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, [EMAIL PROTECTED]
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
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