subsetting data in R

Thank you for all the responses.

Is there a way to do a complete data set from an object in R?
I have a data set with more than 3000 columns.

The subsetting is ok but it could be dangerous if you are using other 
factors to do some analysis as you need to specify the levels for all 
your factors again.

Best,

Manuel

On 24/04/2011 08:30 a.m., Gustavo Carvalho wrote:

pa2<- subset(pa, influencia=="AP")
pa2$influencia<- factor(pa2$influencia)
levels(pa2$influencia)

On Sun, Apr 24, 2011 at 11:24 AM, Manuel Sp??nola<mspinola10 at gmail.com>  wrote:

Thank you very much for your response, Christian, Roman, and Sarah.

Sarah,

I am trying your suggestion but I cannot see the levels:

  >  pa2 = factor(subset(pa, influencia=="AP")$influencia)
  >  levels(pa2$influencia)

Error in pa2$influencia : $ operator is invalid for atomic vectors

Best,

Manuel



On 24/04/2011 07:51 a.m., Sarah Goslee wrote:

By default, read.csv() turns character variables into factors, using all the
unique values as the levels.

subset() retains those levels by default, as they are a vital element of the
data. If you are studying some attribute of men and women, say height,
even if you are only looking at the heights for women it's important to remember
that men still exist.

If you don't want influencia to be a factor, you can change that in the import
stringsAsFactors=FALSE.

If you do want influencia to be a factor, but want the unused levels to be
removed, you can use factor() to do that.

testdata<- data.frame(group=c("A", "B", "C", "A", "B", "C"), value=1:6)
testdata

    group value
1     A     1
2     B     2
3     C     3
4     A     4
5     B     5
6     C     6

str(testdata)

'data.frame': 6 obs. of  2 variables:
   $ group: Factor w/ 3 levels "A","B","C": 1 2 3 1 2 3
   $ value: int  1 2 3 4 5 6

subset(testdata, group=="A")

    group value
1     A     1
4     A     4

subset(testdata, group=="A")$group

[1] A A
Levels: A B C

?subset
factor(subset(testdata, group=="A")$group)

[1] A A
Levels: A

Sarah

On Sun, Apr 24, 2011 at 9:04 AM, Manuel Sp??nola<mspinola10 at gmail.com>    wrote:

Dear list members,

I have a question regarding too subsetting a data set in R.

I created an object for my data:

   >pa = read.csv("espec_indic.csv", header = T, sep=",", check.names = F)

   >    levels(pa$influencia)

[1] "AID" "AII" "AP"

The object has 3 levels for influencia (AP, AID, AII)

Now I subset only observations with influencia = "AID"

   >pa2 = subset(pa, influencia=="AID")

but if I ask for the levels of influencia still show me the 3 levels,
AP, AID, AII.

   >    levels(pa2$influencia)

[1] "AID" "AII" "AP"

Why is that?

I was thinking that I was creating a new data frame with only AID as a
level for influencia.

How can I make a complete new object with only the observations for
"AID" and that the only level for influencia is indeed "AID"?

Best,

Manuel

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