[Rd] precedence (was 'historical NA question')

[Simon Urbanek]

On May 7, 2014, at 5:17 PM, Hervé Pagès <hpages at fhcrc.org> wrote:

> On 05/07/2014 12:52 PM, Simon Urbanek wrote:
>> On May 7, 2014, at 3:37 PM, Hervé Pagès <hpages at fhcrc.org> wrote:
>> 
>>> No big deal. These things can be tricky:
>>> 
>>> https://stat.ethz.ch/pipermail/r-devel/2006-January/036022.html
>>> 
>>> Sorry I couldn't resist ;-)
>>> 
>> 
>> Yeah, but that's just yet another trip down the rabbit hole - why is -2 parsed as `-`(2) and not a single constant?
> 
> You wouldn't want -2 to be parsed as a single constant exactly for
> the reason that you wouldn't want -2^2 to return 4. Having -2^2 treated
> the same way as -x^2 is a sane feature.
> 

On what grounds? -2 is one value - negative two - and if you square it, you get four - so that's not even a question of precedence. It's just a matter of interpretation: do you see the constant -2 or do you see the constant 2 with unary minus? When you print -2 you get -2 - and that's not a positive constant with an unary minus - or is it? ;) - aaah, will we ever know … R is good at hiding that subtlety from us:

> a = quote(-2^2)
> b = bquote(.(-2)^2)
> a
-2^2
> b
-2^2
> eval(a)
[1] -4
> eval(b)
[1] 4


>> Is there a way to express a negative constant in R? Hmm…
> 
> Maybe some people have some use cases for this (speed ?).

Wrong tree ;). You kick-started the trip but failed to follow the path it takes into the depths of the human mind … :P (or was that computer mind? ;))

Cheers,
Simon



> Personally I don't. Of course it would require a special syntax,
> something that would probably be as ugly and confusing as the
> L suffix used for integer constants (L means long int in C).
> 
> H.
> 
>> 
>> 
>>> On 05/07/2014 09:16 AM, John Chambers wrote:
>>>> On 5/7/14, 5:21 AM, Therneau, Terry M., Ph.D. wrote:
>>>>> Hadley asked about the Blue book; my shelf still has the earlier brown
>>>>> book
>>>>>    Becker and Chambers, 1984, S: An interactive environment for data
>>>>> analysis and graphics.
>>>> 
>>>> Historically interesting, but there was never a guarantee that Version 3
>>>> of S (the "blue book") was back-compatible with earlier versions.  We
>>>> gave users some help in "getting on the road" to converting, that was
>>>> all (see Appendix 4 to the blue book).
>>>> 
>>>> For that one brief moment, we felt free to innovate.
>>>> 
>>>> John
>>>> 
>>>>> 
>>>>> The manual page for precedence is
>>>>> 
>>>>> $               component select
>>>>> %x              special operator
>>>>> -               unary minus
>>>>> :               sequence operator
>>>>> ^ **            exponentiation
>>>>> * /             mult/div
>>>>> + -             add/sub
>>>>> < > <= >= == != logical
>>>>> !               not
>>>>> & |             and/or
>>>>> <- ->           assignment
>>>>> 
>>>>> Terry Therneau
>>>>> 
>>>>> ______________________________________________
>>>>> R-devel at r-project.org mailing list
>>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>> 
>>>> ______________________________________________
>>>> R-devel at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>> 
>>> --
>>> Hervé Pagès
>>> 
>>> Program in Computational Biology
>>> Division of Public Health Sciences
>>> Fred Hutchinson Cancer Research Center
>>> 1100 Fairview Ave. N, M1-B514
>>> P.O. Box 19024
>>> Seattle, WA 98109-1024
>>> 
>>> E-mail: hpages at fhcrc.org
>>> Phone:  (206) 667-5791
>>> Fax:    (206) 667-1319
>>> 
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>> 
>> 
> 
> -- 
> Hervé Pagès
> 
> Program in Computational Biology
> Division of Public Health Sciences
> Fred Hutchinson Cancer Research Center
> 1100 Fairview Ave. N, M1-B514
> P.O. Box 19024
> Seattle, WA 98109-1024
> 
> E-mail: hpages at fhcrc.org
> Phone:  (206) 667-5791
> Fax:    (206) 667-1319
>