[datatable-help] Preserving comments in [.data.table?

[Allan Engelhardt]

On 11/09/12 11:00, Matthew Dowle wrote:
> Hi. I didn't know about base::comment. Should use unknownr::unk(),
> shouldn't I!
>
> Please raise a new feature request, or add it to this one :
>
> #2197 "A simple labels attribute like in the Hmisc package for variable
> descriptions"
> https://r-forge.r-project.org/tracker/index.php?func=detail&aid=2197&group_id=240&atid=978
>
> Do we want labels, or comments, or both? Griffith Rees (cc'd)?

I think we want column attributes. As in: all of them. The comment is 
just one attribute. R uses attributes for all sorts of sneaky things so 
I think it is a bad idea if we lose some of them some of the time.

> Since comment<- will copy the whole table,

Even for column comments? I am not quite sure how that works.

> we'd need setcomment() [or
> setlabel(), or both] to avoid that copy; adding to the set* family.

A set attributes that allows setting column attributes may suffice.

>
>> PS: is stopifnot(identical(DT1[DT1]$A, DT1[DT1]$A.1)) a buglet?
> Seems ok :

Sorry, my bad (because str(DT1[DT1]) lists them as having different 
storage classes).

Allan

>
>> DT1 <- data.table(id = seq.int(1, 10), A = LETTERS[1:10], key = "id")
>> stopifnot(identical(DT1[DT1]$A, DT1[DT1]$A.1))
>> comment(DT1$A) <- "A"
>> stopifnot(identical(DT1[DT1]$A, DT1[DT1]$A.1))
> Error: identical(DT1[DT1]$A, DT1[DT1]$A.1) is not TRUE
>> str(DT1[DT1]$A)
>   chr [1:10] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J"
>> str(DT1[DT1]$A.1)
>   atomic [1:10] A B C D ...
>   - attr(*, "comment")= chr "A"
>
>
>> On 11/09/12 09:47, I wrote:
>>> I like comments, but [.data.table drops them, e.g.
>> More specifically, it drops them for the 'outer' data.table but not the
>> inner (!?) when doing the join:
>>
>> DT1 <- data.table(id = seq.int(1, 10), A = LETTERS[1:10], key = "id")
>> comment(DT1$A) <- "A"
>> DT2 <- data.table(id = seq.int(2, 10, 2), b = letters[1:5], key = "id")
>> comment(DT2$b) <- "b"
>> str(DT1[DT2]) # No comment on A
>> str(DT2[DT1]) # No comment on b
>>
>> Allan
>>
>> PS: is stopifnot(identical(DT1[DT1]$A, DT1[DT1]$A.1)) a buglet?
>>
>>> DT1 <- data.table(A = 1:10)
>>> comment(DT1$A) <- "Documenting A column"
>>> DT2 <- DT1[A %% 2]
>>> stopifnot(identical(comment(DT1$A), comment(DT2$A)))
>>>
>>> Is there any way of preserving the comments? (My normal use case is
>>> DT1 <- DT1[...] so copying from one to the other is a little ...
>>> tedious?)
>>>
>>> Allan
>>>
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>