# Functions that converts day of year to month and day and reverse with error checking

There is no error checking in day_of_year or month_day. remedy this defect.

Here is the solution:

int day_of_year(unsigned int year, unsigned int month, int day) {
int leap, i;

leap = ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0));

if(((month >= 1) && (month <= 12)) && ((day >= 1) && (day <= daytab[leap][month]))) {
for(i = 1; i < month; i++) {
day += daytab[leap][i];
}

return day;
}

return -1;

}

void month_day(unsigned int year, unsigned int yearday, int *pmonth, int *pday) {
int leap, i;

leap = ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0));
if((leap == 1 && (yearday >= 1 && yearday <= 366)) || (leap == 0 && (yearday >= 1 && yearday <= 366))) {

for(i = 1; yearday > daytab[leap][i]; i++) {
yearday -= daytab[leap][i];
}
*pday = yearday;
*pmonth = i;
}
else {
printf("error: the yearday is invalid");
}
}


In the day_of_year's case I have to check if the yearday is a valide one. 1 <= yearday <= (365 || 366). I changed the parameters type to unsigned, because a day can't be negative nor a year.

In the month_day's case I check if the month is a valid one, it should be 1 <= month <= 12. After this, I check if the day belongs to a valid interval.

This exercise can be found in K&R2 at page 126.

1. Incorrect yearday limit

// if((leap == 1 ... || (leap == 0 && (yearday >= 1 && yearday <= 366))) {
if((leap == 1 ... || (leap == 0 && (yearday >= 1 && yearday <= 365))) {
//                                                             365

2. month_day() and day_of_year() should use consistent types for month. Suggest int for both.

// int day_of_year(unsigned int year, unsigned int month, int day) {
// void month_day(unsigned int year, unsigned int yearday, int *pmonth, int *pday) {
int day_of_year(unsigned int year,         int month, int day) {

3. month_day() and day_of_year() should use consistent types for yearday. Suggest int for both.

// int day_of_year(unsigned int year, unsigned int month, int day) {
// void month_day(unsigned int year, unsigned int yearday, int *pmonth, int *pday) {
int day_of_year(unsigned int year, int month, int day) {
void month_day(unsigned int year, int yearday, int *pmonth, int *pday) {

4. Leap year calculation leap = ((year... is good back to 1583. For years 4 to 1582 it is leap = (year % 4 == 0); 1582 has other complications. Before 4 has complications.

5. Suggest month_day() return int to indicate success or failure.

• if yearday will be an int the user may pass negative values. Should I modify my function such that it will handle a negativeyearday? Commented Feb 10, 2014 at 20:22
• @Ionut Grt. Suggest that a negative yearday is treated like 0 or 367+. Looks like code yearday >= 1 catches that already. Commented Feb 10, 2014 at 21:04