[Rd] c.factor

Matthew Dowle mdowle at concordiafunds.com
Wed Nov 22 16:17:33 CET 2006

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" <ripley at stats.ox.ac.uk> wrote in message 
news:Pine.LNX.4.64.0611150926070.19618 at 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

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