drop.levels <- function(x, reorder=TRUE, ...)
UseMethod("drop.levels")
drop.levels.default <- function(x, reorder=TRUE, ...)
x
drop.levels.factor <- function(x, reorder=TRUE, ...)
{
x <- x[, drop=TRUE]
if(reorder) x <- reorder(x, ...)
x
}
drop.levels.list <- function(x, reorder=TRUE, ...)
{
lapply(x, drop.levels, reorder=reorder, ...)
}
drop.levels.data.frame <- function(x, reorder=TRUE, ...)
{
x[] <- drop.levels.list(x, reorder=reorder, ...)
x
}
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