Fast Removal of Unused Factor Levels
A substantially faster replacement for droplevels.
fdroplevels(x, ...) ## S3 method for class 'factor' fdroplevels(x, ...) ## S3 method for class 'data.frame' fdroplevels(x, ...)
x |
a factor, or data frame / list containing one or more factors. |
... |
not used. |
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.
x will any unused factor levels removed.
If x is malformed i.e. has too few levels, this function can cause a segmentation fault, thus only use with ordinary / proper factors.
f <- iris$Species[1:100] fdroplevels(f) identical(fdroplevels(f), droplevels(f)) fNA <- na_insert(f) fdroplevels(fNA) identical(fdroplevels(fNA), droplevels(fNA)) identical(fdroplevels(ss(iris, 1:100)), droplevels(ss(iris, 1:100)))
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