[R] Rule for accessing attributes?

[Tribo Laboy]

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
>  >
>