#include<iostream.h>
int main()
{
int a;
cin>>a;
switch(a)
{
case 0:cout<<"Sunday"<<endl;
break;
case 1:cout<<"Monday"<<endl;
break;
case 2:cout<<"Tuseday"<<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;
default:cout<<"Please input 0~6"<<endl;
break;
}
return 0;
}

这个判断可以去掉

int i ;
    printf("请输入一个0~6的数");
    scanf("%d",&i);
    if (i>=0&&i<=6) {
        switch (i) {
            case 0:
                printf("星期日");
                break;
            case 1:
                printf("星期一");
                break;
            case 2:
                printf("星期二");
                break;
            case 3:
                printf("星期三");
                break;
            case 4:
                printf("星期四");
                break;
            case 5:
                printf("星期五");
                break;
            case 6:
                printf("星期六");
                break;
                
            default:
                printf("输入有误");
                break;
        }
    }