Re: [R] Rule for accessing attributes?

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Thu, 27 Mar 2008 08:16:30 +0000 (GMT)

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

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