4.19~4.20

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4.19

#include<iostream>#include<string>using namespace std;class book{public:book(int a){ qu=a;  price=10*a;}void show() { cout<<qu*price<<endl; }private:int qu;int price;};int main(){book book1[5]={book(1),book(2),book(3),book(4),book(5)};    for(int i=0;i<5;i++){book1[i].show();}return 0;}

4.20

#include<iostream>#include<string>using namespace std;class book{public:book(int a){ qu=a;  price=10*a;}void show() { cout<<qu*price<<endl; }private:int qu;int price;};int main(){book *pr;book book1[5]={book(1),book(2),book(3),book(4),book(5)};pr=&book1[4];pr->show();pr--;pr->show();pr--;pr->show();pr--;pr->show();pr--;pr->show();return 0;}

4.21

4.20#include<iostream>#include<string>using namespace std;class student{public:student(string name1,float  score1){name=name1;score=score1;++count;sum=sum+score;ave=sum/count;}static void show(student &stu){cout<<"xingming:"<<stu.name<<endl;cout<<"chenji:"<<stu.score<<endl;}static void show_count_sum_ave(){cout<<"renshu:"<<count<<endl;cout<<"pinchenji:"<<ave<<endl;}private:string name;float score;static int count;static float sum;static float ave;};int student::count=0;float student::sum=0.0;float student::ave=0.0;int main(){student stu1("linming",90);student::show(stu1);student::show_count_sum_ave();student stu2("zhanghao",80);stu2.show(stu2);stu2.show_count_sum_ave();student stu3("wubeiren",95);stu3.show(stu3);stu3.show_count_sum_ave();return 0;}

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