DT[.(, 4L)] assumes "consider all values" for first key column

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Feature
Clarity
Needs clarification
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Reproduce the keyed data.table example and compare the current results for DT[.(unique(x), 4L)] and DT[.(, 4L)]. Determine whether the shorter form is intended to mean all values for the first key column; done when the expected behavior is agreed and covered by a regression test.

Written by the indexing model from the issue text.

Description

feature request
require(data.table)
DT = data.table(x=c(1L,1L,2L,2L), y=c(3:4, 3:4), z=1:4, key=c("x", "y"))
DT[.(unique(x), 4L)]
#    x y z
#1: 1 4 2
#2: 2 4 4

# could just be:
DT[.(, 4L)]

?

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