day number in yaer

This is a discussion on day number in yaer within the C++ Programming forums, part of the General Programming Boards category; I'm conducting self-study C++ and currently reaching functions (void, returning-value function, etc). the requirement is that users will input date ...

  1. #1
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    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
    Code:
    int daysinMonth(int INmonth, int INyear) //Calculate days in months.
    {
    	
    	switch(INmonth)
    	{
    		case(1): case(3): case(5): case(7): 
    		case(8): case(10): case(12): 
    			return 31;
    			break;
    		case(2):
    			if (isLeapYear(INyear))
    			
    				return 29;
    			else
    				return 28;
    			break;
    		case(4): case(6): case(9): case(11):
    			return 30;
    			break;
    	}
    }
    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!
    thanks!

  2. #2
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    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
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    no..i din't implement that.
    Will try.
    I have not reached enum type yet!


    thanks!

  4. #4
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    Just a thought -
    How about an array?

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

  5. #5
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    I thought abt it too.
    It surely will be better. I'm getting there yet, however
    thanks

  6. #6
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    I've use a hybrid of the two:
    Code:
    // 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 wrongness Salem's Avatar
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    > and you still need an if-statement anyways.
    Perhaps
    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
    Code:
    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;
          break;
        // and so on
        default:
        // 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.
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