Alternative solution by Jesse Weinstein; let me know if it seems worth posting on the main page and/or any corrections. -- JesseW 06:44, 19 July 2007 (BST)

/* alternative solution to K&R exercise 5.9, demonstrating method of addressing a 
   2 dimensional array with pointers. 
   Put together by Jesse Weinstein, based on code as listed below. 
   Wed Jul 18 22:45:08 2007 */

#include <stdio.h>
/* based off of code in K&R, p. 111 */
/* exercise is to use pointers, not indexes */

static char daytab[2][13] = {
  {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
  {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};


/*day_of_year: return day of year from year, month and day */
int day_of_year(int year, int month, int day)
{
  int i, leap;
  if (month > 12)
    return -1; /* error */
  leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
  for ( i = 1; i < month; i++)
    day +=  *(*(daytab+leap)+i);
  return day;
}

/* month_day: set month, day from year, and yearday */
int month_day(int year, int yearday, int *pmonth, int *pday)
{
  int i, leap;
  leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
  for (i = 1; i < 13 && yearday > *(*(daytab+leap)+i); i++)
    yearday -= *(*(daytab+leap)+i);
  *pmonth = i;
  *pday = yearday;
  return 0;
}

/* main: test day_of_year and month_day */
/* copied from Lars Wirzenius's solution to K&R exercise 5.8
available at: 
http://www.clc-wiki.net/mediawiki/index.php?title=K%26R2_solutions:Chapter_5:Exercise_8&oldid=3971
*/
int main(void)
{
	int year, month, day, yearday;
	
	for (year = 1970; year <= 2000; ++year) {
		for (yearday = 1; yearday < 366; ++yearday) {
			if (month_day(year, yearday, &month, &day) == -1) {
				printf("month_day failed: %d %d\n",
					year, yearday);
			} else if (day_of_year(year, month, day) != yearday) {
				printf("bad result: %d %d\n", year, yearday);
				printf("month = %d, day = %d\n", month, day);
			}
		}
	}
	
	return 0;
}