On Nov 10, 2009, at 9:49 AM, baptiste auguie wrote:
Dear list,subset has a 'drop' argument that I had often mistaken for the one in [.factor which removes unused levels. Clearly it doesn't work that way, as shown below, d <- data.frame(x = factor(letters[1:15]), y = factor(LETTERS[1:3])) s <- subset(d, y=="A", drop=TRUE) str(s) 'data.frame': 5 obs. of 2 variables: $ x: Factor w/ 15 levels "a","b","c","d",..: 1 4 7 10 13 $ y: Factor w/ 3 levels "A","B","C": 1 1 1 1 1 The subset still retains all the unused factor levels. I wonder how people usually get rid of all unused levels in a data.frame after subsetting? I came up with this but I may have missed a better built-in solution, dropit <- function (d, columns = names(d), ...) { d[columns] = lapply(d[columns], "[", drop=TRUE, ...) d } str(dropit(s)) 'data.frame': 5 obs. of 2 variables: $ x: Factor w/ 5 levels "a","d","g","j",..: 1 2 3 4 5 $ y: Factor w/ 1 level "A": 1 1 1 1 1
There is a page in the R wiki here: http://wiki.r-project.org/rwiki/doku.php?id=tips:data-manip:drop_unused_levels that has some approaches. HTH, Marc Schwartz ______________________________________________ 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.
