[datatable-help] zero length list component in j

[Gabor Grothendieck]

Is this intended?  If we use j = list(x = "X", y = numeric(0)) we get
a row but if we use just list(y = numeric(0)) then we do not get a
row.  In the first case it filled in the zero length component with NA
and in the second case it just omitted the row entirely:

> dd <- data.table(a = 1:3)
> dd
   a
1: 1
2: 2
3: 3
> dd[, list(x = "X", y = numeric(0)), by = a]
   a x  y
1: 1 X NA
2: 2 X NA
3: 3 X NA
> dd[, list(y = numeric(0)), by = a]
Empty data.table (0 rows) of 2 cols: a,y


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