[R] newbie questions: attached data.frames and rm()

[Peter Dalgaard BSA]

Alojz Loui Polic <lpolic at ineural.com> writes:

> Hi
> 
> I'm just starting with R-plus and I have noticed
                          ?????

> the following behaviour.
> 
> If a data.frame is attached and then removed before 
> being detached, the data is still present in the 
> path and can accessed. Is this expected behaviour, 
> and if so why? 

It's designed that way. Whether or not that is a good thing is
debatable - people easily get bitten from attaching a data frame and
modifying it without that affecting the attached version. One upside
is that the argument to attach() can be an arbitrary expression, e.g.

AA <- c(1, 1, 1, 1, 1)
BB <- c(2, 3, 2, 1, 2)
attach(data.frame( AA.data=AA, BB.data=BB)

[It is not a trivial exercise to think up an alternative semantic
model for attached objects. One problem being that modifying an object
in R is more often than not a compute-and-rename operation, so
accessing an object by reference is not possible (although reference
by name is).]
-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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