C while循环 - 代码不起作用

Question

I've been writing a simple program to check if input letter is a vowel, and my code doesn't work. The program should take characters as input one by one until % is entered, which will make it exit. It checks if input chars are vowels, and prints the result. Also it reports an error if input is not a letter. The problem is, it breaks out of the loop on the second step. Thank you for help, in advance. PS Sorry, didn't write that there's no error message, it just breaks out of the loop.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main(void)
{
  char processed='q';
  while(processed != '%')
  {
    printf("Enter letter to check if it's a vowel, %% to quit.\n");
    char input = getchar();
    processed = tolower(input);
    printf("%c\n", processed);
    if (processed == '%')
      break;
    if (processed < 'a' || processed > 'z')
    {
      fprintf(stderr, "Input should be a letter\n");
      exit(1);  
    }
    switch(processed)
    {
      case 'a':
      case 'e':
      case 'i':
      case 'o':
      case 'u':
      case 'y':
        printf ("Vowel\n");
        break;
      default:
        printf ("Non-vowel\n");
    }
  }
  exit(0);
}

Answer 1

Presumably you're entering a character and then hitting [ENTER]. So, in actuality you are entering two characters -- the letter you typed and a line feed ( \\n ). The second time through the loop you get the line feed and find that it's not a letter, so you hit the error case. Perhaps you want to add something like:

if (processed == '\n') {
    continue;
}

Answer 2

Someone else mentioned that you're hitting enter after each letter of input, and thus sending a newline ('\\n') into your program. Since your program doesn't have a case to handle that, it isn't working right.

You could add code to handle the newline, but using scanf would be easier. Specifically, if you replaced

char indent = getchar();

with

char indent;
scanf("%c\n", &indent);

scanf() would handle the newline and just return back the letters you're interested in.

And you should check scanf()'s return value for errors, of course.