[Numpy-discussion] making the distinction between -0.0 and 0.0..

Joe Kington jkington@wisc....
Tue Sep 29 16:49:05 CDT 2009


Using 'd' rather than 'f' doesn't fix the problem...

Python 2.3.4 (#1, Jan  9 2007, 16:40:09)
[GCC 3.4.6 20060404 (Red Hat 3.4.6-3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.pack('d', -0.0)
'\x00\x00\x00\x00\x00\x00\x00\x80'   <-- Correct signbit
>>> struct.pack('>d', -0.0)
'\x00\x00\x00\x00\x00\x00\x00\x00'   <-- No signbit. Unpacks to 0.0
>>> struct.pack('<d', -0.0)
'\x00\x00\x00\x00\x00\x00\x00\x00'   <-- No signbit.
>>>

In python 2.6 it works fine. -0.0 packs to
'\x00\x00\x00\x00\x00\x00\x00\x80' or '\x80\x00...' as it should.

I'm assuming it's a bug that was fixed somewhere in between?


On Tue, Sep 29, 2009 at 4:39 PM, Robert Kern <robert.kern@gmail.com> wrote:

> On Tue, Sep 29, 2009 at 16:37, Joe Kington <jkington@wisc.edu> wrote:
> > I know it's a bit pointless profiling these, but just so I can avoid
> doing
> > real work for a bit...
> >
> > In [1]: import sys, struct, math
> >
> > In [2]: def comp_struct(x):
> >    ...:     # Get the first or last byte, depending on endianness
> >    ...:     # (using '>f' or '<f' loses the signbit for -0.0 in older
> > python's)
>
> Did you try 'd'? I wonder if the extra conversion step from a C double
> to a C float is causing this issue.
>
> --
> Robert Kern
>
> "I have come to believe that the whole world is an enigma, a harmless
> enigma that is made terrible by our own mad attempt to interpret it as
> though it had an underlying truth."
>  -- Umberto Eco
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