dcast() retains key incorrectly when run on .SD and the RHS contains a level matching the input key
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Assessment
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Newbie friendliness
- 50/100
Research direction
Start by running the provided R reproduction using dcast() on .SD and directly on input, then inspect the dcast() and key() entry points. The work is done when both calls produce the same key, as shown in the expected comparison.
Written by the indexing model from the issue text.
Description
Seen while writing #5992.
I'm not sure I've understood the root issue here, but this looks wrong:
input = CJ(a = letters, b = letters, c= letters)[, V := rnorm(.N)][, mean(V), by = .(a, b)]
key(input[, dcast(.SD, a ~ b, value.var = "V1")])
# [1] "a" "b"
key(dcast(input, a ~ b, value.var = "V1"))
# [1] "a"
The key should be the same in both cases.
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