scoping error in xy.coords (PR#932)

Hola!

rm(list=ls(all=TRUE))

x <- 1:20
y <- 1+x+rnorm(x)

xy.coords(y  ~ x,NULL)

... expected output, correct, but when called from inside lowess:

  lowess(y ~ x)

Error in xy.coords(x, y) : x and y lengths differ

If you don't call them x and y it works

x<-1:10
y<-1:10
lowess(y~x)

Error in xy.coords(x, y) : x and y lengths differ

z<-y
w<-x
lowess(z~w)

$x
 [1]  1  2  3  4  5  6  7  8  9 10

$y
 [1]  1  2  3  4  5  6  7  8  9 10

The problem is that the formula is evaluated in the parent frame, where y
evaluates to NULL, and x evaluates to y~x, rather than in the global
environment where they would both evaluate to vectors.

In order to handle formulas correctly I think xy.coords needs to use the
same sort of indirect evaluation as model.frame: eg
   xy <-substitute(xy.coords(X,Y),list(X=substitute(x),Y=substitute(y)))
   xy<-eval(xy,parent.frame())
works as a replacement for the first two lines of lowess().  I hadn't
realised that xy.coords() was supposed to handle formulas, and since it
doesn't have a data= argument it can't do it very well.

This seems a bit drastic, given the number of times xy.coords is used.
Perhaps there's a simpler solution.


	-thomas

Thomas Lumley			Asst. Professor, Biostatistics
tlumley@u.washington.edu	University of Washington, Seattle
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