On Dec 12, 2009, at 1:07 PM, Andy Lee wrote:
> 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.
P.S. If the answer is no, it seems to me there should be an easy
counterexample, but my math isn't good enough to figure one out. I've tried
the following code with f = 0.1, 0.100001, and 0.12345, but I always get back a
bitwise match for the original float, even when stringValue returns a rounded
string.
- (void)testFloat:(float)f
{
NSString * s = [[NSNumber numberWithFloat:f] stringValue];
float f2 = [s floatValue];
NSLog(@"%.20f, %@, %.20f", f, s, f2);
char *cp1 = (char *)(&f);
char *cp2 = (char *)(&f2);
int i;
for (i = 0; i < sizeof(float); i++)
{
NSLog(@"byte %d %@", i, (cp1[i] == cp2[i] ? @"MATCHES" : @"DOES NOT
match"));
}
}
--Andy
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