On Apr 28, 2010, at 12:31 PM, Quincey Morris wrote:
> On Apr 28, 2010, at 11:42, Kyle Sluder wrote:
>
>> Er? Unless there's some documentation about the precision of
>> NSStringFromPoint being less than that of the members,
>> NSStringFromPoint could certainly include just as much precision as
>> the actual float.
>
> Assuming that the string contains a decimal floating point number, then both
> the conversion to and from binary floating point involve (in general)
> imprecision from rounding and/or truncation, since in both cases the result
> could theoretically require an infinite number of decimal or binary places.
>
> Keep in mind there are some binary floating point numbers with a finite
> number of bits that have no exact representation in any finite decimal
> string, and vice versa. It's certainly true that since both representations
> are finite, then with enough decimal digits you could get very close (no more
> error than 1 in the last binary digit), but very close is not the same as
> exact.
>
> Even if you could get back the original binary value by using enough decimal
> digits, you'd still need to be certain that the binary to string conversion
> in the frameworks was just that finicky (as opposed to, say, using a library
> routine that defaults to a smallish maximum number of significant digits).
>
> Plus, you'd have to expect that encoding doubles would require, in the worst
> case, at least 309 significant digits, which is a much longer string than I
> want in my archive.
>
> So my original point stands: I want my original bit pattern back again after
> unarchiving, and I don't believe that a finite decimal string will guarantee
> that.
Then I would recommend using hex-float strings, as they are guaranteed to
represent the exact value of the given floating point value (as binary fp to
hex fp is lossless, unlike binary to dec):
[c...@ccox-macpro:~]% cat test.m
#import <Foundation/Foundation.h>
int main() {
NSPoint p = {3.141592653, 3.141592653};
NSString *s = [NSString stringWithFormat: @"{%A, %A}", p.x, p.y];
NSLog(@"%@", s);
NSPoint p2 = NSPointFromString(s);
if(memcmp(&p, &p2, sizeof p) == 0) {
NSLog(@"Bit-pattern was preserved");
} else {
NSLog(@"Bit-pattern was not preserved");
}
return 0;
}
[c...@ccox-macpro:~]% cc test.m -framework Foundation -fobjc-gc -arch x86_64 &&
./a.out
2010-04-28 14:06:19.396 a.out[90877:903] {0X1.921FB542FE938P+1,
0X1.921FB542FE938P+1}
2010-04-28 14:06:19.399 a.out[90877:903] Bit-pattern was preserved
[c...@ccox-macpro:~]% cc test.m -framework Foundation -fobjc-gc -arch i386 &&
./a.out
2010-04-28 14:06:22.478 a.out[90885:903] {0X1.921FB6P+1, 0X1.921FB6P+1}
2010-04-28 14:06:22.481 a.out[90885:903] Bit-pattern was preserved
[c...@ccox-macpro:~]% cc test.m -framework Foundation -fobjc-gc -arch ppc &&
./a.out
2010-04-28 14:06:26.167 a.out[90893:903] {0X1.921FB6P+1, 0X1.921FB6P+1}
2010-04-28 14:06:26.204 a.out[90893:903] Bit-pattern was preserved
Since NSPointFromString isn't documented to work with hex-floats you may want
to do the parsing yourself with NSScanner, but other than that, the principle
is the same.
_______________________________________________
Cocoa-dev mailing list ([email protected])
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 [email protected]
