你輸入了S後的回車還在緩衝區中，後來被讀取到letter中了，而不是你輸入的字母，所以才會出錯，你在之前再加一個getchar就行了。還有你的第二個else if中不需要再讀取字母了，直接判斷就行了。下面是整理後的程式碼。我建議你直接讀取一段作為字串來判斷比較方便，這樣比較麻煩。

#include

void main()

{

printf("please enter the first letter of someday:\n");

char letter;

while ((letter = getchar()) != "y") {

switch (letter) {

case "S": {

printf("please enter the second letter:");

getchar();

if ((letter = getchar()) == "a") {

printf("It is Saturday!\n");

} else if (letter == "u") {

printf("It is Sunday!\n");

} else {

printf("error\n");

}

break;

}

case "M":

printf("It is Monday!\n");

break;

case "T": {

printf("please enter the second letter:");

getchar();

if ((letter = getchar()) == "h") {

printf("It is Tuesday!\n");

} else if (letter == "u") {

printf("It is Thursday!\n");

}

break;

}

case "F":

printf("It is Friday!\n");

break;

case "W":

printf("It is Wednesday!\n");

break;

}

}

}