[R] confusion about what to expect?
MSchwartz at medanalytics.com
Wed Sep 24 02:02:07 CEST 2003
On Tue, 2003-09-23 at 18:08, A.J. Rossini wrote:
> In playing around with data.frames (and wanting a simple, cheap way to
> use the variable and case names in plots; but I've solved that with
> some hacks, yech), I noticed the following behavior with subsetting.
> testdata <- data.frame(matrix(1:20,nrow=4,ncol=5))
> names(testdata) ## expect labels, get them
> names(testdata[2,]) ## expect labels, get them
> names(testdata[,2]) ## expect labels, but NOT -- STRIPPED OFF??
> testdata[,2] ## would have expect a name (X2) in the front? NOT EXPECTED
> testdata[2,] ## get what I expect
> testdata[2,2] ## just a number, not a sub-data.frame? unexpected
> testdata[2,2:3] ## this is a data.frame
> testdata[2:3,2:3] ## and this is, too.
> > version
> platform i386-pc-linux-gnu
> arch i386
> os linux-gnu
> system i386, linux-gnu
> status alpha
> major 1
> minor 8.0
> year 2003
> month 09
> day 20
> language R
> I don't have 1.7.1 handy at this location to test, but I would've
> expected a data.frame-like object upon subsetting; should I have
> expected otherwise? (granted, a data.frame with just a single
> variable could be thought of as silly, but it does have some extra
> information that might be worthwhile, on occassion?)
> I'm not sure that it is a bug, but I was caught by suprise. If it
> isn't a bug, and someone has a concise way to think through this, for
> my future reference, I'd appreciate hearing about it.
A quick review of what is returned when you subset the data.frame
int [1:4] 5 6 7 8
`data.frame': 1 obs. of 5 variables:
$ X1: int 2
$ X2: int 6
$ X3: int 10
$ X4: int 14
$ X5: int 18
 1 5
Quoting from ?Extract:
"When [.data.frame is used for subsetting rows of a data.frame, it
returns a data frame with unique (and non-missing)row names, if
necessary transforming the names using make.names( * , unique = TRUE)"
What is unstated, but covered by R FAQ 7.7 ("Why do my matrices lose
dimensions?"), a single column in a data.frame resulting from the subset
operation is by default turned into a vector. Hence, no names.
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