Oh, you're right. I was afraid my remembrance would not be right.
I'm not so happy that -0.0 is not printed as -0.0 because mathematically
there is no difference, and the Julia community puts much emphasis on this.
And especially I feel uneasy with these returns:
julia> sign(-0.0)
-0.0
julia> sqrt(-0)
0.0
where `sqrt(-0.0)` and `sqrt(-0)` should print the same?
On Friday, December 19, 2014 3:52:46 PM UTC+1, Tamas Papp wrote:
>
> Nope, as far as I can see Julia is IEEE-compliant. So is R, under the
> hood, it just does not display signed zeros as such. I prefer that Julia
> does it.
>
> Best,
>
> Tamas
>
> On Fri, Dec 19 2014, Stefan Karpinski <[email protected] <javascript:>>
> wrote:
>
> > So is there anything here we're not doing correctly?
> >
> > On Fri, Dec 19, 2014 at 7:54 AM, Tamas Papp <[email protected]
> <javascript:>> wrote:
> >
> >> On Fri, Dec 19 2014, Hans W Borchers <[email protected] <javascript:>>
> wrote:
> >>
> >> > The expression -0.0 means the floating point number that in memory
> has
> >> the
> >> > same
> >> > bit representation as 0.0, but the sign bit set to 1. One can see
> this
> >> > applying
> >> > the 'bits' function to 0.0 and -0.0.
> >> >
> >> > I seem to remember that the IEEE floating point convention says that
> for
> >> > `sqrt(-0.0)` the sign bit has to be reset, so that
> >> >
> >> > julia> sqrt(-0.0)
> >> > -0.0
> >> >
> >> > would be wrong while MATLAB, R,... returning 0.0 are correct.
> >>
> >> I don't think so. Section 6.3 of the IEEE Std 754TM-2008 says that
> >>
> >> "Except that squareRoot(−0) shall be −0, every numeric squareRoot
> result
> >> shall have a positive sign.
> >>
> >> Furthermore, you can't rely on R to display the sign bit, eg
> >>
> >> > 1/(-Inf)
> >> [1] 0
> >>
> >> Best,
> >>
> >> Tamas
> >>
> >>
> >> > The expression -0.0 means the floating point number that in memory
> has
> >> the
> >> > same
> >> > bit representation as 0.0, but the sign bit set to 1. One can see
> this
> >> > applying
> >> > the 'bits' function to 0.0 and -0.0.
> >> >
> >> > I seem to remember that the IEEE floating point convention says that
> for
> >> > `sqrt(-0.0)` the sign bit has to be reset, so that
> >> >
> >> > julia> sqrt(-0.0)
> >> > -0.0
> >> >
> >> > would be wrong while MATLAB, R,... returning 0.0 are correct.
> >> >
> >> > For `1/-Inf` the sign bit is not reset because this expression should
> be
> >> > the
> >> > limit of 1/-x for very large numbers x, i.e. approaching 0 from left.
> >> >
> >> >
> >> > On Friday, December 19, 2014 12:13:57 PM UTC+1, Tamas Papp wrote:
> >> >>
> >> >> Hmmm, did you read the whole blog post, including the update at the
> end?
> >> >> Julia does handle signed zeroes correctly when working with floats.
> >> >>
> >> >> As to what signed zeros "mean" -- they have some applications,
> including
> >> >> indicating the direction of underflow. Probably you don't need to
> worry
> >> >> about them much since 0.0 == -0.0 (unless you specifically want to
> use
> >> >> them).
> >> >>
> >> >> I usually think about -0.0 as equivalent to 0.0 except for +, -, *,
> and
> >> >> /, especially when Inf is involved. Thinking about it as some limit
> is
> >> >> not very intuitive for IEEE floats, eg
> >> >>
> >> >> julia> log(-0.0)
> >> >> -Inf
> >> >>
> >> >> julia> sqrt(-0.0)
> >> >> -0.0
> >> >>
> >> >> all of which are correct IEEE.
> >> >>
> >> >> Best,
> >> >>
> >> >> Tamas
> >> >>
> >> >>
> >> >> On Fri, Dec 19 2014, cdm <[email protected] <javascript:>> wrote:
> >> >>
> >> >> > i stumbled across a post and got to wondering what Julians thought
> of
> >> it
> >> >> ...
> >> >> >
> >> >> > http://www.walkingrandomly.com/?p=5152
> >> >> >
> >> >> >
> >> >> > the author presents the case that Python is not IEEE compliant,
> but
> >> >> > MATLAB is and Julia is close:
> >> >> >
> >> >> > "From the wikipedia page on Division by Zero
> >> >> > <http://en.wikipedia.org/wiki/Division_by_zero>: *“The IEEE 754
> >> >> > standard specifies that every floating point arithmetic operation,
> >> >> > including division by zero, has a well-defined result”.*
> >> >> >
> >> >> >
> >> >> > the author reports that in Julia
> >> >> >
> >> >> > julia> 1/(-0)
> >> >> > Inf
> >> >> >
> >> >> >
> >> >> > but MATLAB "correctly" returns -Inf ...
> >> >> >
> >> >> >
> >> >> > i am not sure that signed zero (-0)
> >> >> > means anything to me ...
> >> >> >
> >> >> > should it?
> >> >> >
> >> >> > many thanks,
> >> >> >
> >> >> > cdm
> >> >>
> >>
>
