
Fast Removal of Unused Factor Levels
fdroplevels.RdA substantially faster replacement for droplevels.
Usage
fdroplevels(x, ...)
# S3 method for factor
fdroplevels(x, ...)
# S3 method for data.frame
fdroplevels(x, ...)Details
droplevels passes a factor from which levels are to be dropped to factor, which first calls unique and then match to drop unused levels. Both functions internally use a hash table, which is highly inefficient. fdroplevels does not require mapping values at all, but uses a super fast boolean vector method to determine which levels are unused and remove those levels. In addition, if no unused levels are found, x is simply returned. Any missing values found in x are efficiently skipped in the process of checking and replacing levels. All other attributes of x are preserved.
Note
If x is malformed e.g. has too few levels, this function can cause a segmentation fault terminating the R session, thus only use with ordinary / proper factors.
Examples
f <- iris$Species[1:100]
fdroplevels(f)
#> [1] setosa setosa setosa setosa setosa setosa
#> [7] setosa setosa setosa setosa setosa setosa
#> [13] setosa setosa setosa setosa setosa setosa
#> [19] setosa setosa setosa setosa setosa setosa
#> [25] setosa setosa setosa setosa setosa setosa
#> [31] setosa setosa setosa setosa setosa setosa
#> [37] setosa setosa setosa setosa setosa setosa
#> [43] setosa setosa setosa setosa setosa setosa
#> [49] setosa setosa versicolor versicolor versicolor versicolor
#> [55] versicolor versicolor versicolor versicolor versicolor versicolor
#> [61] versicolor versicolor versicolor versicolor versicolor versicolor
#> [67] versicolor versicolor versicolor versicolor
#> [ reached getOption("max.print") -- omitted 30 entries ]
#> Levels: setosa versicolor
identical(fdroplevels(f), droplevels(f))
#> [1] TRUE
fNA <- na_insert(f)
fdroplevels(fNA)
#> [1] setosa setosa setosa setosa setosa setosa
#> [7] setosa <NA> setosa setosa setosa setosa
#> [13] setosa <NA> setosa setosa setosa setosa
#> [19] setosa setosa setosa setosa setosa <NA>
#> [25] setosa setosa <NA> setosa setosa setosa
#> [31] setosa setosa setosa setosa setosa setosa
#> [37] setosa setosa setosa setosa <NA> <NA>
#> [43] setosa setosa <NA> setosa setosa setosa
#> [49] setosa setosa versicolor versicolor versicolor versicolor
#> [55] versicolor versicolor versicolor versicolor versicolor <NA>
#> [61] versicolor versicolor versicolor versicolor versicolor versicolor
#> [67] versicolor versicolor versicolor versicolor
#> [ reached getOption("max.print") -- omitted 30 entries ]
#> Levels: setosa versicolor
identical(fdroplevels(fNA), droplevels(fNA))
#> [1] TRUE
identical(fdroplevels(ss(iris, 1:100)), droplevels(ss(iris, 1:100)))
#> [1] TRUE