Feature request: reorder factor levels using a user-supplied sorting function

[russHyde]

Hi,
I was wondering whether you might consider adding a function to forcats that sets up factor levels according to a numerically-aware string-sorting method like gtools::mixedsort.
There's a number of use cases: eg, when you end up with genomes ordered like "chr1", "chr10", "chr11" ... "chr2" rather than "chr1", "chr2", ..., in a plot if you don't hack your factors correctly

eg,

fct_alphameric/fct_alnum/fct_mixedsort <- function(x, ...) {
    factor(x, levels = gtools::mixedsort(unique(x), ...))
    }

[huftis]

I think a better, more general solution would be a function to sort the levels according to a user-defined sort function. Here’s an implementation suggestion:

library(forcats)
fct_sort = function(.f, .fun = sort) {
  f = forcats:::check_factor(.f) # Not needed in dev version
  fct_relevel(f, .fun(levels(f)))
}

x = c("chr1", "chr11", "chr10", "chr2")
fct_sort(x)
#> [1] chr1  chr11 chr10 chr2 
#> Levels: chr1 chr10 chr11 chr2
fct_sort(x, gtools::mixedsort)
#> [1] chr1  chr11 chr10 chr2 
#> Levels: chr1 chr2 chr10 chr11

And perhaps support for a custom order function (e.g. gtools::mixedorder) in addition to a custom sort function would also be useful?

[billdenney]

FYI, naturalsort::naturalfactor does what I think is being requested (working only for natural numbers).

[hadley]

I agree with @huftis that a more general fct_sort() is the better interface here

[russHyde]

Cool. Would you like me to work on a PR? (I'd forgotten I opened this issue)

[hadley]

Sure!