`droplevels()` does not drop levels for empty `data.table`s

[advieser]

Edge case inconsistency: droplevels() does not drop empty levels of factor columns in empty data.tables. This is different from the behavior of data.frame (see reprex). I tried finding out whether this is intentional. However, I could find no mention of it, should that be the case.

As far as I can tell, this is due to an early exit in setdroplevels():

data.table/R/fdroplevels.R

Line 18 in 764809b

if (!nrow(x)) return(invisible(x))

Going by git blame, this was apparently introduced as a fix for #5184.

Reprex

library(data.table)
dt = data.table(x = factor(levels = c("A", "B")))
df = data.frame(x = factor(levels = c("A", "B")))
str(dt)
#> Classes 'data.table' and 'data.frame':   0 obs. of  1 variable:
#>  $ x: Factor w/ 2 levels "A","B": 
#>  - attr(*, ".internal.selfref")=<externalptr>
str(df)
#> 'data.frame':    0 obs. of  1 variable:
#>  $ x: Factor w/ 2 levels "A","B":
str(droplevels(dt))
#> Classes 'data.table' and 'data.frame':   0 obs. of  1 variable:
#>  $ x: Factor w/ 2 levels "A","B": 
#>  - attr(*, ".internal.selfref")=<externalptr>
str(droplevels(df))
#> 'data.frame':    0 obs. of  1 variable:
#>  $ x: Factor w/ 0 levels:

Output of sessionInfo()

sessionInfo()
#> R version 4.5.0 (2025-04-11)
#> Platform: x86_64-pc-linux-gnu
#> Running under: Linux Mint 22
#> 
#> Matrix products: default
#> BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.12.0 
#> LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.12.0  LAPACK version 3.12.0
#> 
#> locale:
#>  [1] LC_CTYPE=de_DE.UTF-8       LC_NUMERIC=C              
#>  [3] LC_TIME=de_DE.UTF-8        LC_COLLATE=de_DE.UTF-8    
#>  [5] LC_MONETARY=de_DE.UTF-8    LC_MESSAGES=de_DE.UTF-8   
#>  [7] LC_PAPER=de_DE.UTF-8       LC_NAME=C                 
#>  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
#> [11] LC_MEASUREMENT=de_DE.UTF-8 LC_IDENTIFICATION=C       
#> 
#> time zone: Europe/Berlin
#> tzcode source: system (glibc)
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] data.table_1.17.4
#> 
#> loaded via a namespace (and not attached):
#>  [1] digest_0.6.37     fastmap_1.2.0     xfun_0.52         glue_1.8.0       
#>  [5] knitr_1.50        htmltools_0.5.8.1 rmarkdown_2.29    lifecycle_1.0.4  
#>  [9] cli_3.6.5         reprex_2.1.1      withr_3.0.2       compiler_4.5.0   
#> [13] rstudioapi_0.17.1 tools_4.5.0       evaluate_1.0.3    yaml_2.3.10      
#> [17] rlang_1.1.6       fs_1.6.6

^{Created on 2025-05-31 with reprex v2.1.1}

[jangorecki]

Thank you for the report