C program days between two dates

Question

I have written a program that should find the days between two dates, but it has some hiccups. The logic makes perfect sense in my head when I read through it, so

Answer 1

First, that leap function feels overly complicated; you don't need to do both dates in one function call, and I'm sure that can be written more succinctly so that it is more obviously correct. Here's a version I've got laying around that isn't succinct but I'm confident it is easy to check the logic:

int is_leap_year(int year) {
        if (year % 400 == 0) {
                return 1;
        } else if (year % 100 == 0) {
                return 0;
        } else if (year % 4 == 0) {
                return 1;
        } else {
                return 0;
        }
}

You could call it like this:

int year1, year2, leap1, leap2;
year1 = get_input();
year2 = get_input();
leap1 = is_leap_year(year1);
leap2 = is_leap_year(year2);

No pointers and significantly less code duplication. Yes, I know that is_leap_year() can be reduced to a single if(...) statement, but this is easy for me to read.

Second, I think you're got a mismatch between 0-indexed arrays and 1-indexed human months:

            if(*month1 < 1 || *month1 > 12)

vs

    int daysPerMonth[] = {31,28,31,30,31,30,31,31,30,31,30,31};

Third, I think that days per month can be calculated slightly nicer:

int days_in_month(int month, int year) {
        int leap = is_leap_year(year);
        /*               J   F   M   A   M   J   J   A   S   O   N   D */
        int days[2][12] = {{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
                           {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}};
        if (month < 0 || month > 11 || year < 1753)
                return -1;

        return days[leap][month];
}

Here, I assume January is 0; you would need to force the rest of the code to match. (I learned this double-array trick from The Elements of Programming Style (page 54).) The best part of using a routine like this is that it removes the leap condition from the difference calculation.

Fourth, you're indexing arrays outside their bounds:

            for(i = month1 + 1; i <= 12; i++)
            {
                if(leap1 == 1)
                    total += daysPerMonthLeap[i];

This is just another instance of the problem with 0-indexed arrays and 1-indexed months -- but be sure that you fix this, too, when you fix the months.

I have a fear that I haven't yet found all the issues -- you may find it easier to sort the first and the second date after input and remove all that validation code -- and then use names before and after or something to give names that are easier to think through in the complicated core of the calculation.