[R] Strange case of partial matching in .[ - possible bug / wrong documentation?

[Duncan Murdoch]

On 19/05/2010 12:14 PM, Hilmar Berger wrote:
> Hi all,
>
> This occurred in R-2.11.0 (WinXP).
>
> The R-help page of .[ says that:
>
> "Character indices can in some circumstances be partially matched (see
> pmatch) to the names or dimnames of the object being subsetted (but never
> for subassignment). Unlike S (Becker et al p. 358)), R has never used
> partial matching when extracting by [, and as from R 2.7.0 partial matching
> is not by default used by [[ (see argument exact)."
>
> My understanding is therefore that .[ should never try partial matching.
>   

See near the top of the page:  "The descriptions here apply only to the 
default methods."  Since indexing is generic, an extraction method can 
do whatever it wants, and you need to read the particular page to find 
the behaviour.  The page for Extract.data.frame says:

"Both |[| and |[[| extraction methods partially match row names. By 
default neither partially match column names, but |[[| will unless 
|exact=TRUE|. If you want to do exact matching on row names use |match 
<http://127.0.0.1:28754/library/base/help/match>| as in the examples."

Duncan Murdoch
> However:
>
> > df = data.frame(a=c(1,2,3,9), b=c(4,5,6,10))
> > rownames(df) = c("ef","gg","hh","fe")
> > df
>    a  b
> ef 1  4
> gg 2  5
> hh 3  6
> fe 9 10
>
>
> > df["e",]
>       a b
> ef 1 4
>
> > rownames(df) = c("ef","gg","hh","efg")
> > df["e",]
>     a  b
> NA NA NA
>
> So, it looks like partial matching is done using pmatch("e",rownames(df))
> for "[". If this is true, the help page is not correct.
>
> Thanks !
> Regards,
> Hilmar
>
> ---
> Hilmar Berger
> Integromics S.L. / CNB-CSIC
> Madrid, Spain
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org 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.
>