如何停止 c 中 char 的重复无效答案？

Question

我正在构建一个长程序，需要在 if/else 语句中使用典型的“y/n”字符函数，它工作正常，但如果用户输入无效的内容，它将重复我的“无效答案”字符串等于他们输入的字符数。我尝试使用“%1s”而不是 %c，但这并不能阻止失控输入。

#include<stdio.h>

int main()
{
 printf("Welcome.I can predict the future\n"
     "I learned this gift from someone in the future.\n"
     "A bright creature with green eyes taught me how.\n"
 "It appeared to me on a Sunday without enthusiam\n"
     "and told me I would end up trapped in a computer,\n"
 "and there was nothing I could do about it.\n"

 "It was really cruel of it to do that.\n"
 "I could have enjoyed the rest of my days\n"
 "without being depressed having known that...\n"
 "I also didn't need to know so many other things I\n"
 "now have learned.\n\n"

 "Having said this, would you like me to predict you\n"
     "future?y/n\n");

 char ansr;
 scanf("%1s", &ansr);


 while (ansr != 'y' && ansr != 'n'){

 printf("Invalid answer, Please try again.");
 scanf("%1s", &ansr);
 }

 if ( ansr == 'y') {
 printf("You've been warned.\n\n");
 }
 else if ( ansr == 'n') {
 printf("Goodbye Then.\n\n");
 }




 return 0;

 }

Answer 1

首先，您不想将%1s 与char 一起使用，因为此格式说明符需要一个数组。在ansr的内存位置写入1个字符后，它将在下一个内存位置写入一个空字节。这导致了未定义的行为。坚持使用%c。

要清除多余的字符，您需要在循环中使用getchar 来读取字符，直到找到换行符为止。这将刷新缓冲区。

while (ansr != 'y' && ansr != 'n') {
    printf("Invalid answer, Please try again.");
    while ( getchar() != '\n');
    scanf("%c", &ansr);
}

输出：

...
Having said this, would you like me to predict you
future?y/n
bogus
Invalid answer, Please try again.wrong
Invalid answer, Please try again.yep
You've been warned.

Answer 2

要处理输入中的字符串，您可以尝试从标准输入读取到ansr 缓冲区，如下所示，然后进行字符串比较。通过用%*[^\n] 丢弃stdin 中的其余内容，扫描不受空格或多个字符的影响，您将获得所需的y/n 字符。也不要忘记你的右大括号，并在main 中返回零。

#include <stdio.h>
#include <string.h>
#define buffLen 32

int main() {        

    char ansr[buffLen] = "";

    printf("...would you like me to predict your future? (y/n) \n");

    while (strcmp(ansr, "y") != 0 && strcmp(ansr, "n") != 0){

        // Read the string, and throw away the rest up to the newline char.
        scanf("%s%*[^\n]", &ansr);

        if (strcmp(ansr, "y") == 0) {
            printf("You've been warned.\n");
        } else if (strcmp(ansr, "n") == 0) {
            printf("Goodbye Then.\n");
        } else {
            printf("Invalid answer, Please try again.\n");
        }
    }

    return 0;
}

编辑： cmets 中有一个很好的建议，即使用 do-while 循环，以免我们用虚拟的东西初始化缓冲区 ansr，因为 do 块初始化在评估条件之前缓冲。我喜欢这个，因为我很少考虑使用这些，但这是何时使用的一个很好的例子......

#include <stdio.h>
#include <string.h>
#define buffLen 32

int main() {

    char ansr[strLen];

    printf("...would you like me to predict your future? (y/n) \n");

    do {
        // Read the string, and throw away the rest up to the newline char.
        scanf("%s%*[^\n]", &ansr);

        if (strcmp(ansr, "y") == 0) {
            printf("You've been warned.\n");
        } else if (strcmp(ansr, "n") == 0) {
            printf("Goodbye Then.\n");
        } else {
            printf("Invalid answer, Please try again.\n");
        }
    } while (strcmp(ansr, "y") != 0 && strcmp(ansr, "n") != 0);

    return 0;
}