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?

The column d[, 3] is already a factor. It is possible to avoid
this using

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

Then, we get

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

  [1] xx   yy   <NA>
  Levels: xx yy

Hope this helps.

Petr Savicky.