[Numpy-discussion] What is the sign of nan?
Pete Forman
pete.forman@westerngeco....
Tue Sep 30 08:42:18 CDT 2008
"Charles R Harris" <charlesr.harris@gmail.com> writes:
> OK, here is what is looks like to me at the moment given that numpy
> requires an IEEE754 machine:
>
> o We need a reliable value for NAN. [...]
>
>
> o Max/min follow the IEEE standard. Given a choice of
> nan/non-nan, return non-nan. [...]
Yes, that follows 754r and C99.
> o Signbit returns the value of the signbit function, but nonzero
> values are set to 1.
Looks okay to me.
> o I am unsure of sign. Should it return signed zeros? Should it
> return nan for nan or return the sign of the nan? I am
> inclined towards returning nan.
How is sign used? If it is in x * sign(y) then it might be better to
use copysign(x, y) which is well defined even with signed zeros and
NaNs. It depends on whether you want special behavior when y is zero.
In copysign y being 0 or +0 is considered positive, so x is returned.
So you could use this as a specification.
def sign(y):
if y == 0: # True for -0 and +0 too
return 0 # or perhaps return y
else
return copysign(1, y)
Your inclination leads to this.
def sign(y):
if y == 0 or isnan(y):
return y
else
return copysign(1, y)
The better choice will be governed by how sign is used in practice.
--
Pete Forman -./\.- Disclaimer: This post is originated
WesternGeco -./\.- by myself and does not represent
pete.forman@westerngeco.com -./\.- the opinion of Schlumberger or
http://petef.22web.net -./\.- WesternGeco.
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