[R] confusion about what to expect?

[Tony Plate]

Have you investigated the drop= argument to "["? (as in the expression 
testdata[,2,drop=F], which will return a dataframe).

"[.data.frame" has somewhat different behavior from "[" on matrices with 
respect to the drop argument: If the result would be a dataframe with a 
single column, the default behavior of "[.data.frame" is to return a vector 
(return a dataframe always if drop=F), but if the result would be a 
dataframe with a single row, the default behavior is to return a dataframe 
(return a list if drop=T).

E.g.:
 > class(data.frame(a=1:3,b=4:6)[,1])
[1] "integer"
 > class(data.frame(a=1:3,b=4:6)[,1,drop=F])
[1] "data.frame"
 > class(data.frame(a=1:3,b=4:6)[1,])
[1] "data.frame"
 > class(data.frame(a=1:3,b=4:6)[1,,drop=T])
[1] "list"
 >

The default behavior is often what you want, but when it isn't it can be 
confusing, especially it's not that easy to find documentation for this (at 
least not in a quick look through the FAQ, ?"[", and "An Introduction to R" 
-- please excuse me if I overlooked something.)

The thing you have going on with names(testdata[...]) is merely a 
consequence of whether or not the result of the subsetting operation is a 
dataframe or a vector.

hope this helps,

Tony Plate


At Tuesday 04:08 PM 9/23/2003 -0700, you 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.
>
>best,
>-tony
>
>--
>rossini at u.washington.edu            http://www.analytics.washington.edu/
>Biomedical and Health Informatics   University of Washington
>Biostatistics, SCHARP/HVTN          Fred Hutchinson Cancer Research Center
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