#include<stdio.h>
void main()
{char letter ;
printf("输入一个字母\n");
while ((letter=getchar())!='Y')
switch(letter)
{
    {case'S':printf("输入第二个字母：\n");
if(letter=getchar()=='a')
printf("saturday\n");
else if ((letter=getchar()=='u'))
printf("sunday\n");
else printf("data error\n");break;}
case'T':printf("是星期2\n");break;
case'M':printf("是星期一\n");break;
case'W':printf("是星期三\n");break;
case'F':printf("是星期5、\n");break;
default:printf("输入错误\n");break;
}
}



输入S  a总显示说 data error  而不是星期四、
求解

#include<stdio.h>
void main()
{
    char letter ;
    printf("输入一个字母\n");
    while ((letter=getchar())!='Y')
    switch(letter)
    {
    case'S':
        printf("输入第二个字母：\n");
        getchar();
        letter=getchar();
        getchar();
        if(letter=='a')
            printf("saturday\n");
        else if (letter=='u')
            printf("sunday\n");
        else
            printf("data error\n");break;
    case'T':printf("是星期2\n");break;
    case'M':printf("是星期一\n");break;
    case'W':printf("是星期三\n");break;
    case'F':printf("是星期5、\n");break;
    default:printf("输入错误\n");break;
    }
}

帮你改好的代码：
#include<stdio.h>
void main()
{char letter ;
printf("输入一个字母\n");
while ((letter=getchar())!='Y')
{ scanf("%*c");//用来吸收回车键 下面的作用也一样；
switch(letter)
{case'S':
    {
        printf("输入第二个字母：\n");
        if(letter=getchar()=='a')
        {scanf("%*c");
        printf("saturday\n");
        }
           else
              if ((letter=getchar()=='u'))
              {
                  scanf("%*c");
                  printf("sunday\n");
              }
              else
                  printf("data error\n");break;
    }
case'T':printf("是星期2\n");break;
case'M':printf("是星期一\n");break;
case'W':printf("是星期三\n");break;
case'F':printf("是星期5、\n");break;
default:printf("输入错误\n");break;
}
}
}

当你要输入字符型数据的时候 一定要考虑下是否要吸收回车键

#include<stdio.h>
void main()
{
    char letter ;
    printf("输入一个字母\n");
    while ((letter=getchar())!='Y')
    {
        getchar();
        switch(letter)
        {
            case'S':
                {
                    printf("输入第二个字母：\n");
                    letter=getchar();
                    getchar();
                    if(letter=='a')
                        printf("saturday\n");
                    else if(letter=='u')
                        printf("sunday\n");
                    else printf("data error\n");break;
                }; break;
            case 'T': printf("是星期二\n"); break;
            case 'M': printf("是星期一\n"); break;
            case 'W': printf("是星期三\n"); break;
            case 'F': printf("是星期五\n"); break;
            default:printf("输入错误\n");
        }
    }
}

LZ可以一下子判断2个字母

#include<stdio.h>
void main()
{
    char letter ;
    printf("输入一个字母\n");
    while ((letter=getchar())!='Y')
    {
        getchar();
        switch(letter)
        {
            case'S':
                {
                    printf("输入第二个字母：\n");
                    letter=getchar();
                    getchar();
                    if(letter=='a')
                        printf("saturday\n");
                    else if(letter=='u')
                        printf("sunday\n");
                    else printf("data error\n");break;
                }; break;
            case 'T': printf("是星期二\n"); break;
            case 'M': printf("是星期一\n"); break;
            case 'W': printf("是星期三\n"); break;
            case 'F': printf("是星期五\n"); break;
            default:printf("输入错误\n");
        }
    }
}

关于getchar()函数的用法，LZ可参考这篇帖子！
https://bbs.bccn.net/thread-303453-1-1.html

#include <stdio.h>
#include <conio.h>

int main(void)
{
    char c;
    printf ("please input the first number:\n");
   
    c = getchar();
    switch(c)
    {
        case 'S':
            getchar();
            printf ("please input the second number:\n");
            if ((c = getchar()) == 'a')
            {
                printf ("Saturday.");
            }
            else if ((c = getchar()) == 'u')
            {
                printf ("Sunday.");
            }
            else
            {
                printf ("Data error!");
            }
            break;

        case 'M':
            printf ("Monday.");
            break;
        case 'T':
            getchar();
            printf ("please input the second number:\n");
            if ((c = getchar()) == 'u')
            {
                printf ("Tuesday.");
            }
            else if ((c = getchar()) == 'h')
            {
                printf ("Thursday.");
            }
            else
            {
                printf ("Data error!");
            }
            break;
        case 'F':
            printf ("Friday.");
            break;
        default:
            printf ("Data error!\n");
            break;
    }

    getch();
    return 0;
}