Is there something wrong with my date code? - c

Question

Description: given a date after START_GREGORIAN_CALENDAR, this function returns the number of days until the next Thursday. For example, for 16 March 2011 (2011,3,16), the function will return 1, and for 17 March 2011 (2011,3,17), the function will return 7.
int daysToNextThursday (int year, int month, int day) {
int Thursday;
Thursday = 7;
return (Thursday - day);
}
The code compiles correctly, but when I input a date, e.g. 16 3 2011, I do not get the right answer. Note this is apart of a larger amount of code that I have written, which works perfectly.
Any ideas?

Answer 1

Yes, I have an idea. My idea is that you go back and rethink the algorithm you've selected for determining that a day is Thursday. It's dead wrong :-)
Now, like a broken clock that's right twice a day, you may find input parameters that give you the correct answer but they'll be the exception rather than the rule.
If you want to figure out when next Thursday is from a given date, C provides date and time functions for exactly that purpose:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
static char *textday[] = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
int main (int argc, char *argv[]) {
int year, month, day, today, thursday;
struct tm *mytm;
time_t mytime;
// Get all arguments (minimal error checks).
if (argc != 3) {
printf ("Usage: next_thursday <year> <month>\n");
return -1;
}
year = atoi (argv[1]);
month = atoi (argv[2]);
// Do first fourteen days of the month.
for (day = 1; day <= 14; day++) {
Up until there, it's just getting parameters and starting the loop. The meat of the calculation is below, setting up a useful struct tm and then forcing our year, month and day into it. Then mktime will fill in the tm_wday (day of week) field for us and we can use that to figure out the number of days till Thursday.
// Make the tm structure based on date (and midday).
mytime = time (0);
mytm = localtime (&mytime);
mytm->tm_year = year - 1900;
mytm->tm_mon = month - 1;
mytm->tm_mday = day;
mytm->tm_hour = 12;
mytime = mktime (mytm);
// Output filled in fields and days till next Thursday.
today = mytm->tm_wday;
thursday = (11 - today) % 7;
if (thursday == 0)
thursday = 7;
printf ("%04d-%02d-%02d, weekday = %d (%s), days till Thu = %d\n",
mytm->tm_year + 1900, mytm->tm_mon + 1, mytm->tm_mday,
today, textday[today], thursday);
}
return 0;
}
Note that the thursday calculation is a bit of modulus trickery - it's simply used to give us the number of days based on the following table:
today thursday
------- --------
0 (sun) 4
1 (mon) 3
2 (tue) 2
3 (wed) 1
4 (thu) 7
5 (fri) 6
6 (sat) 5
If you want a more readable solution, you can use:
if (today < 4) thursday = 4 - today;
else thursday = 11 - today;
This program outputs the following for 2011-03:
2011-03-01, weekday = 2 (Tue), days till Thu = 2
2011-03-02, weekday = 3 (Wed), days till Thu = 1
2011-03-03, weekday = 4 (Thu), days till Thu = 7
2011-03-04, weekday = 5 (Fri), days till Thu = 6
2011-03-05, weekday = 6 (Sat), days till Thu = 5
2011-03-06, weekday = 0 (Sun), days till Thu = 4
2011-03-07, weekday = 1 (Mon), days till Thu = 3
2011-03-08, weekday = 2 (Tue), days till Thu = 2
2011-03-09, weekday = 3 (Wed), days till Thu = 1
2011-03-10, weekday = 4 (Thu), days till Thu = 7
2011-03-11, weekday = 5 (Fri), days till Thu = 6
2011-03-12, weekday = 6 (Sat), days till Thu = 5
2011-03-13, weekday = 0 (Sun), days till Thu = 4
2011-03-14, weekday = 1 (Mon), days till Thu = 3