#include <stdio.h>
int main()
{
    char letter;
    printf("Enter an upper case letter:");
    scanf("%c",&letter);
    if ((letter >= 'A') && (letter <= 'Z'))
    {
        letter = letter + 'a' - 'A';//等价于l=l+'a'-'A'
        printf("You entered an uppercase %c.\n", letter);
    }
    else
        printf("You did not erter an uppercase letter.\n");
    return 0;
}

就是scanf在VS2013里运行需要加上#define _CRT_SECURE_NO_WARNINGS，改成scanf_s就不需要了。在2013里输入任何字符它输出的都是You did not erter an uppercase letter.

好，谢谢