In R devel r71124, if 'x' is a factor, droplevels(x) gives factor(x, exclude = NULL) . In R 3.3.1, it gives factor(x) .
If a factor 'x' has NA and levels of 'x' doesn't contain NA, factor(x) gives the expected result for droplevels(x) , but factor(x, exclude = NULL) doesn't. As I said in https://stat.ethz.ch/pipermail/r-devel/2016-May/072796.html , factor(x, exclude = NULL) adds NA as a level. Using factor(x, exclude = if(anyNA(levels(x))) NULL else NA ) , like in the code of function `[.factor` (in the same file, factor.R, as 'droplevels'), is better. It is possible just to use x[, drop = TRUE] . For a factor 'x' that has NA level and also NA value, factor(x, exclude = NULL) is not perfect, though. It change NA to be associated with NA factor level. ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
