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

[Frank Erickson]

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> 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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