1、四则运算

#include <iostream>
#include <cstdio>
using namespace std;
int main()
{
	float num1,num2;
	char op;
	cout<<"先输入两个数字，请输入四则运算符号'+,-,*,/:"<<endl;
	cin>>num1>>num2>>op;
	switch(op){
		case '+':cout<<num1+num2<<endl;	break;
		case '-':cout<<num1-num2<<endl;	break;
		case '*':cout<<num1*num2<<endl;	break;
		case '/':if(num2!=0) { cout<<num1/num2<<endl; break;}
		else cout <<"Divide by Zero!"<<endl; break;
		default: cout<<"input error!"<<endl;
	} 
}

2、星期几？

# include<cstdio>
# include<iostream>
using namespace std;
int main(){
	int weekday;
	cout<<"请输入1-7代表星期几？"<<endl;
	cin>>weekday;
	switch(weekday){
		case 1:cout<<"Monday"<<endl;	break;
		case 2:cout<<"Tuesday"<<endl;	break;
		case 3:cout<<"Wednesday"<<endl;	break;
		case 4:cout<<"Thursday"<<endl;	break;
		case 5:cout<<"Friday"<<endl;	break;
		case 6:cout<<"Saturday"<<endl;	break;
		case 7:cout<<"Sunday"<<endl;	break;
		default: cout<<"input error!"<<endl;
	}
	return 0;
}

3、输出最大数，

#include <iostream>
#include <cstdio>
using namespace std;
int main()
{
	int a,b,c;
	cin>>a>>b>>c;
	if (a>b && a>c) printf("%d",a);
	else if (b>a && b>c) printf("%d",b);
	else printf("%d",c);
}

4、计算一元二次方程

#include <iostream>
#include <cstdio>
#include <math.h>
using namespace std;
int main()
{
	float a,b,c,q,x1,x2;
	cout<<"请输入a,b,c的值："<<endl;
	cin>>a>>b>>c;
	q=sqrt(b*b-4*a*c);
	if (a==0 && q<0){
		cout<<"输入错误"<<endl;
	}
	else
	{
		x1 = (-b+q)/(2*a);
		x2 = (-b-q)/(2*a);
		cout<<"x1="<<x1<<"\tx2="<<x2<<endl;
	}	
}