R help - Oct 2013 - Automatically Remove Aliased Terms from a Model

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

Hi Thorn,
it is not entirely clear (at least for me) what you want to accomplish.
an easy and fail safe way of extracting used terms in a (g)lm-object is
names(model.frame(l))
if you want to extract terms to finally select a model, have a look at
drop1 and/or MASS::dropterm

Hth

Am 28.10.2013 17:19, schrieb 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
> 
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
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