On Wed, Oct 04, 2006 at 12:42:04AM -0400, Tim Peters wrote:
> [EMAIL PROTECTED]
> > If C90 doesn't distinguish -0.0 and +0.0, how can Python?
>
> With liberal applications of piss & vinegar ;-)
>
> > Can you give a simple example where the difference between the two is
> > apparent
> > to the Python programmer?
>
> Perhaps surprsingly, many (well, comparatively many, compared to none
> ....) people have noticed that the platform atan2 cares a lot:
>
> >>> from math import atan2 as a
> >>> z = 0.0 # postive zero
> >>> m = -z # minus zero
> >>> a(z, z) # the result here is actually +0.0
> 0.0
> >>> a(z, m)
> 3.1415926535897931
> >>> a(m, z) # the result here is actually -0.0
> 0.0
This actually returns -0.0 under linux...
> >>> a(m, m)
> -3.1415926535897931
>
> It work like that "even on Windows", and these are the results C99's
> 754-happy appendix mandates for atan2 applied to signed zeroes. I've
> even seen a /complaint/ on c.l.py that atan2 doesn't do the same when
>
> z = 0.0
>
> is replaced by
>
> z = 0
>
> That is, at least one person thought it was "a bug" that integer
> zeroes didn't deliver the same behaviors.
>
> Do people actually rely on this? I know I don't, but given that more
> than just 2 people have remarked on it seeming to like it, I expect
> that changing this would break /some/ code out there.
Probably!
It surely isn't a big problem though is it?
instead of writing
if (result == 0.0)
returned cached_float_0;
we just write something like
if (memcmp((&result, &static_zero, sizeof(double)) == 0))
returned cached_float_0;
Eg the below prints (gcc/linux)
The memcmp() way
1: 0 == 0.0
2: -0 != 0.0
The == way
3: 0 == 0.0
4: -0 == 0.0
#include <stdio.h>
#include <string.h>
int main(void)
{
static double zero_value = 0.0;
double result;
printf("The memcmp() way\n");
result = 0.0;
if (memcmp(&result, &zero_value, sizeof(double)) == 0)
printf("1: %g == 0.0\n", result);
else
printf("1: %g != 0.0\n", result);
result = -0.0;
if (memcmp(&result, &zero_value, sizeof(double)) == 0)
printf("2: %g == 0.0\n", result);
else
printf("2: %g != 0.0\n", result);
printf("The == way\n");
result = 0.0;
if (result == 0.0)
printf("3: %g == 0.0\n", result);
else
printf("3: %g != 0.0\n", result);
result = -0.0;
if (result == 0.0)
printf("4: %g == 0.0\n", result);
else
printf("4: %g != 0.0\n", result);
return 0;
}
--
Nick Craig-Wood <[EMAIL PROTECTED]> -- http://www.craig-wood.com/nick
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