[R] Rule for accessing attributes?

[Christos Hatzis]

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...

-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.
> 
>