[R] Rule for accessing attributes?

[Tribo Laboy]

Thanks, this really solved my problem.

Actually, I do have a good introduction to R - a  book co-authored by
some W.N. Venables and some B.D. Ripley, colloquially called 'MASS' is
on my desk.  I find it really very helpful. Still, as it is a book on
statistics, some details on R are only mentioned in passing.
I also use some other books and online resources, but obviously will
need to delve a bit deeper. One of the reasons I am moving to R is
because it has a great user community. I hope I am not abusing it with
too many questions.

Regards,

TL


On Thu, Mar 27, 2008 at 5:16 PM, Prof Brian Ripley
<ripley at stats.ox.ac.uk> wrote:
> Use [[ ]].
>
>  It seems it is time for you to study a good introduction to R.
>
>
>
>
>  On Thu, 27 Mar 2008, Tribo Laboy wrote:
>
>  > Now, how is it that I can access the contents of a named list by
>  > dynamically computed name?
>  >
>  > To go back to my previous example I have a list and I know the names.
>  > Now I want do something with that named data in a loop.
>  >
>  > lst <- list(x = 1:3, y = 4:6, z = 7:9)
>  > nm <-names(lst)
>  > nm
>  > [1] "x" "y" "z"
>  >
>  > I can access the list elements by name directly:
>  >
>  > lst$x; lst$y; lst$z,
>  >
>  > But I want to do
>  >
>  > for (k in 1:3) {
>  >     lst$nm[k]
>  > }
>  >
>  > But this doesn't work, basically because
>  > lst$nm[1] returns a NULL.
>  >
>  > So what do I do?
>  >
>  > Thanks for helping,
>  >
>  > TL
>  >
>  >
>  > On Thu, Mar 27, 2008 at 4:30 PM, Tribo Laboy <tribolaboy at gmail.com> wrote:
>  >> So am I to understand that the only realy _correct_ and _recommended_
>  >>  way of accessing the attributes is through
>  >>
>  >>  attr(someobject, "attributename") ?
>  >>
>  >>
>  >>  Regards,
>  >>
>  >>  TL
>  >>
>  >>
>  >>
>  >>  On Thu, Mar 27, 2008 at 4:16 PM, Prof Brian Ripley
>  >>  <ripley at stats.ox.ac.uk> wrote:
>  >> > Oh please don't recommend misuse of @ to those already confused.
>  >> >
>  >> >  @ is for accessing slots in S4 objects.  This 'works' because they happen
>  >> >  to be stored as attributes.  See the help page (and the warning that it
>  >> >  does no checking - we may change that).
>  >> >
>  >> >  Similarly,
>  >> >
>  >> >  plt$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
>  >> >
>  >>
>  >
>
>  --
>
>
> 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
>