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Symbol .I consistency when not grouping #2598
Description
This has come up before I'm sure but I can't find the issue or S.O. post. Anyone remember or have the links please? I seem to remember replying to someone something like ".I is intended for use in grouping as per the documentation, but it would be good to extend it to non-grouping too". The man page still contains the words "while grouping" for .I.
Current behaviour in both v1.10.4-3 and dev :
> X = data.table(c("a","a","b","c","c"), 10:14)
> setkey(X,V1)
> X["b"]
V1 V2
1: b 12 # ok
> X["b", .I]
[1] 1 # expected x's row number 3 (*1)
> X["b", .I, by=.EACHI]
V1 I
1: b 3 # ok
> X["b", .(.I,V2)]
I V2
1: 1 12 # expected x's row number 3 not 1 (*2)
> X["b", .(.I,V2), by=.EACHI]
V1 I V2
1: b 3 12 # ok
>
Now, which=TRUE was intended and works for the first case (*1) :
> X["b", which=TRUE]
[1] 3
but including x's row numbers inside j (*2) isn't currently possible, unless you add x's row numbers explicitly as a column first. It would be nice for .I to do what which=TRUE does in the simple case (*1) and maybe even slowly deprecate which=TRUE argument since my guess is people reach for .I first.
Activity
franknarf1 commentedon Jan 30, 2018
Probably what you are looking for: https://stackoverflow.com/a/34755995/ and .I in DT[i, .I, by] should return row numbers of DT not of DT[i] #1494 Though the title should be ".I in DT[i, .I] should return row numbers of DT not of DT[i]" (without the by) since it already has that behavior when by= is present...?
Another question about .I and by= local and global versions of
.I,.N#1206dracodoc commentedon Feb 7, 2019
Can we use .I for global thus row number without grouping, and .i for local thus only inside grouping?
MichaelChirico commentedon Feb 7, 2019
@dracodoc as mentioned by Frank this is #1206
dracodoc commentedon Feb 7, 2019
Sorry I didn't realize there is a whole thread on this idea! That means this is a good idea right?
jangorecki commentedon Apr 5, 2020
example from #539