[Python-Dev] Caching float(0.0) (original) (raw)

Nick Craig-Wood nick at craig-wood.com
Wed Oct 4 13:52:32 CEST 2006

On Wed, Oct 04, 2006 at 12:42:04AM -0400, Tim Peters wrote:

[skip at pobox.com] > 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 <nick at craig-wood.com> -- http://www.craig-wood.com/nick

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