Sorry for spamming. I swear I had worked on that problem a long time before posting.
But I just figured it out: I have to change the values, which are represented as integers, not strings. So the following code will do it: df <- data.frame ( a = factor( c( "bob", "alice", "bob" ) ), b = factor( c( "kenny", "alice", "alice" ) ) ); allLevels <- unique( c( levels( df$a ), levels( df$b ) ) ) for (c in colnames(df)) { df[,c] <- match( df[,c], allLevels); levels( df[,c] ) <- 1:(length(allLevels)) }; -- View this message in context: http://n4.nabble.com/Sharing-levels-across-multiple-factor-vectors-tp1747714p1747722.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ 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.