Re: [Rd] c.factor

From: Matthew Dowle <>
Date: Wed 22 Nov 2006 - 15:17:33 GMT

I noticed that a new feature in R 2.4 is that unlist of a list of factors already does the operation that I proposed :

> x = factor(letters[1:5])
> y = factor(letters[4:8])
> unlist(list(x,y))
 [1] a b c d e d e f g h
Levels: a b c d e f g h

Therefore, does it not make sense that c(x,y) should return the same as unlist(list(x,y)) ?

Also, the specific "if" for factors inside the definition of unlist, not surprisingly, uses a very similar method to those previously posted. However, it first coerces the factors with as.character, before matching to the new level set. This is inefficient. Here is the c.factor method again that I proposed, which avoids the as.character and is therefore more efficient. Leaving aside the discussion about c.factor, or concat, or whatever, could 'unlist' be changed to use this method instead ? After all one of the key advantages of factors is to save main memory, anything which coerces back to character is going to defeat the benefit.

> c.factor = function(...) {

    args <- list(...)
    if (!all(sapply(args, is.factor))) stop("all arguments must be factor")     newlevels = unique(unlist(lapply(args,levels)))     ans = unlist(lapply(args, function(x) {

        m = match(levels(x), newlevels)

    levels(ans) = newlevels
    class(ans) = "factor"
> identical(c(x,y), unlist(list(x,y)))
[1] TRUE
> version
platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
major          2
minor          4.0
year           2006
month          10
day            03
svn rev        39566
language       R

version.string R version 2.4.0 (2006-10-03)

"Brian Ripley" <> wrote in message news:Pine.LNX.4.64.0611150926070.19618@auk.stats...
> On Tue, 14 Nov 2006, Bill Dunlap wrote:
>> On Tue, 14 Nov 2006, Prof Brian Ripley wrote:
>>> Well, R has managed without a factor method for c() for most of its
>>> decade
>>> of existence (not that it originally had factors as we know them).
>>> I would argue that factors are best viewed as an enumeration type, and
>>> anything which silently changes their level set is a bad idea. I can
>>> see
>>> a case for a c() method for factors that combines factors with the same
>>> level sets, but I can also see this is best done by users who know the
>>> level sets are same (c.factor would have to expend a considerable effort
>>> to check).
>>> You also need to consider the dispatch rules. c.factor will be called
>>> whenever the first argument is a factor, whatever the others are. S4 (I
>>> think, definitely S4-based versions of S-PLUS) has an alternative
>>> concat()
>>> that works differently (recursively) and seems a more natural model.
>> In addition, c() has always had a double meaning of
>> (a) turning an object into a simple "vector" (an object
>> without "attributes"), as in
>> > c(factor(c("Cat","Dog","Cat")))
>> [1] 1 2 1
>> > c(data.frame(x=1:2,y=c("Dog","Cat")))
>> $x
>> [1] 1 2
>> $y
>> [1] Dog Cat
>> Levels: Cat Dog
> To my surprise that was not documented at all on the R help page, and I've
> clarified it. (BTW, at least in R it does not remove names, just all
> other attributes.)
>> (b) concatenating several such vectors into one.
>> The proposed c.factor does only (b).
> (Strictly not, as a factor is not a vector.)
> But the help page explicitly only describes the default method, and some
> of the other methods do preserve some attributes, AFAIR.
>> Should we just
>> throw c() into the ash heap and use as.vector() or
>> concat() instead?
>> The whole concept of concatenating objects of disparate
>> types is suspect.
> I think working on a concat() for R would be helpful. I vaguely recalled
> something like it in the Green Book, but the index does not help (but then
> it is not very complete).
> Brian mailing list Received on Thu Nov 23 17:06:09 2006

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.1.8, at Thu 23 Nov 2006 - 08:30:48 GMT.

Mailing list information is available at Please read the posting guide before posting to the list.