依据首字母判断星期几

根据首字母判断星期几

C/C++ code

#include<stdio.h>
int main(void)
{
    char letter;
    printf("Please input the first letter of someday\n");
    while((letter=getchar())!='y')
    {
        switch(letter)
        {
        case's':printf("Please input second letter\n");
            if((letter=getchar())=='a')
                printf("saturday\n");
            else if((letter=getchar())=='u')
                printf("sunday\n");
            else printf("data error\n");
            break;
        case'f':printf("friday\n");
            break;
        case'm':printf("monday\n");
            break;
        case't':printf("please input second letter\n");
            if((letter=getchar())=='u')
                printf("tuesday\n");
            else if((letter=getchar())=='h')
                printf("thursday\n");
            else
                printf("data error\n");
            break;
        case'w':printf("wednesday\n");
            break;
        default:printf("data error\n");
                }
    }
    return 0;
}

测试输入s,后输入u，首先显示sunday，然后就显示data error，为什么出错呢？

------解决方案--------------------
刚才那个错了、
#include<stdio.h>
int main(void)
{
char letter;
printf("Please input the first letter of someday\n");
letter=getchar();
switch(letter)
{
case's':printf("Please input second letter\n");
letter=getchar();
if((letter=getchar())=='a')
printf("saturday\n");
else if(letter=='u')
printf("sunday\n");
else printf("data error\n");
break;
case'f':printf("friday\n");
break;
case'm':printf("monday\n");
break;
case't':printf("please input second letter\n");
letter=getchar();
if((letter=getchar())=='u')
printf("tuesday\n");
else if(letter=='h')
printf("thursday\n");
else
printf("data error\n");
break;
case'w':printf("wednesday\n");
break;
default:printf("data error\n");
}

return 0;
}