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

[Hervé Pagès]

On 05/07/2014 02:01 PM, peter dalgaard wrote:
>
> On 07 May 2014, at 21:52 , Simon Urbanek <simon.urbanek at r-project.org> 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? Is there a way to express a negative constant in R? Hmm…
>>
>
> It's painful, but
>
>> bquote(.(-2)^2)
> -2^2
>> eval(bquote(.(-2)^2))
> [1] 4
>> bquote(.(-2)^2)[[2]]
> [1] -2
>> mode(bquote(.(-2)^2)[[2]])
> [1] "numeric"
>
> The difficulty is that the tokenizer, which recognizes language elements before the parser goes to work on the grammatical structure, is unable to distinguish the -2 in "-2 + 2" from the one in "-2 ^ y".

Are you saying that the tokenizer could be made smarter and recognize
"-2" as a token in "-2 ^ y", just because you didn't put a space between
"-" and "2"? So in "-2 ^ y" it would be token, but not in "- 2^y".
An therefore precedence would now depend on whether there is a space
after the minus or not, or something like that? I'm confused.

H.

> And since constants are generated by the tokenizer, negative ones are not generated. I don't think it is completely out of reach for the parser to recognize the pattern "unary minus a numeric constant" and fold it into a constant of the opposite sign, but I'm not volunteering... (and anyways, it is part of the bigger issue of general constant folding --- I suppose that Luke has a handle on that.)
>
>
>>
>>> 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
>>>
>>
>> ______________________________________________
>> 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