[Rd] Aggregate factor names

[Prof Brian Ripley]

You seem to be assuming that the argument 'by' to the "data frame" method 
of aggregate() is a call to list() with arguments which are names (and 
evaluate to factors).

When aggregate.data.frame comes to be called, the 'by' argument is a 
promise to the actual argument.  In your example the actual argument is 
the call (in a readable layout)

list(one.iv, another.iv, yet.another.iv)

but that is one of a very large number of possibilities for 'by'.  Trying 
to produce reasonable names for unnamed arguments is hard enough (see 
?cbind), but trying to deduce reasonable names for the elements of a list 
argument is one step further up the chain.  Further, if we did that, 
people who wanted the documented behaviour would not longer be able to get 
it.

I think what you want is a function that takes unnamed arguments and 
returns a named list, to replace your usage of list().  That's not so very 
hard to do, not least as in this context data.frame() will do the job.
So to extend the example on the help page

> aggregate(x = testDF, by = data.frame(by1, by2), FUN = "mean")
    by1  by2 v1 v2
1    1   95  5 55
2    2   95  7 77
3    1   99  5 55
4    2   99 NA NA
5  big damp  3 33
6 blue  dry  3 33
7  red  red  4 44
8  red  wet  1 11

However, note that the grouping variables need NOT be factors, and this 
has made them so.  So you may want to look at data.frame() and
write list_with_names() to do just that.



On Thu, 27 Sep 2007, Mike Lawrence wrote:

> Hi all,
>
> A suggestion derived from discussions amongst a number of R users in
> my research group: set the default column names produced by aggregate
> () equal to the names of the objects in the list passed to the 'by'
> object.
>
> ex. it is annoying to type
>
> with(
> 	my.data
> 	,aggregate(
> 		my.dv
> 		,list(
> 			one.iv = one.iv
> 			,another.iv = another.iv
> 			,yet.another.iv = yet.another.iv
> 		)
> 		,some.function
> 	)
> )
>
> to yield a data frame with names = c
> ('one.iv','another.iv','yet.another.iv','x') when this seems more
> economical:
>
> with(
> 	my.data
> 	,aggregate(
> 		my.dv
> 		,list(
> 			one.iv
> 			,another.iv
> 			,yet.another.iv
> 		)
> 		,some.function
> 	)
> )
>
> --
> Mike Lawrence
> Graduate Student, Department of Psychology, Dalhousie University
>
> Website: http://memetic.ca
>
> Public calendar: http://icalx.com/public/informavore/Public
>
> "The road to wisdom? Well, it's plain and simple to express:
> Err and err and err again, but less and less and less."
> 	- Piet Hein
>
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
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