Adding new columns to a data.table by-reference within a function not always working

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Is this a pointer issue ala this issue, like serializing a data.table destroys the over-allocated pointers?

Yes loading from disk sets the external pointer to NULL. We will have to over-allocate again.

Is there a simple way to restore them?

Yes. You can test for truelength() of the data.table, and if it’s 0, then use setDT() or alloc.col() on it.

truelength(test2) # [1] 0
if (!truelength(test2))
    setDT(test2)
truelength(test2) # [1] 100

foobar(test2, "new")
test2[]
#    id val new
# 1:  a   1   1
# 2:  b   2   1

This should probably go in as a FAQ (can’t remember seeing it there).
Already in FAQ in Warning Messages section.

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