[R] strange behavior of names<-

[Prof Brian Ripley]

Remember tapply with a single factor in R returns a 1D array.  What you
are seeing are the dimnames, not the names: look at attributes() on your
return value (or even name() or str() on it).

I suspect you intended an as.vector() call in the formation of m.

Brian

On Sun, 9 May 2004, Liaw, Andy wrote:

> 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?


-- 
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)
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