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 I'm assuming I have some syntax errors that I keep glancing over or something. Firstly, when entering two dates in different years, the output is always off by about one month (31 in most cases, but 32 in one case...go figure). Second, two dates exactly one month apart will return the number of days in the second month (i.e. 1/1/1 to 2/1/1 yields 28). There are inevitably some other weird things that this program does, but I am hoping that is enough information to help you guys figure out what I'm doing wrong. For the life of me I can't figure this one out on my own. I am relatively new to C, so please be gentle =) Thanks ``` // Calculates the number of calendar days between any two dates in history (beginning with 1/1/1). #include #include void leap(int year1, int year2, int *leap1, int *leap2); void da…

Accepted Answer

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.

C program days between two dates

Solution

Problem