[Rd] Bug or a feature that I completely missed?

[Prof Brian Ripley]

On Wed, 16 Nov 2005, Berwin A Turlach wrote:

> Dear all,
>
> while looking at some R-code submitted by students in a unit that I
> teach, I came across constructs that I thought would lead to an error.
> Much to my surprise, the code is actually executed.
>
> A boiled down version of the code is the following:
>
>> tt <- function(x, i){
> +   mean(x[i,2])/mean(x[i,1])
> + }
>> dat <- matrix(rnorm(200), ncol=2)
>> mean(dat[,2])/mean(dat[,1])
> [1] -1.163893
>> dat1 <- data.frame(dat)
>> tt(dat1)                 ###  Why does this work?
> [1] -1.163893
>> tt(dat)
> Error in mean(x[i, 2]) : argument "i" is missing, with no default
>
> Since the data for the assignment was in a data frame, the students got
> an answer and not an error message when they called the equivalent of
> tt(dat1) in their work.
>
> I tested this code on R 1.8.1, 1.9.1, 2.0.0, 2.0.1, 2.1.0, 2.1.1,
> 2.2.0 and R-devel (2005-11-14 r36330), all with the same result, no
> error message when executing tt(dat1).
>
> I would have expected that tt(dat1) behaves in the same way as tt(dat)
> and would produce an error.  Thus, I think it is a bug, but the fact
> that so many R versions accept this code makes me wonder whether it is
> a misunderstanding on my side.  Can somebody enlighten me why this
> code is working?

[.data.frame is interpreted, [ is internal for a matrix.  The issue is 
what happens to x[i,2] where i is missing.  In [.data.frame you find

     if(missing(i)) { # df[, j] or df[ , ]
         ## handle the column only subsetting ...
         if(!missing(j)) x <- x[j]
 	cols <- names(x)
 	if(any(is.na(cols))) stop("undefined columns selected")
     } ...

so it was deliberate, it seems.  I believe S used to do the same 
thing in its S3 days, but it appears this is now an error.  However, 
missingness is an area of S/R differences (mainly undocumented, I think).
Currently in S there is

> args("[.data.frame")
function(x, ..., drop = T)

whereas in R

> args("[.data.frame")
function (x, i, j, drop = ....

Since [ is primitive you cannot use args() on it, but its argument list is 
more like S's (which is f(x, ..., drop = T) for the generic and all 
methods).

I don't believe this would be easy to change if we wanted to.  Similar 
things happen for the replacement method.

-- 
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