While 循环和嵌套 if 不能一起工作

Question

当没有while循环时，代码运行良好。因为 İt 没有起作用，嵌套 if 中的最后一个 else。声明者是“无效字符”。但如下所示，它不起作用。错误在哪里？我试图删除 while 循环。它跑了。但是，以这种方式，它不起作用。在我看来，这种情况是while循环的一个特定条件。

#include <stdio.h>

int main(void)
{
   unsigned int aCount = 0; 
   unsigned int bCount = 0; 
   unsigned int cCount = 0; 
   unsigned int dCount = 0;
   unsigned int fCount = 0; 

   puts("Enter the letter grades." );
   puts("Enter the EOF character to end input." );
   int grade; // one grade 

   // loop until user types end-of-file key sequence
    while (( grade = getchar()) != EOF) { //while

        if(grade == 'A'|| grade == 'a')
        {
            ++aCount;
        }
        else if(grade == 'B' || grade == 'b')
        {
            ++bCount;
        }
        else if(grade == 'C' || grade == 'c')
        {
            ++cCount;
        }
        else if(grade =='D' || grade == 'd')
        {
            ++dCount;
        }
        else if(grade == 'F' || grade =='f')
        {
            ++fCount;
        }
        else
        {
            puts("invalid character");

        }

   }//while





   // output summary of results
   puts("\nTotals for each letter grade are:");
   printf("A: %u\n", aCount);
   printf("B: %u\n", bCount); 
   printf("C: %u\n", cCount); 
   printf("D: %u\n", dCount); 
   printf("F: %u\n", fCount); 

return 0;
} 

Answer 1

在`while 中使用if 没有问题。

正如评论所述，等级之间的空格和换行符将导致它打印“无效字符”。您应该忽略这些字符而不是打印消息。

当用户在行首键入 EOF 字符时，循环将结束。

#include <stdio.h>
#include <ctype.h>

int main(void)
{
   unsigned int aCount = 0; 
   unsigned int bCount = 0; 
   unsigned int cCount = 0; 
   unsigned int dCount = 0;
   unsigned int fCount = 0; 

   puts("Enter the letter grades." );
   puts("Enter the EOF character to end input." );
   int grade; // one grade 

   // loop until user types end-of-file key sequence
    while (( grade = getchar()) != EOF) { //while
        grade = tolower(grade);
        if(grade == 'a')
        {
            ++aCount;
        }
        else if(grade == 'b')
        {
            ++bCount;
        }
        else if(grade == 'c')
        {
            ++cCount;
        }
        else if(grade == 'd')
        {
            ++dCount;
        }
        else if(grade =='f')
        {
            ++fCount;
        }
        else if (isspace(grade)) {
            // ignore whitespace
        }
        else
        {
            printf("invalid character '%c'\n", grade);
        }

   }//while

   // output summary of results
   puts("\nTotals for each letter grade are:\n");
   printf("A: %u\n", aCount);
   printf("B: %u\n", bCount); 
   printf("C: %u\n", cCount); 
   printf("D: %u\n", dCount); 
   printf("F: %u\n", fCount); 

return 0;
} 

Answer 2

'\n' 也算作一个字符，' ' 也是如此。总而言之，您永远不会检查这些字符。 代码如下：

#include <stdio.h>

int main(void)
{
   unsigned int aCount = 0; 
   unsigned int bCount = 0; 
   unsigned int cCount = 0; 
   unsigned int dCount = 0;
   unsigned int fCount = 0; 

   puts("Enter the letter grades." );
   puts("Enter the EOF character to end input." );
   int grade; 


    while (( grade = getchar()) != EOF  ) { 

        if(grade == 'A'|| grade == 'a')
        {
            ++aCount;
        }
        else if(grade == 'B' || grade == 'b')
        {
            ++bCount;
        }
        else if(grade == 'C' || grade == 'c')
        {
            ++cCount;
        }
        else if(grade =='D' || grade == 'd')
        {
            ++dCount;
        }
        else if(grade == 'F' || grade =='f')
        {
            ++fCount;
        }
        else if(grade =='\n' || grade==' ')
        {
            continue;

        }
        else
        {
        puts("invalid character");
        }
   }


   puts("\nTotals for each letter grade are:");
   printf("A: %u\n", aCount);
   printf("B: %u\n", bCount); 
   printf("C: %u\n", cCount); 
   printf("D: %u\n", dCount); 
   printf("F: %u\n", fCount); 

return 0;
}