[R] removing factor values in the main data frame

[hoda rahmati]

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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