【Luogu】UVA12412 A Typical Homework (a.k.a 师兄帮帮忙)

本人 AC 的第一道大模拟，写篇题解纪念一下。

---

这是一道要你写班级系统的题目。

对于每个学生，先用一个结构体写下。

struct stu{
	int rk,s1,s2,s3,s4,sum,id;
	string sid,cid,name;
	double avg;//avg 为平均分
	bool in()//内嵌输入函数
	{
		cin>>sid;
		if(sid=="0") return false;
		cin>>cid>>name>>s1>>s2>>s3>>s4;
		sum=s1+s2+s3+s4;
		avg=sum/4.00;//即时算出平均分
		id=++_id;
		return true;
	}
};
vector<stu>v;//出于方便，使用 STL vector

可以发现，每次开始，都有一次 Welcome... 的语句。那我们使用一个函数存起来，方便使用。

void wel()
{
	puts("Welcome to Student Performance Management System (SPMS).\n\n1 - Add\n2 - Remove\n3 - Query\n4 - Show ranking\n5 - Show Statistics\n0 - Exit\n");
}

那么主程序，我们可以这么写：

int main()
{
	wel();
	int op;
	cin>>op;
	while(1)
	{
		switch(op)
		{
			case 0://结束操作
				exit(0);
			case 1://添加学生
				add();
				break;
			case 2://删除学生
				del();
				break;
			case 3://查找学生
				qry();
				break;
			case 4://显示排行
				shw();
				break;
			case 5://显示成绩
				stc();
		}
		wel();//每一次操作完都要一次输出
		cin>>op;
	}
}

添加学生

使用之前的 in() 函数，判断为零时退出，否则判重来确定可否添加。

bool add_sid(string sid)
{
    for(int i=0;i<v.size();i++)
	{
        if(sid==v[i].sid) return false;//判重（名字是可以重复的）
    }
    return true;
}

void add()
{
	while(1)
	{
    	puts("Please enter the SID, CID, name and four scores. Enter 0 to finish.");
   		stu t;
    	if(!t.in()) break;//退出
    	else if(!add_sid(t.sid)) puts("Duplicated SID.");//重复
        else v.push_back(t);//合法
    }
}

删除学生

利用 vector 的 pop_back() 函数，再排一遍序，使得高效删除。

int del_rem(string s)
{
	int res=0;
	for(int i=0;i<v.size();i++)
	{
		if(v[i].name==s||v[i].sid==s)//匹配成功
		{
			swap(v[i--],v[(int)v.size()-1]);
			v.pop_back();
			res++;//删除人数增加
			sort(v.begin(),v.end(),cmp);//再次排序
		}
	}
	return res;
}

void del()
{
	while(1)
	{
		puts("Please enter SID or name. Enter 0 to finish.");
		string s;
		cin>>s;
		if(s=="0") break;
		else cout<<del_rem(s)<<" student(s) removed."<<endl;//直接调用函数
	}
}

查找学生

最难的部分之一。

一边查找，一边处理排名。

同时浮点数精度会有误差，需要增加 $10^{-5}$。

void qry_fnd(string s)
{
	vector<stu>vt;
	for(int i=0;i<v.size();i++)
	{
		if(v[i].name==s||v[i].sid==s)//匹配成功
		{
			stu t;
			t=v[i];
			t.rk=1;
			for(int j=0;j<v.size();j++)
			{
				if (v[j].sum>t.sum) t.rk++;//处理排名
			}
			vt.push_back(t);
		}
	}
	for(int i=0;i<vt.size();i++)
	{
    	cout<<vt[i].rk<<' '<<vt[i].sid<<' '<<vt[i].cid<<' '<<vt[i].name<<' '<<vt[i].s1<<' '<<vt[i].s2<<' '<<vt[i].s3<<' '<<vt[i].s4<<' '<<vt[i].sum<<' '<<fixed<<setprecision(2)<<vt[i].avg+exp/*这里的 exp 就是 10^(-5)*/<<endl;
	}
}

void qry()
{
	while(1)
	{
    	puts("Please enter SID or name. Enter 0 to finish.");
    	string s;
    	cin>>s;
   		if(s=="0") break;
    	else qry_fnd(s);
	}
}

显示排名

输出那句话即可。

void shw()
{
	puts("Showing the ranklist hurts students' self-esteem. Don't do that.");
}

显示成绩

最难的部分之二。

样例有误，一些地方是有空行的（详见代码）。

暴力查找计数，当然也要处理精度误差。

void stc_out(string s)
{
	int s1s,s2s,s3s,s4s,s1p,s2p,s3p,s4p,s1f,s2f,s3f,s4f,p[10],k;//p_i 为通过 i 科人数
	double s1a,s2a,s3a,s4a;
	s1s=s2s=s3s=s4s=s1p=s2p=s3p=s4p=s1f=s2f=s3f=s4f=k=0;
	memset(p,0,sizeof(p));
	for(auto i:v)
	{
		if(i.cid==s||s=="0")
		{
			int ps=0;
 			k++;
			s1s+=i.s1;
			s2s+=i.s2;
			s3s+=i.s3;
			s4s+=i.s4;
			if(i.s1>=60)
			{
				s1p++;
				ps++;
			}
			else s1f++;
			if(i.s2>=60)
			{
				s2p++;
				ps++;
			}
			else s2f++;
			if(i.s3>=60)
			{
				s3p++;
				ps++;
			}
			else s3f++;
			if(i.s4>=60)
			{
				s4p++;
				ps++;
			}
			else s4f++;
			p[ps]++;
        }
    }
    s1a=(double)s1s/k;
    s2a=(double)s2s/k;
    s3a=(double)s3s/k;
    s4a=(double)s4s/k;
    printf("Chinese\nAverage Score: %.2f\nNumber of passed students: %d\nNumber of failed students: %d\n\nMathematics\nAverage Score: %.2f\nNumber of passed students: %d\nNumber of failed students: %d\n\nEnglish\nAverage Score: %.2f\nNumber of passed students: %d\nNumber of failed students: %d\n\nProgramming\nAverage Score: %.2f\nNumber of passed students: %d\nNumber of failed students: %d\n\nOverall:\nNumber of students who passed all subjects: %d\nNumber of students who passed 3 or more subjects: %d\nNumber of students who passed 2 or more subjects: %d\nNumber of students who passed 1 or more subjects: %d\nNumber of students who failed all subjects: %d\n\n",s1a+exp,s1p,s1f,s2a+exp,s2p,s2f,s3a+exp,s3p,s3f,s4a+exp,s4p,s4f,p[4],p[3]+p[4],p[2]+p[3]+p[4],p[1]+p[2]+p[3]+p[4],p[0]);
}

void stc()
{
	puts("Please enter class ID, 0 for the whole statistics.");
   	string s;
   	cin>>s;
   	stc_out(s);
}

那么这样子整个代码就写完啦。

最后放上正确的样例：

Welcome to Student Performance Management System (SPMS).

1 - Add
2 - Remove
3 - Query
4 - Show ranking
5 - Show Statistics
0 - Exit

Please enter the SID, CID, name and four scores. Enter 0 to finish.
Please enter the SID, CID, name and four scores. Enter 0 to finish.
Please enter the SID, CID, name and four scores. Enter 0 to finish.
Duplicated SID.
Please enter the SID, CID, name and four scores. Enter 0 to finish.
Please enter the SID, CID, name and four scores. Enter 0 to finish.
Welcome to Student Performance Management System (SPMS).

1 - Add
2 - Remove
3 - Query
4 - Show ranking
5 - Show Statistics
0 - Exit

Please enter SID or name. Enter 0 to finish.
2 0022334455 1 Tom 59 72 60 81 272 68.00
Please enter SID or name. Enter 0 to finish.
1 0011223344 1 John 79 98 91 100 368 92.00
3 2423475629 2 John 60 80 30 99 269 67.25
Please enter SID or name. Enter 0 to finish.
Welcome to Student Performance Management System (SPMS).

1 - Add
2 - Remove
3 - Query
4 - Show ranking
5 - Show Statistics
0 - Exit

Please enter class ID, 0 for the whole statistics.
Chinese
Average Score: 69.00
Number of passed students: 1
Number of failed students: 1

Mathematics
Average Score: 85.00
Number of passed students: 2
Number of failed students: 0

English
Average Score: 75.50
Number of passed students: 2
Number of failed students: 0

Programming
Average Score: 90.50
Number of passed students: 2
Number of failed students: 0

Overall:
Number of students who passed all subjects: 1
Number of students who passed 3 or more subjects: 2
Number of students who passed 2 or more subjects: 2
Number of students who passed 1 or more subjects: 2
Number of students who failed all subjects: 0

Welcome to Student Performance Management System (SPMS).

1 - Add
2 - Remove
3 - Query
4 - Show ranking
5 - Show Statistics
0 - Exit

Please enter SID or name. Enter 0 to finish.
1 student(s) removed.
Please enter SID or name. Enter 0 to finish.
Welcome to Student Performance Management System (SPMS).

1 - Add
2 - Remove
3 - Query
4 - Show ranking
5 - Show Statistics
0 - Exit

Please enter class ID, 0 for the whole statistics.
Chinese
Average Score: 59.50
Number of passed students: 1
Number of failed students: 1

Mathematics
Average Score: 76.00
Number of passed students: 2
Number of failed students: 0

English
Average Score: 45.00
Number of passed students: 1
Number of failed students: 1

Programming
Average Score: 90.00
Number of passed students: 2
Number of failed students: 0

Overall:
Number of students who passed all subjects: 0
Number of students who passed 3 or more subjects: 2
Number of students who passed 2 or more subjects: 2
Number of students who passed 1 or more subjects: 2
Number of students who failed all subjects: 0

Welcome to Student Performance Management System (SPMS).

1 - Add
2 - Remove
3 - Query
4 - Show ranking
5 - Show Statistics
0 - Exit

Showing the ranklist hurts students' self-esteem. Don't do that.
Welcome to Student Performance Management System (SPMS).

1 - Add
2 - Remove
3 - Query
4 - Show ranking
5 - Show Statistics
0 - Exit