Using value.var = "." in a table with a column named "." gives wrong results

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by running the reproducible R examples in the issue and compare dcast() when value.var is "." with the renamed-column case. Trace how dcast handles a column named "." and add coverage for this input; done when both examples produce the expected counts.

Written by the indexing model from the issue text.

Description

reshape
DT = data.table(a = rep(1:3, 10), b = 2, rep_len(letters, 30))
dcast(DT, a + b ~ ., fun.aggregate = length)[, dcast(.SD, a ~ b, value.var = ".")]
#    a 2
# 1: 1 .
# 2: 2 .
# 3: 3 .

# vs. renaming the column

dcast(DT, a + b ~ ., fun.aggregate = length)[, x := .][, dcast(.SD, a ~ b, value.var = "x")]
#    a  2
# 1: 1 10
# 2: 2 10
# 3: 3 10

Glanced at ?dcast and didn't see anything relevant, but it definitely feels like a bug anyway.

Dominant language
R
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