data.table objects assigned with := from within function not printed

Question

I would like to modify a data.table within a function. If I use the := feature within the function, the result is only printed for the second call.

Answer 1

As David Arenburg mentions in a comment, the answer can be found here. There was a bug fixed in the version 1.9.6 but the fix introduced this downside.

One should call DT[] at the end of the function to prevent this behaviour.

myfunction <- function(dt) {
    dt[, z := y - x][]
}
myfunction(mydt)  # prints immediately
#    x y z
# 1: 1 5 4
# 2: 2 6 4
# 3: 3 7 4