RE: Recommendation for keeping track of money in Core Data
RE: Recommendation for keeping track of money in Core Data
- Subject: RE: Recommendation for keeping track of money in Core Data
- From: Jeff Laing <email@hidden>
- Date: Tue, 17 Jan 2006 14:53:25 +1100
> On Jan 16, 2006, at 9:44 PM, Jeff Laing wrote:
>
> > You end up dealing with "23.29999999999999999" or "23.300000000001"
> > instead
> > of "23.3".
>
> But do you get the wrong result if you're using a number formatter
> (using the new behavior with 10.4) to display the number as currency
> using two decimal places? If my understanding is correct,
> the answer
> is no. The loss of precision would take dozens or maybe even
> hundreds
> of calculations between roundings in order to cause a discrepancy of
> even $0.01.
As I said before, the problem is not that the discrepancies are "so small
they won't bother anyones bank manager". The problem is that you will
compute your "income - expenses" and can come out with a non-zero number
(-.0000002) - then if you have code elsewhere that says "whoop, whoop, we
made a loss" because that difference is less than zero, you have users
complaining.
You very quickly spend all your application logic dealing with these niggly
rounding errors, and inevitably end up with paths through your application
that never get checked, or that still fail.
> Or am I missing something (I certainly could be)? While in memory,
> the value is stored as an NSNumber, not as a raw float, so only when
> the number is stored or loaded will the loss of precision come into
> play, right?
As far as I know, NSNumber still uses raw double under the covers. I'm
quite prepared to be corrected on that.
> I'm not a mathematician, so I'll defer to you if you say it matters
> with Core Data, but I have trouble seeing many situations where the
> loss of precision that comes into play when the data gets read from
> or written to disk would actually cause a wrong result ...
You also have problems when you "edit the ascii representation", and then
try to decide "did anything change?".
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