From ggrothendieck at gmail.com  Sat Sep  1 03:25:10 2007
From: ggrothendieck at gmail.com (Gabor Grothendieck)
Date: Fri, 31 Aug 2007 21:25:10 -0400
Subject: [Rd] Friday question: negative zero
In-Reply-To: <46D8B4A6.4040800@stats.uwo.ca>
References: <46D8B4A6.4040800@stats.uwo.ca>
Message-ID: <971536df0708311825w1cda0aa3l8946ad48aaa8f0fb@mail.gmail.com>

On 8/31/07, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> The IEEE floating point standard allows for negative zero, but it's hard
> to know that you have one in R.  One reliable test is to take the
> reciprocal.  For example,
>
>  > y <- 0
>  > 1/y
> [1] Inf
>  > y <- -y
>  > 1/y
> [1] -Inf
>
> The other day I came across one in complex numbers, and it took me a
> while to figure out that negative zero was what was happening:
>
>  > x <- complex(real = -1)
>  > x
> [1] -1+0i
>  > 1/x
> [1] -1+0i
>  > x^(1/3)
> [1] 0.5+0.8660254i
>  > (1/x)^(1/3)
> [1] 0.5-0.8660254i
>
> (The imaginary part of 1/x is negative zero.)
>
> As a Friday question:  are there other ways to create and detect
> negative zero in R?

I think the only other way is the IEEE CopySign function but I don't think
R has implemented it -- R probably should.  Google CopySign .



[Rd] Friday question: negative zero
Gabor Grothendieck