[R] Rule for accessing attributes?

[Prof Brian Ripley]

On Thu, 27 Mar 2008, Christos Hatzis wrote:

> Yes, indeed.  The help page says that @ extracts the contents of a slot in
> S4 objects.  But you mention below that this 'works' for S3 objects because
> S4 slots are stored as attributes.  Doesn't this mean that @ is currently
> implemented to access attributes of objects in general (attributes of S3
> objects or slots of S4 objects that are implemented as attributes)?  I
> realize that this might change in the future...

Yes, but it will change, quite possibly for 2.7.0.

>
> -Christos
>
>> -----Original Message-----
>> From: r-help-bounces at r-project.org
>> [mailto:r-help-bounces at r-project.org] On Behalf Of Prof Brian Ripley
>> Sent: Thursday, March 27, 2008 3:17 AM
>> To: Christos Hatzis
>> Cc: r-help at r-project.org
>> Subject: Re: [R] Rule for accessing attributes?
>>
>> Oh please don't recommend misuse of @ to those already confused.
>>
>> @ is for accessing slots in S4 objects.  This 'works' because
>> they happen to be stored as attributes.  See the help page
>> (and the warning that it does no checking - we may change that).
>>
>> Similarly,
>>
>> plt$title <- "My Title"
>>
>> works because the package maintainer (of ggplot2,
>> unmentioned?) has chosen to set things up that way.  R is
>> very flexible, and there is plenty of scope for package
>> authors to do confusing things.
>>
>> On Thu, 27 Mar 2008, Christos Hatzis wrote:
>>
>>> You need to use the '@' operator to directly access attributes (not
>>> elements) of objects:
>>>
>>>> lst at names
>>> [1] "x" "y" "z"
>>>
>>> See ?'@' for more details.
>>>
>>> -Christos
>>>
>>>> -----Original Message-----
>>>> From: r-help-bounces at r-project.org
>>>> [mailto:r-help-bounces at r-project.org] On Behalf Of Tribo Laboy
>>>> Sent: Thursday, March 27, 2008 12:16 AM
>>>> To: r-help at r-project.org
>>>> Subject: [R] Rule for accessing attributes?
>>>>
>>>> Hi !
>>>>
>>>> I am a new user and quite confused by R-indexing.
>>>>
>>>> Make a list and get the attributes
>>>> lst <- list(x = 1:3, y = 4:6, z = 7:9)
>>>> attributes(lst)
>>>>
>>>> This returns:
>>>>
>>>> $names
>>>> [1] "x" "y" "z"
>>>>
>>>> I can easily do:
>>>>
>>>> nm <-names(lst)
>>>>
>>>> or
>>>>
>>>> nm <-attr(lst,"names")
>>>>
>>>> which both return the assigned names of the named list
>> 'lst', but why
>>>> then this doesn't work:
>>>>
>>>> lst$names
>>>>
>>>> ?
>>>>
>>>> I am confused ... Moreover, I noticed that some of the
>> objects (e.g.
>>>> plot objects returned by ggplot) also have attributes when
>> queried by
>>>> the 'attributes' function, but they are accessible by the
>> $ notation.
>>>> (e.g.
>>>>
>>>> xydf <- data.frame(x = 1:5, y = 11:15) plt <- ggplot(data = xydf,
>>>> aes(x = x,y = y)) + geom_point()
>>>> attributes(plt)
>>>>
>>>> Now we can change the title:
>>>>
>>>> plt$title <- "My Title"
>>>> plt
>>>>
>>>> So is it some inconsistency or am I missing something important?
>>>>
>>>> ______________________________________________
>>>> R-help at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide
>>>> http://www.R-project.org/posting-guide.html
>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>
>>>>
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>> --
>> Brian D. Ripley,                  ripley at stats.ox.ac.uk
>> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
>> University of Oxford,             Tel:  +44 1865 272861 (self)
>> 1 South Parks Road,                     +44 1865 272866 (PA)
>> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595