oooh, damn... I was afraid someone was going to say that. I just hoped
there might be some way to force a float to conform to what NSLog %f
prints... That seems like it might be a useful function - something
like pround(aFloat), for "print-round", to force any float number to
round as the printf %f would round it. Anyway, that's obviously not
available.
Is there any C function that can compare within a given margin of
error? (I seem to remember seeing something like that, as some
point...??)
If not, no big deal - I'll just calculate the comparison within a
margin of error by hand.
thanks,
J.
On 11-Apr-09, at 10:15 AM, Michael Ash wrote:
On Sat, Apr 11, 2009 at 12:46 PM, James Maxwell
<jbmaxw...@rubato-music.com> 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
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