Returning only groups
Nobody has claimed this yet.
Assessment
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Newbie friendliness
- 42/100
Research direction
Start with the linked dplyr discussion and the data.table grouping example in this issue, then locate the grouping and .SD entry points in the repository. Done means providing a supported way to return only the unique group columns, matching the expected x-only result shown here, with coverage for grouped rows.
Written by the indexing model from the issue text.
Description
From https://github.com/hadley/dplyr/issues/1073#issuecomment-131119903
require(data.table)
DT = data.table(x=1:2, y=1:6)
# x y
#1: 1 1
#2: 2 2
#3: 1 3
#4: 2 4
#5: 1 5
#6: 2 6
DT[, .SD, by=x] # rows for each group are collected together
# x y
#1: 1 1
#2: 1 3
#3: 1 5
#4: 2 2
#5: 2 4
#6: 2 6
Currently, there's no way of returning just the groups:
# expected result
# x
#1: 1
#2: 2
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