[R] Splicing factors without losing levels

ONKELINX, Thierry Thierry.ONKELINX at inbo.be
Tue Jun 9 11:23:36 CEST 2009


Dear Titus,

Your first function can be simplified to

  splice <- function(x, y) {
    as.vector(rbind(x, y))
  } 

For factors, you better convert them first back to character strings.

  splice <- function(x, y) {
	x <- levels(x)[x]
	y <- levels(y)[y]
	factor(as.vector(rbind(x, y)))
  } 

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Titus von der Malsburg
Verzonden: dinsdag 9 juni 2009 11:12
Aan: r-help at r-project.org
Onderwerp: [R] Splicing factors without losing levels


Hi list!

An operation that I often need is splicing two vectors:

  > splice(1:3, 4:6)
  [1] 1 4 2 5 3 6

For numeric vectors I use this hack:

  splice <- function(x, y) {
    xy <- cbind(x, y)
    xy <- t(xy)
    dim(xy) <- length(x) * 2
    return(xy)
  }

So far, so good (?).  But I also need splicing for factors and I tried
this:

  splice <- function(x, y) {
    xy <- cbind(x, y)
    xy <- t(xy)
    dim(xy) <- length(x) * 2
    if (is.factor(x) && is.factor(y)) {
      xy <- as.factor(xy)
      levels(xy) <- levels(x)
    }
    return(xy)
  }

This, however, doesn't work because the level name to integer mapping
gets mixed up when copying the levels from x to xy.

My questions:

 1.) How can this be fixed?
 2.) What's the best way to do splicing of vectors and factors in R?
 (I couldn't find a prefdefined function for this although it seems to
be  such a basic and useful operation.)

Thanks!!

 Titus

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