修改下面的练习，使菜单选项由字符代替数字进行标记：
用switch选择工资等级。程序运行的开头应该像这样：
Enter the number corresponding to the desired pay rate or action: 1) $8.75/hr 2) $9.33/hr 3) $lO.OO/hr 4) $11.20/hr 5) quit
如果选择l到4．那么程序应该请求输入工作小时数。程序应该一直循环运行，直到输入5。如果输入l到5以外的选项，那么程序应该提醒用户合适的选项是哪些，然后再循环。用#define为各种工资等级和税率定义常量。
#include<stdio.h> #include<ctype.h> char get_first(void); //b.加班 #define TIME 40 //加班（超过TIME小时） = #define ADD 1.5 //ADD倍的时间 //c.税率 #define LIMIT1 300 //前LIMIT1美元为RATE1 #define RATE1 0.15 #define LIMIT2 150 //下一个LIMIT2美元为RATE2 #define RATE2 0.20 #define RATE3 0.25 //余下的位RATE3 int main(void) { double basic,hours,gross,tax; printf("Enter the number corresponding to the desired pay rate or action:\n"); printf("1) $8.75/hr\t\t\t2) $9.33/hr\n"); printf("3) $10.00/hr\t\t\t4) $11.20/hr\n"); printf("5) quit\n"); switch( get_first() ) { case '1': basic = 8.75; break; case '2': basic = 9.33; break; case '3': basic = 10.00; break; case '4': basic = 11.20; break; default: printf("quit\n"); return(0); //退出程序 } printf("you have select $%.2lf\n",basic); printf("input the work hours of a week:"); scanf("%lf",&hours); if (hours > 40) hours = 40 + (hours - 40) * 1.5; gross = hours * basic; printf("gross income:\t\t%lf\n",gross); if (gross <= LIMIT1) tax = gross * RATE1; else if (gross <= LIMIT2) tax = LIMIT1 * RATE1 + (gross - LIMIT1) * RATE2; else tax = LIMIT1 * RATE1 + LIMIT2 * RATE2 + (gross - LIMIT1 - LIMIT2) * RATE3; printf("tax:\t\t\t%lf\n",tax); printf("net income:\t\t%lf\n",gross - tax); return(0); } char get_first(void) //得到字符串中的第一个非空字符 { int ch; while( isspace( ch = getchar() ) ); while ( getchar() != '\n'); return ch; }