return the day of the week with day, month, year and first of January provided?

Question

I have the C99 as follow:

int dayOfWeek(int day, int month, int year, int firstJan);

The first parameter, day, provides the day of interest − range from 1 to 31 (inclusive). The second parameter, month, provides the month of interest − range from 1 to 12 (inclusive). The third parameter, year, provides the year of interest − any integer value of 1970 or greater. The fourth parameter, firstJan, indicates the day of the week on which the first of January falls in the provided year.

The function will return the day of the week on which the indicated date falls. For example, the call:

dayOfWeek(13, 11, 2017, 0);

will return the integer 1 (representing Monday).

How can I approach the solution? Its permitted values are 0 (representing Sunday), 1 (representing Monday), and so on, up to 6 (representing Saturday). Code has been edit:

  1 #include <stdio.h>
  2 #include <stdlib.h>
  3 
  4 int dayOfweek(int day, int month, int year, int firstJan)
  5 {
  6         int mth[] = {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
  7         int mth_leap[] = {0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335};
  8 
  9         if(year <1970 || month < 1 || month > 12 || day < 1 || day > 31 || firstJan < 0 || firstJan > 6 ){
 10         printf("invalid input");
 11         //return -1;
 12         }
 13 
 14         if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)){
 15                 day = mth_leap[month - 1] + day ;
 16         }else{
 17                 day = mth[month - 1] + day;
 18         }
 19 
 20         int dow = (day - firstJan + 7)%7;
 21         printf("Day of week is %i.\n", dow);
 22         //return 1;
 23 
 24 }

Use mktime and forget the nonsense about writing this function :D — Antti Haapala -- Слава Україні, Nov 06 '18 at 16:36
@AnttiHaapala writing such a function is OK as an exercise, but of course not for real production code. — Jabberwocky, Nov 06 '18 at 16:40
@Jabberwocky Yes, it is OK as an exercise, but I must refuse. — Antti Haapala -- Слава Україні, Nov 06 '18 at 16:40
Duplicate of https://stackoverflow.com/questions/6054016/c-program-to-find-day-of-week-given-date — Antti Haapala -- Слава Україні, Nov 06 '18 at 16:41
" first of January" is not needed. The DOW is independent of Jan 1. Perhaps it is used only to establish if 0 is a Sunday, Monday, or .... — chux - Reinstate Monica, Nov 06 '18 at 17:41
@chux - Hi Chux, it is part of the practice to use first of January, any idea for it? — Jim Ye, Nov 07 '18 at 01:19
@AnttiHaapala - it is a exercise from tutor for extra. Is there any simple approach to this question? Or I need to include calculation because of the first of January? — Jim Ye, Nov 07 '18 at 02:18
@JimYe I see that `int dayOfWeek(int day, int month, int year);` woudl provide the same answer as `int dayOfWeek(int day, int month, int year, int firstJan);` so it is unclear why `int firstJan` is part of the function signature. — chux - Reinstate Monica, Nov 07 '18 at 03:07
`firstJan` makes it a lot easier for an exercise, but it's not necessary — phuclv, Nov 07 '18 at 03:24
@chux - instead of using mktime, using firstJan might be easier to understand I guess? just another way to solve the issue? — Jim Ye, Nov 07 '18 at 03:47
@phuclv - Yes phuclv, it will be easier for calculation I guess? any idea to approach the solution? — Jim Ye, Nov 07 '18 at 03:48

chux - Reinstate Monica · Accepted Answer · 2018-11-07T04:32:07.927

2

Day of the week is found simply with mktime().

The mktime function

On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time,

#include <time.h>

int dayOfWeek(int day, int month, int year, int /*firstJan*/) {
   // struct tm members domain are: years from 1900, and months since January
   // Important to set tm_isdst = -1 to let the function determine dst setting.
   struct tm ymd = { .tm_year - 1900, .tm_mon = month - 1, .tm_mday = day, .tm_isdst = -1);

   time_t t = mktime(&ymd);  // this will fill in .tm_wday
   if (t == -1) return -1;  // Failed to find a valid calender time (and day-of-the-week)
   return ymd.tm_wday;
}

How can I approach the solution?

Yet OP has a function that provides the day-of-the-week for Jan 1st.

Some pseudo code for that approach:

int dayOfWeek(int day, int month, int year, int firstJan) {
  days_since_jan1 = table[month] + day;
  if (month > Feb and isleap(year)) days_since_jan1++;
  dow = (days_since_jan1 - firstJan + 7)%7
}

edited Nov 07 '18 at 04:32

answered Nov 07 '18 at 04:24

chux - Reinstate Monica

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for the approach with firstJan, what is the meaning of table[month]? – Jim Ye Nov 07 '18 at 08:58
@JimYe Some how code needs to convert `month` to "days since January." Perhaps a table look-up? 0, 31, 31+28, 31+28+31, ... – chux - Reinstate Monica Nov 07 '18 at 12:22
i have add my code. Could you help me? I got the error of control reaches end of non-void function? – Jim Ye Nov 07 '18 at 12:50
@JimYe 1) It is poor Stack-overflow etiquette to change the question once answers start arriving. Recommend to roll back the edit. 2) `int dayOfweek()` needs to return _something_ or make it `void dayOfweek()`. – chux - Reinstate Monica Nov 07 '18 at 13:11

return the day of the week with day, month, year and first of January provided?

1 Answers1