sol1.c File Reference

Problem 19 solution More...

#include <stdio.h>

Include dependency graph for sol1.c:

Functions
char	get_month_days (short month)
char	is_leap_year (short year)
int	main (int argc, char **argv)

Detailed Description

Problem 19 solution

Author

Krishna Vedala

Function Documentation

get_month_days()

char get_month_days

(

short

month

)

returns number of days in a month. Month is identified by an integer -

0 = Jan and 11 = December

For February, adjust for leap year outside the function.

{
    if (month == 1) /* February has 28 days. Adjust leap year in the loop */
        return 28;
    else if (month <= 6) /* odd months till July have 30 days - Jan = 0 (even)*/
    {
        if (month & 0x01)
            return 30;
        else
            return 31;
    }
    else if (month >= 7) /* odd months after July have 31 days*/
    {
        if (month & 0x01)
            return 31;
        else
            return 30;
    }
    /* should never reach here! */
    perror("Should never have reached this point!\n");
    return -1;
}

is_leap_year()

char is_leap_year

(

short

year

)

return 1 if input year is a leap year otherwise, return 0

{
    if ((year % 400 == 0) || ((year % 4 == 0) && (year % 100 != 0)))
        return 1;
 
    return 0;
}

main()

int main	(	int	argc,
		char **	argv
	)

Main function

Let us identify days i.e., Sunday thru Saturday with integers - 0 thru 6 respectively Jan 1 1901 was a Tuesday

These two for-loops count the start of day for the next month. Hence, we have to skip the last December count

Main Algorithm: every week has 7 days hence, the start of next day would be modulo 7 add to this, the current start date and ensure the result is still modulo 7!

{
    int count_sundays = 0;
    const short start_year = 1901;
    const short end_year = 2000;
 
    /**
     * Let us identify days i.e., Sunday thru Saturday with integers - 0 thru 6
     *respectively Jan 1 1901 was a Tuesday
     **/
    char start_day = 2;
 
    for (int year = start_year; year <= end_year; year++)
    {
        char is_leap = is_leap_year(year);
        for (char month = 0; month < 12; month++)
        {
            /**
             * These two for-loops count the start of day for the next month.
             * Hence, we have to skip the last December count */
            if (year == end_year && month == 11)
                continue;
 
            int days = get_month_days(month);
 
            if (is_leap && month == 1) /* for a leap year february, add a day */
                days++;
 
#ifdef DEBUG
            if (year == end_year)
            {
                printf("Year: %d\t Month: %d\t Days: %d\t First of day: %s\n",
                       year, month, days, day_string(start_day));
            }
#endif
 
            /** Main Algorithm:
             * every week has 7 days hence, the start of next day would be
             *modulo 7 add to this, the current start date and ensure the result
             *is still modulo 7!
             **/
            start_day = ((days % 7) + start_day) % 7;
 
            /* If start-day is a Sunday, increment counter */
            if (start_day == 0)
                count_sundays++;
        }
    }
 
    printf(
        "Total number of Sundays that happened on the 1st of a month in the "
        "last century: %d\n",
        count_sundays);
 
    return 0;
}

Here is the call graph for this function: