Factor function

Dear All,

I just want to remove ?NA? from the levels of a factor.  For example:

d<-data.frame(matrix(c("ww","ww","xx","yy","ww","yy","xx","yy","NA"),
ncol=3, byrow=TRUE))

factor(d[, 3], exclude=NA)

[1] xx yy NA
Levels: NA xx yy

But ?NA? is still listed in the levels. How can I solve this problem? Thanks
in advance.

Lisa


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