removing factor values in the main data frame

On 01 Mar 2016, at 18:33 , David L Carlson <dcarlson at tamu.edu> wrote:

You need to learn how to send emails in plain text since html gets mangled on r-help. See your message below. If I understand your question, it has to do with what happens to factor levels when you subset your data. Subsetting a factor does not remove empty factor levels. This is documented on the manual page for subset():

"Factors may have empty levels after subsetting; unused levels are not automatically removed. See droplevels for a way to drop all unused levels from a data frame."

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David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352


-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of hoda rahmati via R-help
Sent: Tuesday, March 1, 2016 10:32 AM
To: r-help at r-project.org
Subject: [R] Fw: removing factor values in the main data frame

Hi all,I have the following main data frame:(mydata)        $ TE : num 40 40 20 20 20 20 20 20 20 40 ...        $ TR : num 49 49 28 28 28 28 28 28 28 49 ...        $ COUNTRY : Factor w/ 27 levels "","AU","BA","BE",..: 8 8 8 8 8 ...among the COUNTRY I just need US and AU,first I get a subset to contain just these two countries:        submydata=subset(mydata,COUNTRY%in%c("US","AU"))       factor(submydata$COUNTRY) but after this when I get str of mydata again I have the same data frame with no changes in COUNTRY, however I want mydata to be like: 
       $ TE : num 40 40 20 20 20 20 20 20 20 40 ...        $ TR : num 49 49 28 28 28 28 28 28 28 49 ...        $ COUNTRY : Factor w/ 2 levels "","AU","US",..: 8 8 8 8 8 8 8 8 8 8Thanks for any help


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