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Similar to base::droplevels but much faster.


fdroplevels(x, exclude = if (anyNA(levels(x))) NULL else NA, ...)

# S3 method for data.table
droplevels(x, except = NULL, exclude, = FALSE, ...)



factor or data.table where unused levels should be dropped.


A character vector of factor levels which are dropped no matter of presented or not.


An integer vector of indices of data.table columns which are not modified by dropping levels.

logical (default is FALSE). If TRUE levels of factors of data.table are modified in-place.


further arguments passed to methods


fdroplevels returns a factor.

droplevels returns a data.table where levels are dropped at factor columns.


# on vectors
x = factor(letters[1:10])
#> [1] a b c d e
#> Levels: a b c d e
# exclude levels from drop
fdroplevels(x[1:5], exclude = c("a", "c"))
#> [1] <NA> b    <NA> d    e   
#> Levels: b d e

# on data.table
DT = data.table(a = factor(1:10), b = factor(letters[1:10]))
#> [1] a b c d e f
#> Levels: a b c d e f
# exclude levels
droplevels(head(DT), exclude = c("b", "c"))[["b"]]
#> [1] a    <NA> <NA> d    e    f   
#> Levels: a d e f
# except columns from drop
droplevels(head(DT), except = 2)[["b"]]
#> [1] a b c d e f
#> Levels: a b c d e f g h i j
droplevels(head(DT), except = 1)[["b"]]
#> [1] a b c d e f
#> Levels: a b c d e f