Thread: day number in yaer

  1. #1
    Registered User
    Join Date
    May 2008

    Lightbulb day number in yaer

    I'm conducting self-study C++ and currently reaching functions (void, returning-value function, etc).
    the requirement is that users will input date in form of "month day year" like 12 25 2002 and the output will show that date's day number, which is day 359 ( 1 1 2002 to be the reference, or day 1).
    I'm done with this, but I'm wondering is there any more efficient way to determine how many days in a month

    I used this function
    int daysinMonth(int INmonth, int INyear) //Calculate days in months.
    		case(1): case(3): case(5): case(7): 
    		case(8): case(10): case(12): 
    			return 31;
    			if (isLeapYear(INyear))
    				return 29;
    				return 28;
    		case(4): case(6): case(9): case(11):
    			return 30;
    I've been thinking about a loop but don't know how to alternate return value between 30 and 31

    Any insight will be so great!

  2. #2
    Registered User
    Join Date
    Jan 2005
    Looks good to me. I wouldn't worry too much about efficiency here beyond not doing anything horribly inefficient. Worry more about clarity. For example, perhaps use an enum for the months so it is clearer which months are in which case.

    BTW, do you check for an invalid month elsewhere in the code?

  3. #3
    Registered User
    Join Date
    May 2008
    no..i din't implement that.
    Will try.
    I have not reached enum type yet!


  4. #4
    Registered User
    Join Date
    Dec 2006
    Just a thought -
    How about an array?

    daysInMonth_noleap[12] = { 31, 28, 31 .... };
    daysInMonth_leap[12] = { 31, 29, 31 .... };
    Remember to make it 0 - 11 (January = 0, December = 11).

  5. #5
    Registered User
    Join Date
    May 2008
    I thought abt it too.
    It surely will be better. I'm getting there yet, however

  6. #6
    Kernel hacker
    Join Date
    Jul 2007
    Farncombe, Surrey, England
    I've use a hybrid of the two:
    // This function assumes that month and year are correct values. 
    // Month is 1..12!
    int daysInMonth(int month, int year)
        static const int dayArray[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
        int days = dayArray[month-1];
        if (month == 2 && leapYear(year))
           days = 29;
        return days;
    The reason for the hybrid is that the extra check of month == 2 is probably no bigger than the overhead of the second array, and you still need an if-statement anyways.

    Edit: Fix up bug with off-by-one.
    Last edited by matsp; 05-24-2008 at 03:03 AM.
    Compilers can produce warnings - make the compiler programmers happy: Use them!
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  7. #7
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    > and you still need an if-statement anyways.
    return days + (month == 2 && leapYear(year));
    since the result of a boolean expression is 0 or 1

    > but I'm wondering is there any more efficient way to determine how many days in a month
    Maybe, but there are plenty of increasingly obscure (see above) shorter answers.

    Consider your code, written as
    enum {
      January = 1, February, March, April, // you get the idea
    int daysInMonth ( int month, int year ) {
      int result = 0;
      switch ( month ) {
        case January: case March:
          result = 31;
        // and so on
        // error trap
    How easy would it be for the average reader of the code to say whether the function was correct or not?

    Arrays need bound checking, and as cyberfish points out, there's always the 'off-by-1' problem.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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