# [Rd] scoping error in xy.coords (PR#932)

**Thomas Lumley
**
tlumley@u.washington.edu

*Fri, 4 May 2001 08:40:48 -0700 (PDT)*

On Fri, 4 May 2001 kjetilh@umsanet.edu.bo wrote:
>* 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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