aggregate same variable as groupby on-the-fly expression won't make it length 1

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Assessment

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

Research direction

Start with the two reproducible data.table expressions in the issue and compare the grouped results. Read the FAQ section “Inside each group, why are the group variables length-1?” and trace the relevant grouping behavior. Done means grouping by the on-the-fly factor expression produces the same TotalA value as grouping by SomeNumberA.

Written by the indexing model from the issue text.

Description

consistency

Reported by @geofflazzarini in https://github.com/Rdatatable/data.table/issues/3103#issuecomment-439286088
There should be no difference in TotalA column in results

library(data.table)
dt = data.table(SomeNumberA=c(1,1,1),SomeNumberB=c(1,1,1))
dt[, .(.N, TotalA=sum(SomeNumberA), TotalB=sum(SomeNumberB)), by=SomeNumberA]
#   SomeNumberA     N TotalA TotalB
#         <num> <int>  <num>  <num>
#1:           1     3      1      3
dt[, .(.N, TotalA=sum(SomeNumberA), TotalB=sum(SomeNumberB)), by=as.factor(SomeNumberA)]
#   as.factor     N TotalA TotalB
#      <fctr> <int>  <num>  <num>
#1:         1     3      3      3

This might be tricky because of "Inside each group, why are the group variables length-1?" in FAQ.

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