From: Jeff Schwab <jeff@...>
Date: 2009-04-30T00:45:35+09:00
Subject: Re: Detecting -0.0

Robert Klemme wrote:
> On 28.04.2009 22:27, Jeff Schwab wrote:
>> Thomas B. wrote:
>>> Hello.
>>>
>>> I have a simple question - how to detect if a variable contains a Float
>>> of value -0.0? Is there any special method for this, or any comparison
>>> that distinguishes negative zero from a the regular zero?
>>
>> NB: I'm not sure how portable this is.
>>
>> def negative_zero?(number)
>>   inverse = 1 / number.to_f
>>   inverse < 0 && inverse.infinite? ? true : false
> 
> Why do you use the ternary operator to convert a boolean into a boolean?

I don't.  Float#infinite? can return nil, -1, or +1, but never true or 
false. Without the ternary, the original client code produces the 
following, far less meaningful output, including the blank line:

false
-1

false

>> end
>>
>> [ 0.0, # false
>>  -0.0, # true
>>  -1.0, # false
>>   1.0  # false
>> ].each do |f|
>>   puts negative_zero?(f)
>> end

> My 0.02 EUR: normally there should not be any distinction between 0.0 
> and -0.0.  Even though computer math is not the same as real math, the 
> distinction does not seem to make sense to me.

"Real" math?  Whatever you say.  In EE college courses, professors often 
use -0 to represent the limit of an asymptotic function that approaches 
zero from the negative side, e.g. the voltage decay of a negatively 
charged capacitor.  The use has nothing to do with computers or IEEE floats.

Anyway, take it up with the OP; AFAIK, his question was academic, but 
maybe he has an interesting use case.