stu)
{
    char temp[50];
    double te;
    is.getline(temp,50);
    cout<<"请输入学号："<<endl;
    is.getline(temp,50);
    stu.change_id(atoi(temp)); 
    cout<<"请输入姓名："<<endl; 
    is.getline(temp,20);
    stu.change_name(temp);
    cout<<"请输入性别："<<endl;
    is.getline(temp,10);
    if(!strcmp(temp,"男")||!strcmp(temp,"女"))
    {
        stu.change_sex(temp);
    } else
    {
        cout<<"性别输入错误  Σ( ° △ °|||)︴"<<endl;
    }

    cout<<"科目成绩："<<endl;
    cout<<"输入数学成绩："<<endl;
    cin>>te;
    stu.set_mathscore(te);
    cout<<"输入英语成绩："<<endl;
    cin>>te;
    stu.set_engscore(te);
    cout<<"输入C++成绩："<<endl;
    cin>>te;
    stu.set_cppscore(te);
    return is;
}
void add_stu()
{
    int choice=0;
    Stu stu;
    cout<<"1.添加大一学生"<<endl;
    cout<<"2.添加大二学生"<<endl;
    cout<<"请输入你的选择："<<endl;
    cin>>choice;
    switch (choice) 
    {
    case 1: 
        {
            cin >> stu;
            stu.set_type(1);
            Stu_map.insert(map<int, Stu>::value_type(stu.get_id(), stu));
            cout<<"添加成功"<<endl;
            break;
        }

    case 2: 
        {

            cin >> stu;
            stu.set_type(2);
            Stu_map.insert(map<int, Stu>::value_type(stu.get_id(), stu));
            cout<<"添加成功"<<endl;
            break;
        }

    default:
        break;
    }
}




int main()
{
    //调用函数添加4个数据
    for(int i=0;i<4;i++)
        add_stu();
    map<int,Stu>::iterator it = Stu_map.begin();
    vector< pair<int,Stu> > arr;
    for (; it != Stu_map.end(); it++)
    {
        arr.push_back(make_pair(it->first,it->second) );
    }
    //排序
    sort(arr.begin(),arr.end(),cmp);
    //输出
    vector< pair<int,Stu> >::iterator is = arr.begin();
    for (; is != arr.end(); is++)
    {
        cout << is->first << " " << is->second.getname() << " " << is->second.getmathscore()<<endl;
    }
    return 0;
}