[R] elements that appear only once

From: Robin Hankin <r.hankin_at_noc.soton.ac.uk>
Date: Wed 22 Feb 2006 - 20:11:51 EST


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
> a

[1] oak oak oak oak oak ash elm beech beech beech beech Levels: ash beech elm oak
> table(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!


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

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Received on Wed Feb 22 20:16:18 2006

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