Allow inversion of columns for update by reference
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Assessment
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Newbie friendliness
- 35/100
Research direction
Start by reproducing the requested dt[j = !(cols) := NULL] behavior and compare it with column selection using !c("col1") and !..cols. Trace the update-by-reference path for j and :=; done means inversion removes every column except the specified set without copying the data table.
Written by the indexing model from the issue text.
Description
In data.table the ! operator generally serves as a inversion operator. E.g. the operation dt[!condition], dt[!(1:3)], A[!B, on = ...] will invert the condition (naturally), invert the row indices (so !(1:3) does not evaluate to c(FALSE, FALSE, FALSE), but to c(4, 5, ...)) and do an anti-join. Even in column selection this works: dt[j = !c("col1")] will select all columns but col1, and dt[j = !..cols] will select all but the columns specified in cols.
But this does not work in combination with update by reference. In my opinion this would be useful to have. There are often occasions where you want to keep only a certain set of columns from the data table. For this it would be useful to be able to do
dt[j = !(cols) := NULL]
instead of having to do
dt <- dt[j = ..cols] # copies memory
or
dt[j = setdiff(names(dt), cols) := NULL] # Very verbose
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