[Numpy-discussion] What is the sign of nan?

[Robert Kern]

On Mon, Sep 29, 2008 at 23:02, Charles R Harris
<charlesr.harris at gmail.com> wrote:
>
>
> On Mon, Sep 29, 2008 at 9:02 PM, David Cournapeau
> <david at ar.media.kyoto-u.ac.jp> wrote:
>>
>> Charles R Harris wrote:
>> >
>> > So the proposition is, sign, max, min return nan when any of the
>> > arguments is nan.
>>
>> Note that internally, signbit (the C function) returns an integer.
>
> That is the signature of the ufunc. It could be changed... I believe the
> actual signbit of nan is undefined but I suppose we could return -1 in the
> nan case. That would be a fairly typical error signal for integers.

numpy.signbit() should work like C99 signbit() (where possible), IMO.
It can only return (integer) 0 or 1, and it does differentiate between
NAN and -NAN. I don't think we should invent new semantics if we can
avoid it. I think we can change what the platform provides, but only
in the direction of C99, IMO. I see signbit() as more along the lines
of functions like isnan() than log().

There is no C99 cognate for numpy.sign(), and it is a float->float
function, so I think we could make it return NAN. C99's copysign(x,y)
is almost a cognate (e.g. numpy.sign(y) == copysign(1.0,y) except for
y==+/-0.0), but since it does fall down on y==0, I don't think it's
determinative for y==NAN.


[~]$ man copysign
COPYSIGN(3)              BSD Library Functions Manual              COPYSIGN(3)

NAME
     copysign -- changes the sign of x to that of y

SYNOPSIS
     #include <math.h>

     double
     copysign(double x, double y);
...
[~]$ gcc --version
i686-apple-darwin9-gcc-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5465)
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[~]$ cat foo.c
#include <stdio.h>
#include <math.h>

int main(int argc, char **argv)
{
    printf("signbit(NAN) = %d\n", signbit(NAN));
    printf("signbit(-NAN) = %d\n", signbit(-NAN));
    printf("copysign(1.0, NAN) = %g\n", copysign(1.0, NAN));
    printf("copysign(1.0, -NAN) = %g\n", copysign(1.0, -NAN));
    return 0;
}
[~]$ gcc -std=c99 -o foo foo.c -lm
[~]$ ./foo
signbit(NAN) = 0
signbit(-NAN) = 1
copysign(1.0, NAN) = 1
copysign(1.0, -NAN) = -1

-- 
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