[R] Automatically Remove Aliased Terms from a Model

[Thaler,Thorn,LAUSANNE,Applied Mathematics]

Dear all,

I am trying to implement a function which removes aliased terms from a model. The challenge I am facing is that with "alias" I get the aliased coefficients of the model, which I have to translate into the terms from the model formula. What I have tried so far:

------------------8<------------------
d <- expand.grid(a = 0:1, b=0:1)
d$c <- (d$a + d$b)  %% 2
d$y <- rnorm(4)
d <- within(d, {a <- factor(a); b <- factor(b); c <- factor(c)})
l <- lm(y ~ a * b + c, d)

removeAliased <- function(mod) {
  ## Retrieve all terms in the model
  X <- attr(mod$terms, "term.label")
  ## Get the aliased coefficients  
  rn <- rownames(alias(mod)$Complete)
  ## remove factor levels from coefficient names to retrieve the terms
  regex.base <- unique(unlist(lapply(mod$model[, sapply(mod$model, is.factor)], levels)))
  aliased <- gsub(paste(regex.base, "$", sep = "", collapse = "|"),  "", gsub(paste(regex.base, ":", sep = "", collapse = "|"), ":", rn))
  uF <- formula(paste(". ~ .", paste(aliased, collapse = "-"), sep = "-"))
  update(mod, uF)
}

removeAliased(l)
------------------>8------------------

This function works in principle, but this workaround with removing the factor levels is just, well, a workaround which could cause problems in some circumstances (when the name of a level matches the end of another variable, when I use a different contrast and R names the coefficients differently etc. - and I am not sure which other cases I am overlooking).

So my question is whether there are some more intelligent ways of doing what I want to achieve? Is there a function to translate a coefficient of a LM back to the term, something like:

termFromCoef("a1") ## a1
termFromCoef("a1:b1") ## a:b

With this I could simply translate the rownames from alias into the terms needed for the model update.

Thanks for your help.

Kind Regards,

Thorn Thaler 
NRC Lausanne
Applied Mathematics