Rule for accessing attributes?

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?

 --
 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