Re: Best way to compare CGFloats (Rick B.)
I am stuck with floats (doubles, to be precise) comparison.
Machine 'real-numbers' such as floats and doubles should be thought of
as intervals or neighborhoods near the mathematical number. Tests for
'equality' of machine reals should never use machine equality '=='
operators. 'Equality' of two machine reals , in this case, means that
the two numbers are separated by a distance less than some small
number, such as '(fabs(a-b) epsilon)'. Exactly what the value of
epsilon should be gets a little tricky.
Hope that I am not being too pedantic but imo you should never test
for 'equality' ('==') when using machine real data types.
Rick B.
*-
* Stop spam before it gets to your mailbox! Support blocklists.
* I use http://www.spamcop.net/.
* my PGP key id: 0x63fa758 keyserver: http://keyserver1.pgp.com
*/
___
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
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:
http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
Re: Best way to compare CGFloats (Rick B.)
On 30 Jun 2010, at 15:56, Frederick Bartram wrote:
Hope that I am not being too pedantic but imo you should never test for
'equality' ('==') when using machine real data types.
Sometimes you really do want binary equivalence, and in that case == may be the
right thing to use. Normally, though, you're quite right, you need to be
careful with floating point comparisons (not just equality tests, actually;
sometimes for greater-than or less-than comparisons you might want two floats
to be un-ordered if they're close in value).
I thought it'd be useful to link to the excellent What Every Computer
Scientist Should Know About Floating-Point Arithmetic, since that's usually
the best thing to give to people who are worrying about this kind of thing:
http://dlc.sun.com/pdf/800-7895/800-7895.pdf
http://docs.sun.com/source/806-3568/ncg_goldberg.html
Anyone who hasn't read it should.
Kind regards,
Alastair.
--
http://alastairs-place.net
___
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
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:
http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
Re: Best way to compare CGFloats (Rick B.)
On Wed, Jun 30, 2010 at 11:11 AM, Alastair Houghton
alast...@alastairs-place.net wrote:
On 30 Jun 2010, at 15:56, Frederick Bartram wrote:
Hope that I am not being too pedantic but imo you should never test for
'equality' ('==') when using machine real data types.
Sometimes you really do want binary equivalence, and in that case == may be
the right thing to use.
While we're being pedantic, note that == is not always the same as
binary equivalence. For example, 0.0 == -0.0, and x != x when x =
NAN.
Using == can make sense when you know that your values are exact
integers, which is not an uncommon scenario, but neither is it the
usual one.
Mike
___
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
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:
http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
Re: Best way to compare CGFloats (Rick B.)
On 30 Jun 2010, at 17:37, Michael Ash wrote: While we're being pedantic, note that == is not always the same as binary equivalence. For example, 0.0 == -0.0, and x != x when x = NAN. That's a good point. I spoke inaccurately. Using == can make sense when you know that your values are exact integers, which is not an uncommon scenario, but neither is it the usual one. It may also make sense sometimes with floating point values. An example might be where you have an object that you're using as the key for an NSDictionary; in that case, your -isEqual: method may well want to do a proper equality test. These are, however, special situations. Kind regards, Alastair. -- http://alastairs-place.net ___ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) 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: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
