[Rd] identical(0, -0)

[Duncan Murdoch]

Petr Savicky wrote:
> On Sat, Aug 08, 2009 at 10:39:04AM -0400, Prof. John C Nash wrote:
>   
>> I'll save space and not include previous messages.
>>
>> My 2 cents: At the very least the documentation needs a fix. If it is 
>> easy to do, then Ted Harding's suggestion of a switch (default OFF) to 
>> check for sign difference would be sensible.
>>
>> I would urge inclusion in the documentation of the +0, -0 example(s) if 
>> there is NOT a way in R to distinguish these.
>>     
>
> It is possible to distinguish 0 and -0 in R, since 1/0 == Inf and
> 1/(-0) == -Inf.
>
> I do not know, whether there are also other such situations. In particular
>   (0)^(-1) == (-0)^(-1) # [1] TRUE
>   log(0) == log(-0) # [1] TRUE
>
>   
>> There are occasions where 
>> it is useful to be able to detect things like this (and NaN and Inf and 
>> -Inf etc.). They are usually not of interest to users, but sometimes are 
>> needed for developers to check edge effects. For those cases it may be 
>> time to consider a package FPIEEE754 or some similar name to allow 
>> testing and possibly setting of flags for some of the fancier features. 
>> Likely used by just a few of us in extreme situations.
>>     
>
> I think that distinguishing 0 and -0 may be useful even for nonexpert
> users for debugging purposes. Mainly, because x == y does not imply
> that x and y behave equally as demonstrated above or by
>   x <- 0
>   y <-  - 0
>   x == y # [1] TRUE
>   1/x == 1/y # [1] FALSE
>
> I would like to recall the suggestion
>   On Sat, Aug 08, 2009 at 03:04:07PM +0200, Martin Maechler wrote:
>   > Maybe we should introduce a function that's basically
>   > isTRUE(all.equal(..., tol=0))  {but faster},  or
>   > do you want a 3rd argument to identical, say 'method'
>   > with default  c("oneNaN", "use.==", "strict")
>   > 
>   > oneNaN: my proposal of using  memcmp() on doubles as its used for
>   >        other types already  (and hence distinguishing +0 and -0;
>   >      otherwise keeping the feature that there's just one NaN
>   >      which differs from 'NA' (and there's just one 'NA').
>   > 
>   > use.==: the previous R behaviour, using '==' on doubles 
>   >   (and the "oneNaN" behavior)
>   > 
>   > strict: be even stricter than oneNaN:  Use  memcmp()
>   >   unconditionally for doubles.  This would be the fastest
>   >   version of all three.
>
> In my opinion, for debugging purposes, the option identical(x,y,method="strict"),
> which implies that x and y behave equally, could be useful, if it is available
> in R base, 
>
> At the R interactive level, negative zero as the value of -0 could possibly
> be avoided. However, negative zero may also occur in numerical calculations,
> since it may be obtained as x * 0, where x is negative. So, i think, negative
> zero cannot be eliminated from consideration as something too infrequent.

I wouldn't mind a "strict" option.   It would compare bit patterns, so 
would distinguish +0 from -0, and different NaN values. But having the 
value of  identical(x-y, -(y-x)) depend on whether x and y are equal or 
not would just lead to confusion.W

Duncan Murdoch