[R] Thougt I understood factors but??

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Nicholas Lewin-Koh
> Sent: Monday, March 01, 2010 9:08 AM
> To: r-help at r-project.org
> Subject: [R] Thougt I understood factors but??
> 
> Hi,
> consider the following
> > a<-gl(3,3,9)
> > a
> [1] 1 1 1 2 2 2 3 3 3
> Levels: 1 2 3
> > levels(a)<-3:1
> > a
> [1] 3 3 3 2 2 2 1 1 1
> Levels: 3 2 1

A very similar question came up last week.  Your
call to levels<-, with a [non-recursive] vector on the right
side, just relabels the levels.  It does
not alter the integer codes in the factor.  It does
not compare the new level labels with the existing
ones.  E.g.,
  > a <- gl(3,2)
  > str(a)
   Factor w/ 3 levels "1","2","3": 1 1 2 2 3 3
  > levels(a) <- c("Dog", "Cat", "Chicken")
  > str(a)
   Factor w/ 3 levels "Dog","Cat","Chicken": 1 1 2 2 3 3
  > # you can increase number of levels (but not decrease)
  > levels(a) <- c("Dog", "Cat", "Chicken", "Goat")
  > str(a)
    Factor w/ 4 levels "Dog","Cat","Chicken",..: 1 1 2 2 3 3
 
You can use a list on the right side of levels<- to reorder
or combine levels (and thus change the integer codes).
  > # combine levels
  > levels(a) <- list(small=c("Cat", "Chicken"), large=c("Dog","Goat"))
  > str(a)
   Factor w/ 2 levels "small","large": 2 2 1 1 1 1
  > a
  [1] large large small small small small 
  Levels: small large
  # next reorder
  > levels(a) <- list(largeFirst = "large", smallLast = "small")
  > str(a)
   Factor w/ 2 levels "largeFirst","smallLast": 1 1 2 2 2 2
You might find it easier to use factor(), along with ifelse(),
and is.element(), for this sort of thing.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
  
> > a<-gl(3,3,9)
> > factor(a,levels=3:1)
> [1] 1 1 1 2 2 2 3 3 3
> Levels: 3 2 1
> It is probably something obvious I missed, but reading the 
> documentation
> of factor, and levels I would have thought
> that both should produce the same output as 
> factor(a,levels=3:1)
> [1] 1 1 1 2 2 2 3 3 3
> Levels: 3 2 1
> The closest I could find in a quick search was this
> http://tolstoy.newcastle.edu.au/R/e5/help/08/09/2503.html
> 
> Thanks
> Nicholas
> 
> sessionInfo()
> R version 2.10.1 Patched (2009-12-20 r50794) 
> x86_64-unknown-linux-gnu 
> 
> locale:
>  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
>  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
>  [5] LC_MONETARY=C              LC_MESSAGES=en_US.UTF-8   
>  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
>  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
> [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C    
> attached base packages:
> [1] splines   tcltk     stats     graphics  grDevices utils   
>   datasets 
> [8] methods   base     
> 
> other attached packages:
> [1] mvtnorm_0.9-9      latticeExtra_0.6-9 RColorBrewer_1.0-2
> lattice_0.18-3    
> [5] nlme_3.1-96        XML_2.6-0          gsubfn_0.5-0       
> proto_0.3-8 
> 
> loaded via a namespace (and not attached):
> [1] grid_2.10.1  tools_2.10.1
> 
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