factor level issue after subsetting

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-----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. 
That is the nature of factors.  Once created, unused levels must be xplicitly dropped

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

Hope this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

A work around is exporting and importing the new subset. Then it's
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)
	[[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-
guide.html
and provide commented, minimal, self-contained, reproducible code.
first of all, the subsetting line is overly complicated.

dat.sub<-dat[dat$treat!='cont',]

will work just fine.  R does exactly what you're describing.  It knows
the levels of the factor.  Once you remove 'cont' from the data, that
doesn't mean that the level is removed from the factor:
df<-data.frame(let=factor(sample(letters[1:5],100,replace=T)),num=rnorm(100))
str(df)
'data.frame':	100 obs. of  2 variables:
 $ let: Factor w/ 5 levels "a","b","c","d",..: 1 5 1 4 3 5 2 2 1 3 ...
 $ num: num  0.224 -0.523 0.974 -0.268 -0.61 ...
df.sub<-df[df$let!='a',]
str(df.sub)
'data.frame':	82 obs. of  2 variables:
 $ let: Factor w/ 5 levels "a","b","c","d",..: 5 4 3 5 2 2 3 3 5 3 ...
 $ num: num  -0.523 -0.268 -0.61 -1.383 -0.193 ...
unique(df.sub$let)
[1] e d c b
Levels: a b c d e
df.sub$let<-factor(df.sub$let)
unique(df.sub$let)
[1] e d c b
Levels: e d c b
str(df.sub$let)
Factor w/ 4 levels "e","d","c","b": 1 2 3 1 4 4 3 3 1 3 ...

by redefining your factor you can eliminate the problem.  the other
option, if you don't want factors to begin with is:

options(stringsAsFactors=FALSE)  # to set the global option

or

dat<-read.csv("~/MyFiles/data.csv",stringsAsFactors=FALSE)  # to set
the option locally for this single read.csv call.

On Tue, Nov 1, 2011 at 2:28 PM, Schreiber, Stefan
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. A work around is exporting and importing the new subset. Then it's
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)
? ? ? ?[[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

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