Re: NSLog with %f and comparisons using ==
Re: NSLog with %f and comparisons using ==
- Subject: Re: NSLog with %f and comparisons using ==
- From: Michael Ash <email@hidden>
- Date: Sat, 11 Apr 2009 13:15:36 -0400
On Sat, Apr 11, 2009 at 12:46 PM, James Maxwell
<email@hidden> wrote:
> I've got a strange problem.
> I have a list of float constants that I need to compare to the result of a
> new calculation.
>
> I derived the constants by performing the calculation, and printing using
> NSLog with %f, then just writing down the results in my array of constants.
> So, the result of the calculation used to derive the float constants was run
> through NSLog, using the %f token.
>
> Now, when I run the calculation "live" and try to compare to my stored
> constants, I'm not getting matches. I'm assuming this is because the result
> of the calculation *isn't* actually what I stored in my constants, since the
> constants were rounded during the NSLog. So, how do I make sure my "live"
> calculation returns a value that will be rounded in the same way as an
> NSLog(@"%f", aFloat)?
>
> That's a confusing question, I know... I hope it makes sense to somebody.
Read "What Every Computer Scientist Should Know About Floating-Point
Arithmetic":
http://docs.sun.com/source/806-3568/ncg_goldberg.html
But basically, never depend on floating point calculations to be
exact. As a consequence, never compare floats or doubles for equality.
Instead, compute a delta from the desired value, and see if the delta
is within an acceptable range.
Mike
_______________________________________________
Cocoa-dev mailing list (email@hidden)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden