1.最好不要string和char数组混用
2.string数组初始化可写为:string mars1[13]={"tret", "jan", "feb", "mar", "apr", "may", "jun", "jly", "aug", "sep", "oct", "nov", "dec"};
3.string类的比较函数s1.compare(t)
4.string不能之间用等号赋值,用assign函数,first.assign(str, 0,3)将str从0开始长度为3的部分赋给first