[Rd] Why is there no c.factor?

[John Fox]

Dear Thomas and Hadley,

I'd propose the following: If the sets of levels of all arguments are the
same, then c.factor() would return a factor with the common set of levels;
if the sets of levels differ, then, as Hadley suggests, the level-set of the
result would be the union of sets of levels of the arguments, but a warning
would be issued.

Best,
 John


> -----Original Message-----
> From: r-devel-bounces at r-project.org [mailto:r-devel-bounces at r-project.org]
On
> Behalf Of Thomas Lumley
> Sent: February-04-10 12:07 PM
> To: Hadley Wickham
> Cc: r-devel at r-project.org
> Subject: Re: [Rd] Why is there no c.factor?
> 
> On Thu, 4 Feb 2010, Hadley Wickham wrote:
> 
> > Hi all,
> >
> > Is there are reason that there is no c.factor method?  Analogous to
> > c.Date, I'd expect something like the following to be useful:
> >
> > c.factor <- function(...) {
> >  factors <- list(...)
> >  levels <- unique(unlist(lapply(factors, levels)))
> >  char <- unlist(lapply(factors, as.character))
> >
> >  factor(char, levels = levels)
> > }
> >
> > c(factor("a"), factor("b"), factor(c("c", "b","a")), factor("d"))
> > # [1] a b c b a d
> > # Levels: a b c d
> >
> 
> It's well established that different people have different views on what
> factors should do, but this doesn't match mine.   I think of factors as
> enumerated data types where the factor levels already specify all the
valid
> values for the factor, so I wouldn't want to be able to combine two
factors
> with different sets of levels.
> 
> For example:
>    A <- factor("orange",levels=c("orange","yellow","red","purple"))
>    B <- factor("orange", levels=c("orange","apple","mango", "banananana"))
> 
> On the other hand, I think the current behaviour, which reduces them to
> numbers, is just wrong.
> 
> 
>        -thomas
> 
> Thomas Lumley			Assoc. Professor, Biostatistics
> tlumley at u.washington.edu	University of Washington, Seattle
> 
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