c - 函数获取垃圾值

Question

 #include <stdio.h>
   void clearKeyboard(void)
   {
       while (getchar() != '\n')   ; // empty execution code block on purpose
   }
   int yes(void)
   {

       char a,b;
       printf("<Please enter a character>: ");
       scanf("%c%c", &a,&b);

       while ((a !='Y' && a !='y' && a !='N' && a!='n') || (b!='\n'))
       { 
          if (b!='\n') ungetc(b, stdin),scanf("%*[^\n]%c", &b);
          a='n',b='\n';
          clearKeyboard();
          printf("*** INVALID ENTRY *** <Only (Y)es or (N)o are acceptable>: ");
          scanf("%c%c", &a,&b);

        }
        if (a =='Y' || a=='y')
        {
            printf("Contact Management System: terminated\n");
            return 1;

        }
        else 
        {
            if (a =='N' || a=='n')
            ContactManagerSystem();
            return 0;
        }

   }

   int menu(void)

   {
       int i;
       printf("0. Exit\n\nSelect an option:> ");
       scanf("%d", &i);
       while (i<0 || i>6)
       {
           printf("*** OUT OF RANGE *** <Enter a number between 0 and 6>: ");
           scanf("%d", &i);
       }
       return i;
   }

   void ContactManagerSystem(void)

   {
     int i=menu();

      switch(i)
      {
         case 0 :
          printf("Exit the program? (Y)es/(N)o: ");
          yes();


       }

   }
   int main(void)
   {
       ContactManagerSystem();
   }







So my "yes" function is working fine on it's own but when I call it within 

ContactManagerSystem() case 0 由于某种原因它变得疯狂 “yes”功能实际上是一个验证检查功能我需要用户 输入“yes”或“no”，如 'Y''y''N' 或 'n' 等待用户输入“Y”、“y”、“N”或“n”(表示"is"或“否”)。 如果用户回答"is"(“Y”，“y”)，则会结束程序，显示 以下消息:

Contact Management System: terminated< (followed by a newline) Otherwise, if the user entered No (“N”,”n”), the application continues to display the menu.

Answer 1

我认为您需要重新考虑对 && 和 || 的使用，并使用括号。此代码还负责处理 EOF 和键入的单词(如“yes”)。

#include <stdio.h>

int main(void)
{
    char a, b;
    printf("<Please enter a character>: ");
    if (scanf("%c%c", &a, &b) == 2)
    {
        while ((a != 'Y' && a != 'y' && a != 'N' && a != 'n') || b != '\n')
        {
            printf("*** INVALID ENTRY *** <Only (Y)es or (N)o are acceptable>: ");
            if (b != '\n')
            {
                int c;
                while ((c = getchar()) != EOF && c != '\n')
                    ;
            }
            if (scanf("%c%c", &a, &b) != 2)
                break;
        }
        if (a == 'Y' || a == 'y')
        {
            printf("Got yes!\n");
            return 1;
        }
        else if (a == 'N' || a == 'n')
        {
            printf("Got no!\n");
            return 0;
        }
    }
    printf("Got EOF\n");
    return 0;
}

如果a中的字母既不是'Y'也不是'y'也不是'N'也不是'n' 或者如果 b 中的值不是换行符，则报告问题，吞噬换行符(或 EOF)之前的任何杂散字符，然后恢复。通过打印报告状态以及退出状态。

运行示例:

$ ./scan13; echo $?
<Please enter a character>: yes
*** INVALID ENTRY *** <Only (Y)es or (N)o are acceptable>: no
*** INVALID ENTRY *** <Only (Y)es or (N)o are acceptable>: pdq
*** INVALID ENTRY *** <Only (Y)es or (N)o are acceptable>: y
Got yes!
1
$ scan13; echo $?
<Please enter a character>: n
Got no!
0
$ ./scan13; echo $?
<Please enter a character>: Got EOF
0
$

正如下面的评论中所指出的，scanf() 的问题之一是确保错误时的行为符合预期。使用 fgets() 将整行读入缓冲区通常更容易或 POSIX getline() ，然后用 sscanf() 解析数据.