我以大一初学者的视角编写,虽有不足,但容易理解

#include <stdio.h>

int main() {
    char num;
    
    while (scanf("%c", &num) != EOF) { 
        getchar();//换行但不结束输入
        if(num>='a'&&num<='z')
        {
            printf("%c is an alphabet.\n",num);
        }
        else if (num>='A'&&num<='Z')
        {
            printf("%c is an alphabet.\n",num);
        }   //判断字母
        else 
        {
            printf("%c is not an alphabet.\n",num);
        }   //判断非字母
    }
    return 0;
}