[R] elements that appear only once

[Robin Hankin]

Hi.

I have a factor and I want to extract just those elements that appear  
exactly once.
How to do this?

Toy example follows.

 > a <- as.factor(c(rep("oak",5) ,rep("ash",1),rep("elm",1),rep 
("beech",4)))
 > a
[1] oak   oak   oak   oak   oak   ash   elm   beech beech beech beech
Levels: ash beech elm oak
 > table(a)
a
   ash beech   elm   oak
     1     4     1     5
 >

So I would want "ash" and "elm", because there is only one ash and
only one elm in my wood.

My Best Effort:


 > names(table(a)[table(a)==1])
[1] "ash" "elm"
 >

This doesn't seem particularly elegant to me; there must be a better  
way!

anyone?




--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743