Inconsistent lengths of col when in both by and .SDcols

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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 by running the minimal R example in the issue and inspect how data.table handles columns shared by by and .SDcols. Determine the intended scalar behavior for by columns while preserving the column in .SD, then add a regression test covering the shown grouped output.

Written by the indexing model from the issue text.

Description

consistency

When a col is present in both by and .SDcols, it might get more than one values in j, which seems inconsistent with my expectation that all cols in all cols in by should only get a scalar value in j, and .SDcols should only make col appear in .SD, and col in j should not necessarily be the same vector (of multiple values) in .SD.

Following is a minimal example:

library(data.table)

dt <- data.table(
  grp1 = c(1, 1, 1, 2, 2, 2),
  grp2 = c(1, 2, 1, 2, 1, 2),
  x = c(1, 2, 3, 1, 2, 3),
  y = c(1, 2, 3, 4, 5, 6))

print_expr <- function(x) {
  expr <- substitute(x)
  cat("> ")
  print(expr)
  print(x)
}

dt[, {
  print_expr(grp1)
  print_expr(grp2)
  print_expr(.SD)
  NULL
}, by = .(grp1, grp2), .SDcols = c("grp2", "x", "y")]

In this example, grp2 appear in both by and .SDcols and get multiple values in some groups.

> grp1
[1] 1
> grp2
[1] 1 1
> .SD
    grp2     x     y
   <num> <num> <num>
1:     1     1     1
2:     1     3     3
> grp1
[1] 1
> grp2
[1] 2
> .SD
    grp2     x     y
   <num> <num> <num>
1:     2     2     2
> grp1
[1] 2
> grp2
[1] 2 2
> .SD
    grp2     x     y
   <num> <num> <num>
1:     2     1     4
2:     2     3     6
> grp1
[1] 2
> grp2
[1] 1
> .SD
    grp2     x     y
   <num> <num> <num>
1:     1     2     5
Empty data.table (0 rows and 2 cols): grp1,grp2
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