使用toupper()和tolower()函数分别将字符转换为大写和小写

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

int main(void)
{
    char letter = 0;                          // Stores a character
    printf("Enter an uppercase letter:");     // Prompt for input
    scanf("%c", &letter);                     // Read a character

    if(isalpha(letter) && isupper(letter))
       printf("You entered an uppercase %c./n", tolower(letter));
    else
       printf("You did not enter an uppercase letter./n");

    return 0;
}