#include <stdio.h>

 int main ()
 {
     int grade;
     int aCount = 0, bCount = 0;

     printf("Enter the letter grades.\n");
     printf("Enter the EOF character to end input.\n");
     while ((grade = getchar ()) != EOF)
     {
         switch (grade)
         {

         case 'A': case 'a':
             ++aCount;
             break;
         case 'B': case 'b':
             ++bCount;
             break;


         case '\n' :case' ':
             break;

         default:
             printf("Incorrect letter grade entered.");
             printf("Enter a new grade.\n");
             break;
         }
     }
     printf("\nTotals for each letter grade are: \n");
     printf("A:%d\n", aCount);
     printf("B:%d\n", bCount);

     return 0;
 }
这个程序中的caseA casea 还有最后的  case '\n' :case' ':不能理解为什么会有大写的A还有小写的a 多谢大神