R help - Aug 2007 - Does anyone else think this might be worth a warning?!?

Hi,

I was *very* surprised by this little trick for new players: mean() only 
considers its first argument!

 > mean(1,1,2)
[1] 1
 > mean(2,1,1)
[1] 2


I found this very different behaviour to max():

 > max(1,1,2)
[1] 2
 > max(2,1,1)
[1] 2



Perhaps this is the wrong list to ask, but does anyone else think this a 
little on the interesting side?  Is it not possible to detect a first 
argument of length one in the presence of other un-named arguments and 
at least produce a warning?


Cheers,


Matthew

First, note that functions in R match named arguments first, followed by 
the position of the arguments in the call.

Second, have a look at how mean and max are defined

    mean <- function (x, trim = 0, na.rm = FALSE, ...){

    max <- function (..., na.rm = FALSE){

It's the difference in the position of "..." argument or catchall 
argument (sorry, I don't know its formal name) that determines the 
different behaviour. The "..." is often converted to a list
internally.

So when you type in mean(1,1,2), it is treated as
  "mean( x=1, trim=1, na.rm=2 )".

and when you type in max(1,1,2), it is treated as
  "max( as.list(1,1,2), na.rm = FALSE )"


However, you do raise a good point. Reading mean.default(), I do not see 
how and when the "..." argument in mean() comes to play. Perhaps 
redefine mean to be mean <- function (..., trim = 0, na.rm = FALSE) so 
that it is similar to max, sum, range etc.

But there might be a philosopphical counter argument for this as well. 
Functions like mean() and sd() are supposed to summarise a single vector 
whereas max, sum, range can work on several vectors by concatenating 
them into a single list. Consider max( c(1,2,3), c(2,3,4) ).

Regards, Adai



Matthew Walker wrote:
> Hi,
> 
> I was *very* surprised by this little trick for new players: mean() only 
> considers its first argument!
> 
>  > mean(1,1,2)
> [1] 1
>  > mean(2,1,1)
> [1] 2
> 
> 
> I found this very different behaviour to max():
> 
>  > max(1,1,2)
> [1] 2
>  > max(2,1,1)
> [1] 2
> 
> 
> 
> Perhaps this is the wrong list to ask, but does anyone else think this a 
> little on the interesting side?  Is it not possible to detect a first 
> argument of length one in the presence of other un-named arguments and 
> at least produce a warning?
> 
> 
> Cheers,
> 
> 
> Matthew
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
> 
>

I do think this is worth a warning.
mean.default could do something like
if (length(list(...)) > 0) warning("extra arguments ignored")

The same could also apply to many other methods of S3 generic
functions which are forced to include the formal argument `...` in the
signature but do not use it.

Felix


On 8/19/07, Matthew Walker <m.g.walker at massey.ac.nz>
wrote:
> Hi,
>
> I was *very* surprised by this little trick for new players: mean() only
> considers its first argument!
>
>  > mean(1,1,2)
> [1] 1
>  > mean(2,1,1)
> [1] 2
>
>
> I found this very different behaviour to max():
>
>  > max(1,1,2)
> [1] 2
>  > max(2,1,1)
> [1] 2
>
>
>
> Perhaps this is the wrong list to ask, but does anyone else think this a
> little on the interesting side?  Is it not possible to detect a first
> argument of length one in the presence of other un-named arguments and
> at least produce a warning?
>
>
> Cheers,
>
>
> Matthew
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Felix Andrews / ???
PhD candidate
Integrated Catchment Assessment and Management Centre
The Fenner School of Environment and Society
The Australian National University (Building 48A), ACT 0200
Beijing Bag, Locked Bag 40, Kingston ACT 2604
http://www.neurofractal.org/felix/
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