Allow intermediate `by` state

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#3,100 5 comments 1 reaction 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
30/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by reproducing the two examples in the issue and inspect how data.table currently handles by when no i or j is supplied. Determine the intended behavior for saving an intermediate grouped result and for chained by expressions; the examples should work while existing grouping behavior remains unchanged.

Written by the indexing model from the issue text.

Description

feature request

It would be nice if DT allowed an intermediate by state when no i or j arguments are provided. For example:

data(mtcars)
setDT(mtcars)
mtcars_by_cyl <- mtcars[ by=cyl ]

There are two reasons for the request.

1. It would be nice to then just supply i or j to retrieve a results:

mtcars_by_cyl[ , .(mpg = mean(mpg) ] 
mtcars_by_cyl[ , median(wt) ]

2. It allows a more legible chaining syntax, especially for long chains since the by clause can be part of a chain.

mtcars[ 
   ][ by=cyl 
   ][ wt <= 4 
   ][ , mean(mpg) 
   ]
Dominant language
R
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Forks
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Avg merge
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Merged PRs (30d)
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  4. Open a pull request that references the issue number.

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