[R] using variables in obj$model

[Prof Brian Ripley]

Model frames do not contain interactions, they contain variables.
`a:b' is not the name of a variable whereas A is.

On Mon, 27 Oct 2003, Vito Muggeo wrote:

> dear all,
> for some reason I am intersted in updating a glm taking variables from its
> model argument, namely:
> 
> > dati<-data.frame(y=runif(10),x=1:10)
> > obj<-glm(y~x,data=dati)
> > obj$model[,c("A","a:b")]<-cbind(rnorm(10),runif(10))
> > names(obj$model)
> [1] "y"   "x"   "A"   "a:b"
> > update(obj,.~.+A,data=obj$model) #it works
> 
> Call:  glm(formula = y ~ x + A, data = obj$model)
> ............[SNIP]
> 
> > update(obj,.~.+A+a:b,data=obj$model) #it does not work
> Error in eval(expr, envir, enclos) : Object "a" not found

-- 
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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