factor level issue after subsetting

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Schreiber, Stefan
Sent: Tuesday, November 01, 2011 2:29 PM
To: r-help at r-project.org
Subject: [R] factor level issue after subsetting

Dear list,

I cannot figure out why, after sub-setting my data, that particular
item
which I don't want to plot is still in the newly created subset (please
see example below). R somehow remembers what was in the original data
set. 

all fine; but I don't like this idea and was wondering what am I
missing
here?

Thanks!
Stefan

P.S. I am using R 2.13.2 for Mac.

dat<-read.csv("~/MyFiles/data.csv")
class(dat$treat)

[1] "factor"

dat

   treat yield
1   cont  98.7
2   cont  97.2
3   cont  96.1
4   cont  98.1
5     10 103.0
6     10 101.3
7     10 102.1
8     10 101.9
9     30 121.1
10    30 123.1
11    30 119.7
12    30 118.9
13    60 109.9
14    60 110.1
15    60 113.1
16    60 112.3

plot(dat$treat,dat$yield)
dat.sub<-dat[which(dat$treat!='cont')]
class(dat.sub$treat)

[1] "factor"

dat.sub

   treat yield
5     10 103.0
6     10 101.3
7     10 102.1
8     10 101.9
9     30 121.1
10    30 123.1
11    30 119.7
12    30 118.9
13    60 109.9
14    60 110.1
15    60 113.1
16    60 112.3

plot(dat.sub$treat,dat.sub$yield)

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