[R-Forge #4856] rbind, rbindlist, and merge don't preserve "ordered" factor columns; convert them to "factor"

[arunsrinivasan]

Submitted by: Arun ; Assigned to: Eduard Antonyan; R-Forge link

dt1 <- data.table(x=1:5, y=factor(letters[1:5], ordered=TRUE), z=factor(letters[1:5]))
dt2 <- data.table(x=6:10, y=sample(letters, 5), z=sample(letters, 5))

dt1$y is an ordered factor here. Doing:

sapply(rbind(dt1, dt2) , class) gives:
x y z
"integer" "factor" "factor"

sapply(rbindlist(list(dt1, dt2)) , class) gives:
x y z
"integer" "factor" "factor"

---

After an internal discussion, rbinding ordered with any not-ordered class should result in an ordered column (with the given ordered order and some, maybe first come first serve, order for the unordered part). ordered+ordered should result in ordered iff the orders are compatible and should result in a factor + a warning if the orders are not compatible.