>> Let's see if I understand this. Do I iterate through >> x <- factor(x, levels(c(levels(x), NA), exclude=NULL) >> for each of the few hundred variables (x) in my data frame? > > > Yes, for all being factors.
Wouldn't addNA() be the preferred method? To do it for all variables is pretty simple: cat <- sapply(df, is.factor) df[cat] <- lapply(df[cat], addNA, ifany = TRUE) Hadley -- http://had.co.nz/ ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
