droplevels for empty table and ordered factors

NEWS.md

@@ -139,7 +139,7 @@
 
 25. `setcolorder()` gains `before=` and `after=`, [#4385](https://github.com/Rdatatable/data.table/issues/4358). Thanks to Matthias Gomolka for the request, and both Benjamin Schwendinger and Xianghui Dong for implementing.
 
-26. `base::droplevels()` gains a fast method for `data.table`, [#647](https://github.com/Rdatatable/data.table/issues/647). Thanks to Steve Lianoglou for requesting, and Jan Gorecki and Benjamin Schwendinger for the PR. `fdroplevels()` for use on vectors has also been added.
+26. `base::droplevels()` gains a fast method for `data.table`, [#647](https://github.com/Rdatatable/data.table/issues/647). Thanks to Steve Lianoglou for requesting, Boniface Kamgang and Martin Binder for testing, and Jan Gorecki and Benjamin Schwendinger for the PR. `fdroplevels()` for use on vectors has also been added.
 
 27. `shift()` now also supports `type="cyclic"`, [#4451](https://github.com/Rdatatable/data.table/issues/4451). Arguments that are normally pushed out by `type="lag"` or `type="lead"` are re-introduced at this type at the first/last positions. Thanks to @RicoDiel for requesting, and Benjamin Schwendinger for the PR.
 

R/fdroplevels.R

@@ -4,12 +4,13 @@ fdroplevels = function(x, exclude = if (anyNA(levels(x))) NULL else NA, ...) {
   lev = which(tabulate(x, length(levels(x))) & (!match(levels(x), exclude, 0L)))
   ans = match(as.integer(x), lev)
   setattr(ans, 'levels', levels(x)[lev])
-  setattr(ans, 'class', 'factor')
+  setattr(ans, 'class', class(x))
   return(ans)
 }
 
 droplevels.data.table = function(x, except = NULL, exclude, in.place = FALSE, ...){
-    stopifnot(length(x) > 0L, is.logical(in.place))
+    stopifnot(is.logical(in.place))
+    if (nrow(x)==0L) return(x)
     ix = vapply(x, is.factor, NA)
     if(!is.null(except)){
         stopifnot(is.numeric(except), except <= length(x))

inst/tests/tests.Rraw

@@ -18187,17 +18187,24 @@ test(2213, identical(fread(text="A\n0.8060667366\n")$A, 0.8060667366))
 # droplevels.data.table method, and fdroplevels, #647
 x = factor(letters[1:10])
 DT = data.table(a = x)[1:5]
-test(2214.1, fdroplevels(factor()), droplevels(factor()))
-test(2214.2, fdroplevels(x[1:5]), droplevels(x[1:5]))
+test(2214.01, fdroplevels(factor()), droplevels(factor()))
+test(2214.02, fdroplevels(x[1:5]), droplevels(x[1:5]))
 if (base::getRversion() >= "3.4.0") {
   # bug fix in R 3.4.0: "droplevels(f) now keeps <NA> levels when present."
-  test(2214.3, fdroplevels(x[1:5], c("b", "d")), droplevels(x[1:5], c("b", "d")))
-  test(2214.4, fdroplevels(x[1:5], letters[1:5]), droplevels(x[1:5], letters[1:5]))
-  test(2214.5, droplevels(DT, exclude=c("b", "d"))[["a"]], droplevels(DT[1:5,a], c("b", "d")))
+  test(2214.03, fdroplevels(x[1:5], c("b", "d")), droplevels(x[1:5], c("b", "d")))
+  test(2214.04, fdroplevels(x[1:5], letters[1:5]), droplevels(x[1:5], letters[1:5]))
+  test(2214.05, droplevels(DT, exclude=c("b", "d"))[["a"]], droplevels(DT[1:5,a], c("b", "d")))
 }
-test(2214.6, droplevels(DT)[["a"]], droplevels(DT[1:5,a]))
-test(2214.7, droplevels(DT, 1)[["a"]], x[1:5])
-test(2214.8, droplevels(DT, in.place=TRUE), DT)
+test(2214.06, droplevels(DT)[["a"]], droplevels(DT[1:5,a]))
+test(2214.07, droplevels(DT, 1)[["a"]], x[1:5])
+test(2214.08, droplevels(DT, in.place=TRUE), DT)
+# support ordered factors in fdroplevels
+o = factor(letters[1:10], ordered=TRUE)
+test(2214.09, fdroplevels(o[1:5]), droplevels(o[1:5]))
+# edge case for empty table #5184
+test(2214.10, droplevels(DT[0]), DT[0])
+test(2214.11, droplevels(data.table()), data.table())
+
 
 # factor i should be just like character i and work, #1632
 DT = data.table(A=letters[1:3], B=4:6, key="A")