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trouble with 'double' in Visual C++ 5.0

Posted on 1998-08-12
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Last Modified: 2013-11-20
I am creating a service report and the users have to enter in the time they started
   to the time they finished and I want to calculate the amount of hours they have
   worked.  The method I am using seems to work the odd time but I am having
   trouble getting it to work all the time.  I am using double variables and I am not
   sure if I should be using them. I am also changing the time into 24 hr clock if the
   starttime is larger than the endtime.
   eg. starttime  11.00  endtime 2.00->14.00
   This works all the time but it is when you subtract the time such as 1.15 to 2.00,
   you get 0. 45  but I want it to be .75 because you have worked 3/4 of an hour.  I
   am only doing it in 15 minute intervals so I didn't think it woul be that difficult.  This
   is how I am doin it.
   if (m_hourswork==0.45) //45 minutes worked
       m_hourswork=0.75;
   if m_hourswork==0.15)  //15 minutes worked
      m_hourswork=0.25)
   I do this for all 15 min intervals and every hour upto 10 hours because no one will
   work more than 10.  The method I am using seems to work from time to time so I
   think it may be the 'double' varibles I am using.

   Thank You for the help!!
   Greatly Needed
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Question by:troy24
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jstolan earned 10 total points
ID: 1320642
The problem is with rounding in the double numerical system.  Since doubles are binaries, they cannot exactly represent all decimal numbers.  If you really want to do it this way, you need to allow for this.  for example your first test for the 45 minutes could be expressed as:

if( ( m_hourswork - 0.45 ) < 0.001 )   // 45 minutes worked
    m_hourswork = 0.75.

I must say though that this is a really clumsy way of approaching your problem.  MFC has some wonderful time handling functions.  Why don't you check the CTime class?
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Author Comment

by:troy24
ID: 1320643
Thank you very much for the help, I am going to try it in a few minutes.
I will also look at the CTime class.
Thanks again.

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