Re: [R] Rule for accessing attributes?

From: Tribo Laboy <tribolaboy_at_gmail.com>
Date: Thu, 27 Mar 2008 16:59:04 +0900

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
> >
>



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