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Sorting Dates

Posted on 2003-10-30
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Last Modified: 2010-04-17
I have an array full of birthdays but am having trouble using srtcmp to sort them.
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Question by:Toy_Toy
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brettmjohnson earned 300 total points
ID: 9655414
You can use strcmp to sort on dates if the dates are of the form YYYYMMDD .
It still works if there is delimiters between year month and day  2003/10/29 .

You could attempt to a triple-sort on year, month, and day.  However, I would
simply convert the dates to a numerical time - either time_t or simply number
of days since some baseline.

/* returns number of days since year 0 for year.
 * understands leapyears, but ignores julian
 * calendar changes.  this is dumb and simplistic.
 */
static int numdays (int year, int month, int day)
{
  static int mdays[] = { 0, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 };
  int days;

  /* as a convenience, assume a two digit year is modern */
  if (year < 100)
    year += (year < 10)? 2000 : 1900;

  if ((month < 1) || (month > 12))
    month = 1;
  if ((day < 1) || (day > 31))
    day = 1;

  days = year*365 + year/4 - (year/100 - year/400);
  days += mdays[month];
  days += day;

  /* add a day if current year is leap year */
  if ((month > 2) && !(year % 4) && (year %100) && !(year % 400))
    days++;

  return days;
}
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