On Saturday, December 12, 2009, at 11:32AM, "Ben Haller" <bhcocoa...@sticksoftware.com> wrote: >>> You should not compare floating point numbers for equality in most >>> cases. This is true of any language on any platform. >> >> Indeed, some floating-point numbers (such as the one represented by >> the integer 0x7fc00000) will compare as not equal to themselves: > > I think what the OP really wanted to know (and I'm interested in >the answer too) is whether going out to the stringValue and back to >the doubleValue is guaranteed to yield a float that is bitwise >identical to the original float, or whether there is "drift" in the >least significant bit or two due to the changes in representation. >Anyway, even if that's not the OP meant, that's what I'd like to >ask. :-> >
It is absolutely not guaranteed to work - see <http://docs.sun.com/source/806-3568/ncg_goldberg.html#1251>. To be able to convert base 2 (the binary representation) to base 10 (the textual representation) without a loss of precision is possible (since 2 is a factor of 10), though it potentially requires at least 17 digits (quick back of the envelope calculation of how many digits are in 1/2**24, given that a float has a 24 bit mantissa), and stringValue isn't documented as providing any specific number of digits of precision. Going the other way, 0.1 (base 10) isn't possible to be represented exactly as a floating point number. _______________________________________________ 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
