[datatable-help] Discrepancy between as.data.frame & as.data.table when handling nested lists

[Matthew Dowle]

Agreed, intentional.

 > L = list(1,2,3)
 > as.data.table(L)
    V1 V2 V3             #  3 columns, not one list column
1:  1  2  3

 > L = list(1,2,3,list("a",4L,3:10))    # the one nested list here 
creates one list column
 > as.data.table(L)
    V1 V2 V3           V4
1:  1  2  3            a
2:  1  2  3            4
3:  1  2  3 3,4,5,6,7,8,

Rick - are you asking for use cases of list columns full stop or use 
cases of converting nested lists to data.table containing list columns ?


On 08/08/13 04:30, Ricardo Saporta wrote:
> Hey Frank,
>
> Thanks for pointing out that SO link, I had missed it.
>
> All,
>
> I'm curious as to which used cases this functionality would be used in 
> (used for?)
>
> thanks,
> Rick
>
>
>
> On Wed, Aug 7, 2013 at 8:14 PM, Frank Erickson <FErickson at psu.edu 
> <mailto:FErickson at psu.edu>> wrote:
>
>     Hi Rick,
>
>     I guess it's intentional: Matthew saw this SO question (since he
>     edited one of the answers):
>     http://stackoverflow.com/questions/9547518/creating-a-data-frame-where-a-column-is-a-list
>
>     Some musings: Of course, to reproduce as.data.frame-like behavior,
>     you can un-nest the list, so both functions treat it the same way.
>
>         Z <- unlist(Y,recursive=FALSE)
>
>         identical(as.data.table(Z),as.data.table(as.data.frame(Z))) # TRUE
>         # or, equivalently (?)
>         identical(do.call(data.table,Z),data.table(do.call(data.frame,Z)))
>         # TRUE
>
>
>     On the other hand, going back the other direction (getting
>     data.table-like behavior when data.frame's is the default) is more
>     awkward, as seen in that SO question (where they mention
>     protecting each sublist with the I() function). Besides, I'm with
>     @flodel, who asked the SO question, in expecting data.table's
>     behavior: one top-level item in the list mapping to one column in
>     the result...
>
>     --Frank
>
>     On Wed, Aug 7, 2013 at 4:56 PM, Ricardo Saporta
>     <saporta at scarletmail.rutgers.edu
>     <mailto:saporta at scarletmail.rutgers.edu>> wrote:
>
>         Hi all,
>
>         Note the following discrepancy in structure between
>         as.data.frame & as.data.table when called on a nested list.
>         as.data.frame converts the sublist into individual columns
>         whereas as.data.table stacks them into a single column and
>         creates additional rows.
>
>         Is this intentional?
>         -Rick
>
>
>         as.data.frame(X)
>         #        start       type      end data.editDist data.second
>         # 1 start_node is_similar end_node         1  HelloWorld
>
>         as.data.table(X)
>         #         start       type      end   data
>         # 1: start_node is_similar end_node      1
>         # 2: start_node is_similar end_node HelloWorld
>
>
>
>
>         ### Copy+Paste'able Below ###
>
>         # Example 1:
>         X <-  structure(list(start = "start_node", type =
>         "is_similar", end = "end_node",
>             data = structure(list(editDist = 1, second =
>         "HelloWorld"), .Names = c("editDist",
>             "second"))), .Names = c("start", "type", "end", "data"))
>
>         as.data.frame(X)
>         as.data.table(X)
>
>         as.data.table(as.data.frame(X))
>
>
>         # Example 2, with more elements:
>         Y <- structure(list(start = c("start_node", "start_node"),
>         type = c("is_similar", "is_similar"), end = c("end_node",
>         "end_node"), data = structure(list(editDist = c(1, 1), second
>         = c("HelloWorld", "HelloWorld")), .Names = c("editDist",
>         "second"))), .Names = c("start", "type", "end", "data"))
>
>         as.data.frame(Y)
>         as.data.table(Y)
>
>
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>
>
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