转换字母为大小写

#include <stdio.h>

int main(voID){
  char letter = 0;                          // Stores a character

  printf("Enter an uppercase letter:");     // Prompt for input
  scanf("%c", &letter);                     // Read a character

  // Check whether the input is uppercase
  if(letter >= 'A')                         // Is it A or greater?
    if(letter <= 'Z')                       // and is it Z or lower?
    {                                       // It is uppercase
      letter = letter - 'A' + 'a';          // Convert from upper- to lowercase
      printf("You entered an uppercase %c\n", letter);
    }
    else                                    // It is not an uppercase letter
      printf("Try using the shift key! I want a cAPItal letter.\n");
  return 0;
}