[R] strange behavior of names<-

[Gabor Grothendieck]

Execute these two commands:
   ans <- fun(x,f)
   attributes(ans)

and you get this:

   $dim
   [1] 20

   $dimnames
   $dimnames[[1]]
[1] "a" "a" "b" "c" "a" "d" "a" "b" "d" "d" "a" "b" "d" "c" "c" "c" "b" "c" "b"
[20] "d"

so ans does not have names, it has dimnames.  If you try dimnames(ans) <- NULL
then its dimnames do get nulled out.  


Liaw, Andy <andy_liaw <at> merck.com> writes:

: 
: Dear R-help,
: 
: I've encounter what seems to me a strange problem with "names<-".  Suppose I
: define the function:
: 
: fun <- function(x, f) {
:     m <- tapply(x, f, mean)
:     ans <- x - m[match(f, unique(f))]
:     names(ans) <- names(x)
:     ans
: }
: 
: which subtract out the means of `x' grouped by `f' (which is the same as,
: e.g., resid(lm(x~f)) if `f' is a factor).  If `x' does not have names, then
: I'd expect the output of the function not to have names, as names(x) would
: be NULL, and assigning NULL to names(ans) should wipe out the names of
: `ans'.  However, I get:
: 
: > x = rnorm(20)
: > f = factor(sample(rep(letters[1:4], 5)))
: > fun(x, f)
:           a           b           c           b           c           c
: d 
: -0.53791639  1.03704065  0.95727411  0.89219177 -0.04218746  0.57976675
: -2.15799919 
:           a           c           d           a           d           b
: d 
:  1.28422452 -0.92881186  0.40526262 -0.13471983 -0.72599709  1.68726680
: -0.95420354 
:           a           c           a           b           b           d 
: -2.28013373  1.02522037  0.07728352  0.54321899  0.95742354 -1.68420455 
: 
: What am I missing?
: 
: [BTW, this is using the tip that Thomas Lumley posted about forming the
: group means.  I've wanted to write a `tsweep' function that's sort of the
: cross of tapply() and sweep().]
: 
: Best,
: Andy Liaw, PhD
: Biometrics Research      PO Box 2000, RY33-300     
: Merck Research Labs           Rahway, NJ 07065
: mailto:andy_liaw <at> merck.com        732-594-0820
: 
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