On Dec 12, 2009, at 12:51 PM, David Rowland wrote: > On Dec 12, 2009, at 9:32 AM, Ben Haller 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. :-> >> > > The problem with floating point is that the binary form almost always uses > binary arithmetic. That means that a number like 0.1 has an infinitely > repeating binary form. It is likely that converting the string "0.1" to a > float and then back will yield "0.1" because it will be rounded, but there > are marginal cases where you won't get what you put in. > > Also, the string can carry more information than the float. "8.336" is > different from "8.336000". The latter implies that you know the precision to > six decimal places, the former does not.
If I understand the question, it's not about converting an arbitrary decimal string to a float, but specifically a string that was generated from a float in the first place. As glenn pointed out, that string most certainly *can* be a string that can be converted back to a bitwise equivalent of the original float. Ben's question was whether in practice it is guaranteed to be so, or whether stringValue uses a maximum number of decimal places that would lead to rounding error. I can't tell from a quick look at the docs. --Andy _______________________________________________ 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
