[R] Object attributes in R

[Prof Brian Ripley]

This issue is discussed in much more detail in the manuals for 2.4.0, in 
particular in the new 'R Internals' manual.  You will find the basic rules 
in the Blue Book (Becker, Chambers, Wilks, 1988) but they have not always 
been applied consistently.

On Wed, 11 Oct 2006, Michael Toews wrote:

> Hi,
> I have questions about object attributes, and how they are handled when
> subsetted. My examples will use:
> tm <- (1:10)/10
> ds <- (1:10)^2
> attr(tm,"units") <- "sec"
> attr(ds,"units") <- "cm"
> dat <- data.frame(tm=tm,ds=ds)
> attr(dat,"id") <- "test1"
>
> When a "primitive class" object (numeric, character, etc.) is subsetted,
> the attributes are often stripped away, but the rules change for more
> complex classes, such as a data.frame, where they 'stick' for the
> data.frame, but attributes from the members are lost:
> tm[3:5]        # lost
> ds[-3]         # lost
> str(dat[1:3,]) # only kept for data.frame
>
> Is there any way of keeping the attributes when subsetted from primitive
> classes, like a fictional "attr.drop" option within the "[" braces? The
> best alternative I have found is to make a new object, and copy the
> attributes:
> tm2 <- tm[3:5]
> attributes(tm2) <- attributes(tm)
>
> However, for the data.frame, how can I copy the attributes over (without
> using a for loop -- I've tried a few things using sapply but no success)?
> Also I don't see how this is consistent with an empty index, "[]", where
> attributes are always retained (as documented):
> tm[]
>
> I have other concerns about the evaluation of objects with attributes
> (e.g. ds/tm), where the attributes from the first object are retained
> for the output, but this evaluation of logic is a whole other can of
> worms I'd rather keep closed for now.
>
> +mt
>
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>

-- 
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