Scoping / Documentation / Context issues -- How to be specific?

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
55/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Quiet
Tech stack
r
Domain
documentation

Research direction

Start with the Introduction to data.table vignette and compare its scoping guidance with the data.table programming vignette's env interface examples. Clarify how callers pass column names and outer values in i, including the character-value cases described here; done when the introductory documentation gives a correct, usable note and links to the fuller programming guidance.

Written by the indexing model from the issue text.

Description

programming question

Hello,

I have been trying to find a good documentation on how to avoid potential scoping issues.

For instance, consider

library("data.table")
dt = data.table(a = 1:3, b = 4:6)
dt[a %in% 1:3]

So far so good, a is interpreted within the dt. But what if we set a = 4:6?
Documentation often talks about the . and .. calls, but those are specific to j and not i part of data.table.

a = 4:6
dt[a %in% a]

obviously wouldn't have desired effect.

But the Introduction to data.table doesn't really offer a solution.
https://cran.r-project.org/web/packages/data.table/vignettes/datatable-intro.html

Most stackoverflow or AI answers are incorrect, often they will say to use the get() solution, but it doesn't work either.

The solution is introduced in https://cran.r-project.org/web/packages/data.table/vignettes/datatable-programming.html, use the env = list(...), such as:

dt[col %in% value, env = list(col = "a", value = a)

Yet, in some cases, wrapping in I is also required if the value is of character, because it would be interpreted as a column.

dt[] = lapply(dt, as.character)
a = "1"
dt[col %in% value, env = list(col = "a", value = a))] # fails because "1" is not a column
dt[col %in% value, env = list(col = "a", value = I(a))] # works
dt[col %in% value, env = list(col = as.name("a"), value = I(a))] # safest?

All these things are imo required when he user wants to be specific, like this variable names a column in a data.table, while this_one names a variable comming from outer environment. This way wires won't be crossed.

This should IMO be in the "introduction to data.table" as a simple case or much more complex "Programming on data.table`

Apparently, env is a new interface and get(), mget() etc. were at one point interfaces of data.table but were discontinued (likely because they were buggy).

https://stackoverflow.com/a/54800108

tl,dr: Add a note in "Introduction to data.table" about the env interface and how to pass column names/values as variables.

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