R help - Apr 2011 - modify particular factor levels

Dear list,

I wish to modify programmatically only a few factor levels, according
to a named list. I came up with this function,

modify.levels <- function(f, modify=list()){

  ## levels that will not be changed
  names.old.levels <- setdiff(levels(f), unlist(modify))
  ## as a named list
  old.levels <- as.pairlist(names.old.levels)
  names(old.levels) <- names.old.levels
  ## union of unchanged levels and modified ones
  levels(f) <-  modifyList(old.levels, modify)
  f
}

f <- factor(LETTERS[1:4])
f2 <- modify.levels(f, list(aa = "A", cc="C"))

It seems to work, but the original order of the levels is changed.
Have I missed a better way of doing this manipulation?

Best regards,

baptiste

baptiste auguie-5 wrote:
> 
> 
> I wish to modify programmatically only a few factor levels, according
> to a named list. I came up with this function..
> ....
> .....
> It seems to work, but the original order of the levels is changed.
> 
> 
The split-and-unite policy you use makes it a bit difficult to re-unite. You
could keep a copy of the original sequence, but it's probably easier to do
it via match. Note also that it also works for simple named vectors, which I
would prefer (I hate these generic lists, lapply).

You should probably think of the error case below. Maybe it is best to keep
it an error.

Dieter



modify.levels <- function(f, modify=list()){
  levs = levels(f)
  m = match(modify,levs)
  levs[m] = names(modify)
  factor(f,labels=levs)
}

f <- factor(c(LETTERS[1:4],LETTERS[1:2]))# Added a few more to avoid special
case
modify.levels(f, list(aa = "A", cc="C"))
modify.levels(f, c(aa = "A", cc="C"))
modify.levels(f, c(aa = "G", cc="C")) # Gives an error



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