R help - Oct 2003 - using variables in obj$model

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


Please, how can I solve this problem?
many thanks in avance,
best,
vito

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
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