add a 'having' parameter to `[.data.table`

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start at the [.data.table entry point and read how the existing i, j, and by parameters are evaluated. Use the issue's grouped mean example to define the proposed behavior, including a scalar logical condition for each group and preserving existing syntax; the work is done when the design is implemented and its behavior is verified.

Written by the indexing model from the issue text.

Description

feature request top request

Currently, to have the equivalent (or something similar) of the SQL having clause you need to write a [.data.table first using by and then feed the result into the i parameter of a second [.data.table, like in:

dt <- data.table(id   = rep(1:2, each = 2),
                 var  = c(0.2, 0.5, 1.5, 1.3))

dt[dt[, mean(var) > 1, by = id]$id]
   id var
1:  2 1.5
2:  2 1.3

Another option is to use conditional statement inside j, very powerful, I do all the time, and so far there is nothing that the current syntax did not allow me to do. However having a having parameter I believe will allow writing much more clear and readable codes. For example the above can be written as:

dt[, if(mean(var) > 1) .SD, by = id]

What I propose is something like:

dt[, .SD, by = id, having = mean(var) > 1]

The idea is to have an expression that always evaluates to a logical of length 1 which would tell whether or not j has to be evaluated for the current group.

Thanks,
Michele

Dominant language
R
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Merged PRs (30d)
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