[R] Weird behavior with names()
Prof Brian D Ripley
ripley at stats.ox.ac.uk
Thu Jul 15 08:13:30 CEST 1999
On Wed, 14 Jul 1999, Christian Posse wrote:
> Dear R fellows,
>
> Is the following a feature or a bug of names() ??
It is a feature of matrix indexing.
> It does not seem to be reported as a bug in the R Bug Tracking System.
>
>
> (RedHat 6.0, R-0.64.2 compiled from source, not RPM).
>
> # a trivial test
> x <- 1:5
> y <- 6:10
> names(x) <- c("a", "b", "c", "d", "e")
> names(y) <- c("a", "b", "c", "d", "e")
> xy <- cbind(x,y)
>
> ## test names()
>
> > names(x)
> [1] "a" "b" "c" "d" "e"
> > names(xy)
> NULL
> > names(xy[,1])
> [1] "a" "b" "c" "d" "e"
>
>
> So far, so good. But now look at what happens when I tried
> to extract from the array:
>
> > names(x[-(1:3)])
> [1] "d" "e"
> > names(xy[-(1:3),1]) it works
> [1] "d" "e"
> > names(xy[,1])[-(1:3)] it works too.....
> [1] "d" "e"
>
> > names(x[-(1:4)])
> [1] "e"
> > names(xy[-(1:4),1])
> NULL ??????? should be
> "e"
No, it should not:
> xy[-(1:4),1]
[1] 5
has no names, so names is reporting correctly. I believe you think it
should have names, but it does not in R nor S. Try (in S)
> xy[-(1:4),1, drop=F]
x
e 5
Now, should the name be "e" (xy[, 1][-(1:4)]) or "x" (xy[-(1:4),][1])?
Sensibly, S does not try to choose.
In R, cbind works slightly differently: use xy <- cbind(x=x, y=y)
to see the dilemma more clearly.
[BTW, I think this is a bug in R's cbind and should be fixed.]
--
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 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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