练习5-8 函数day_of_year和month_day中没有进行错误检查，请解决该问题。
#include <stdio.h>
#define PPP printf("what does it for?\n");static char daytab[2][13] = {{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},{0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};int day_of_year(int year, int month, int day) {int i, leap;if (year < 0) {printf("Error: invalid year input\n");return -1;}if (month > 12 || month < 1) {printf("Error: invalid month input\n");return -1;}leap = year % 4 == 0 && year % 100 != 0 ||year % 400 == 0;if (day < 0 || day > daytab[leap][month]) {printf("Error: invalid day input\n");return -1;}for (i = 1; i < month; i++)day += daytab[leap][i];return day;
}int month_day(int year, int yearday, int *pmonth, int *pday) {int i, leap;if (year < 0) {printf("Error: invalid year input\n");return -1;}leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;if (yearday < 1 || yearday > 365 + leap) {printf("Error: invalid year day input\n");return -1;}for (i = 1; yearday > daytab[leap][i]; i++)yearday -= daytab[leap][i];PPP*pmonth = i;*pday = yearday;return 1;
}int main() {int choice;int year, month, day, day_of_month, yearday, ret;int pmonth, pday;printf("This program is intended to display day of the year,\n""or the exact month and day of the year.\n""Choose '1': Input year, month, day of the month,\n""and it'll automatically show the day of year.\n""Choose '2': Input day_of_year, and it'll display\n""month and day_of_month.\n");printf("input function choice:\n");printf("1. day of year\n2. month and day of month\n");scanf("%d", &choice);switch(choice) {case 1:printf("year:\n");scanf("%d", &year);printf("month:\n");scanf("%d", &month);printf("day:\n");scanf("%d", &day);ret = day_of_year(year, month, day);if (ret == -1)break;printf("The day of year is: %d\n", ret);break;case 2:printf("Input the year:\n");scanf("%d", &year);printf("The day of year:\n");scanf("%d", &yearday);ret = month_day(year, yearday, &pmonth, &pday);if (ret == -1)break;printf("month and day: %d.%d\n", pmonth, pday);break;default:printf("Unknown instruction, program exits..\n");break;}return 1;
}