c.factor

On Tue, 2006-11-14 at 16:36 +0000, Matthew Dowle wrote:

Hi,

Given factors x and y,  c(x,y) does not seem to return a useful result :

x

[1] a b c d e
Levels: a b c d e

y

[1] d e f g h
Levels: d e f g h

c(x,y)

 [1] 1 2 3 4 5 1 2 3 4 5

Is there a case for a new method c.factor as follows?  Does something
similar exist already?  Is there a better way to write the function?

c.factor = function(x,y)

{
    newlevels = union(levels(x),levels(y))
    m = match(levels(y), newlevels)
    ans = c(unclass(x),m[unclass(y)])
    levels(ans) = newlevels
    class(ans) = "factor"
    ans
}

c(x,y)

 [1] a b c d e d e f g h
Levels: a b c d e f g h

as.integer(c(x,y))

 [1] 1 2 3 4 5 4 5 6 7 8

Regards,
Matthew

I'll defer to others as to whether or not there is a basis for c.factor,
however:

c.factor <- function(...)
{
  args <- list(...)

  # this could be optional
  if (!all(sapply(args, is.factor)))
   stop("All arguments must be factors")

  factor(unlist(lapply(args, function(x) as.character(x))))
}