[Rd] value returned by by()

[Seb]

On Wed, 15 Sep 2010 11:29:23 +0200,
peter dalgaard <pdalgd at gmail.com> wrote:

> On Sep 15, 2010, at 10:55 , Uwe Ligges wrote:

> > 
> > 
> > On 14.09.2010 20:50, Seb wrote:
> >> On Tue, 14 Sep 2010 12:02:04 +0200,
> >> Uwe Ligges<ligges at statistik.tu-dortmund.de>  wrote:
> >> 
> >>> It returns a list with athe class attribut set to "by", just use: x<-
> >>> by(.....)  unclass(x)
> >> 
> >> Thanks Uwe, however, that still returns an array when using the
> >> data.frame method for by():
> >> 
> >> R>  class(unclass(with(warpbreaks, by(warpbreaks[, 1:2], tension, summary))))
> >> [1] "array"
> >> 
> >> It seems as if the only way to really ensure a list:
> >> 
> >> R>  class(lapply(unclass(with(warpbreaks, by(warpbreaks[, 1:2], tension, summary))), function(x) x))
> >> [1] "list"
> >> 
> >> but it seems like a waste to call another function just to do this.
> >> 
> >> 
> > 
> > Then you could still do
> > 
> > x <- by(.....)
> > attributes(x) <- NULL
> > 

> Or just use c() instead of unclass(). (The root cause is that even with simplify=FALSE, tapply() will always create an array, in this case a 1d array with dim=3. The _contents_ of the array will be a list, though.)

> Notice that in the relevant cases, what you get really _is_ a list, and both walks and quacks like one.  E.g.

> > L <- with(warpbreaks, by(warpbreaks[, 1], tension, mean, simplify=FALSE))
> > is.list(L)
> [1] TRUE
> > L$M
> [1] 26.38889

But if one tries to include this list dressed in 'by' clothes into an S4
class slot declared as a list, then we have problems.  In that case, I
propose this simple patch to by.Rd, which simply removes the statement
about the result being *always* a list.


Index: by.Rd
===================================================================
--- by.Rd	(revision 52375)
+++ by.Rd	(working copy)
@@ -36,8 +36,6 @@
 }
 \value{
   An object of class \code{"by"}, giving the results for each subset.
-  This is always a list if \code{simplify} is false, otherwise a list or
-  array (see \code{\link{tapply}}).
 }
 \seealso{\code{\link{tapply}}}


Thanks!

-- 
Seb