#include<string>
#include<cmath>
#include<iostream>
#include<cstdlib>
using namespace std;
class Course
{
private:
    friend class Systemgrade;
    Course *next;
    string subject;    //课程名称
    double credit;        //学分
    double score;         //成绩
    string quality;    //成绩性质
    double GPA;         //绩点？
public:
    //创建构造与析构函数
    Course(string sub="",double cre=0,double s=0,string q="");   
    Course(const Course& temp)
    {
        subject=temp.subject;
        credit=temp.credit;
        score=temp.score;
        quality=temp.quality;
        GPA=temp.GPA;
    }
    ~Course() {}
    void output();
    string getSubject() const  {return subject;}      
    int getCredit() const {return credit;}
    double getScore() const  {return score;}
    string getQuality() const   {return quality;}
    double getGPA() const {return GPA;}
    //计算绩点
   
};


Course::Course(string sub,double cre,double s,string q)   
{
    subject=sub;
    credit=cre;
    score=s;
    quality=q;
    if(score-50>=1e-5)
        GPA=(score-50)*0.1;
    else
        GPA=0;
    if(quality=="优秀") GPA=4.0;
    if(quality=="良好") GPA=3.0;
    if(quality=="中等") GPA=2.0;
    if(quality=="及格") GPA=1.0;
    if(quality=="不及格") GPA=0.0;
}

void Course::output()
{
    cout<<subject<<"  "<<credit<<"  "<<GPA<<endl;
}
class Systemgrade
{
    Course *first;
    double ave;
    int tot;
public:
    Systemgrade()
    {
        first=new Course;
        first->next=NULL;
    }
    void add(string sub="",double cre=0,double s=0,string q="");
    void OUT();
    void Add();
    void Ave();
};
void Systemgrade::OUT()
{
    cout<<"   科目  学分  绩点"<<endl;
    Course *p=first->next;
    while(p!=NULL)
    {
        p->output();
        p=p->next;
    }
    cout<<"平均学分绩点"<<endl;
    cout<<ave<<endl;
}

void Systemgrade::add(string sub,double cre,double s,string q)
{
    Course *p=new Course(sub,cre,s,q);
    if(first->next==NULL)
    {
        first->next=p;
    }
    else
    {
        Course *q=first->next;
        first->next=p;
        p->next=q;
    }
}

void Systemgrade::Ave()
{
    Course *p=first->next;
    double sum1=0,sum2=0;
    while(p!=NULL)
    {
        sum1+=p->credit*p->GPA;
        sum2+=p->credit;
        p=p->next;
    }
    ave=sum1/sum2;
}

void Systemgrade::Add()
{
    cout<<"请输入成绩相关信息,按   科目  学分   成绩(等级)"<<endl;
    string sub;
    double cre;
    char a[20];
    cin>>sub>>cre>>a;
    if(a[0]>='0' && a[0]<='9')
    {
        double s=atoi(a);          //将字符串里的数字强制转换成整形数
        add(sub,cre,s);
    }
    else
    {
        string ss=a;
        add(sub,cre,0,ss);
    }
}



int main()
{
//    freopen("in.txt","r",stdin);
    Systemgrade System;
    int menu,sonmenu;
repeat0:
    cout<<"显示所有信息，请按：1"<<endl;
    cout<<"添加信息，请按：2"<<endl;
    cin>>menu;
    switch(menu)
    {
    case 1:
        System.OUT();
        cout<<"返回主菜单，请按：0"<<endl;
        cout<<"直接退出，请按：-1"<<endl;
        cin>>sonmenu;
        if(sonmenu==-1)
            goto repeat11;
        if(sonmenu==0)
            goto repeat0;
    case 2:
repeat1:
        System.Add();
        cout<<"继续，添加请按：1"<<endl;
        cout<<"返回主菜单，请按：0"<<endl;
        cin>>sonmenu;
        if(sonmenu==1)
            goto repeat1;
        if(sonmenu==0)
            goto repeat0;
    }
repeat11:
    return 0;
}



   


上面的能运行 输入课程名、学分和成绩，但是要根据  平均学分绩点（GPA）=    ∑(课程学分数×绩点数×K)
    ________________________________________       ∑课程学分数
计算相应的积点   还要判断所获得的绩点是否合格，能给出相应的结果提示 这三个不会 求帮忙

哪里不会？怎么不会？直接说出来，