[R] recursive relevel

[ONKELINX, Thierry]

Dear Baptiste,

You can avoid the recursive stuff. And it will run about twice as fast.

>  order.factor <- function (x, ref)
+  {
+  last.index <- length(ref) # convenience for matlab's end keyword
+  if(last.index == 1) return(relevel(x, ref)) # end case, normal case  
+  my.new.list <- list(x=relevel(x, ref[last.index]), ref=ref[-last.index])
+  return(do.call(order.factor, my.new.list)) # recursive call
+  }
> 
> order.factor2 <- function(x, ref){
+     factor(x, levels = c(ref, sort(levels(x)[!levels(x) %in% ref])))
+ }
> order.factor3 <- function(x, ref){
+     factor(x, levels = c(ref, sort(levels(x)[!levels(x) %in% ref])), labels = c(ref, sort(levels(x)[!levels(x) %in% ref])))
+ }
> 
> x <- factor(sample(LETTERS[1:5], 10000000, replace = TRUE))
> y <- factor(sample(LETTERS[1:20], 10000000, replace = TRUE))
> system.time(order.factor(x, c("D", "B")))
   user  system elapsed 
   5.69    0.38    6.09 
> system.time(order.factor2(x, c("D", "B")))
   user  system elapsed 
   3.90    0.20    4.12 
> system.time(order.factor3(x, c("D", "B")))
   user  system elapsed 
   3.26    0.19    3.46 
> system.time(order.factor(y, c("D", "B")))
   user  system elapsed 
  17.43    0.39   17.84 
> system.time(order.factor3(y, c("D", "B")))
   user  system elapsed 
   8.25    0.17    8.46 


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 

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~ Sir Ronald Aylmer Fisher

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~ Roger Brinner

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-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Namens baptiste auguie
Verzonden: vrijdag 9 januari 2009 15:11
Aan: R R-help
Onderwerp: [R] recursive relevel

Dear list,

I'm having second thoughts after solving a very trivial problem: I  
want to extend the relevel() function to reorder an arbitrary number  
of levels of a factor in one go. I could not find a trivial way of  
using the code obtained by getS3method("relevel","factor"). Instead, I  
thought of solving the problem in a recursive manner (possibly after  
reading Paul Graham essays on Lisp too recently). Here is my attempt :

>
> order.factor <- function (x, ref)
> 	{
> 		
> 	last.index <- length(ref) # convenience for matlab's end keyword
> 	if(last.index == 1) return(relevel(x, ref)) # end case, normal case  
> of relevel
> 	my.new.list <- list(x=relevel(x, ref[last.index]),  # creating a  
> list with updated parameters,
> 														# going through the list in reverse order
> 							ref=ref[-last.index]) # chop the vector from its last level
> 	return(do.call(order.factor, my.new.list)) # recursive call
> }
>
> ff <- factor(c("a", "b", "c", "d"))
> ff
> relevel(ff, levels(ff)[1])
> relevel(ff, levels(ff)[2]) # that's the usual case: you want to put  
> a level first
>
> order.factor(x=ff, ref=c("a", "b"))
> order.factor(x=ff, ref=c("c"))
> order.factor(x=ff, ref=c("c", "d")) # that's my wish: put c and d in  
> that order as the first two levels
>


I'm hoping this can be improved in several aspects:

- there is probably already a better function I missed or overlooked  
(I'd still be curious about the following points, though)

- after reading a few threads, it appears that some recursive  
functions are fragile in some sense, and I'm not sure what this means  
in practice. (Should I use Recall, somehow?)

- it's probably quite slow for large data.frames

- I could not think of a good name, this one might clash with some S3  
method perhaps?

- any other thoughts welcome!


Best wishes,

Baptiste
_____________________________

Baptiste Auguié

School of Physics
University of Exeter
Stocker Road,
Exeter, Devon,
EX4 4QL, UK

Phone: +44 1392 264187

http://newton.ex.ac.uk/research/emag

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