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Why is data.table not making a data.table from a table although data.frame does?
I have following data and code: 
mydf
  grp categ condition value
1   A     X         P     2
2   B     X         P     5
3   A     Y         P     9
4   B     Y         P     6
5   A     X         Q     4
6   B     X         Q     5
7   A     Y         Q     8
8   B     Y         Q     2
> 
> 
mydf = structure(list(grp = structure(c(1L, 2L, 1L, 2L, 1L, 2L, 1L, 
2L), .Label = c("A", "B"), class = "factor"), categ = structure(c(1L, 
1L, 2L, 2L, 1L, 1L, 2L, 2L), .Label = c("X", "Y"), class = "factor"), 
    condition = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L), .Label = c("P", 
    "Q"), class = "factor"), value = c(2, 5, 9, 6, 4, 5, 8, 2
    )), .Names = c("grp", "categ", "condition", "value"), out.attrs = structure(list(
    dim = structure(c(2L, 2L, 2L), .Names = c("grp", "categ", 
    "condition")), dimnames = structure(list(grp = c("grp=A", 
    "grp=B"), categ = c("categ=X", "categ=Y"), condition = c("condition=P", 
    "condition=Q")), .Names = c("grp", "categ", "condition"))), .Names = c("dim", 
"dimnames")), row.names = c(NA, -8L), class = "data.frame")

However, following works for data.frame but not for data.table: 
> data.frame(with(mydf, table(grp, categ, condition)))
  grp categ condition Freq
1   A     X         P    1
2   B     X         P    1
3   A     Y         P    1
4   B     Y         P    1
5   A     X         Q    1
6   B     X         Q    1
7   A     Y         Q    1
8   B     Y         Q    1
> 
> data.table(with(mydf, table(grp, categ, condition)))
   V1
1:  1
2:  1
3:  1
4:  1
5:  1
6:  1
7:  1
8:  1
> 

Am I making some mistake here or do I need to correct the data.table command to get other variables? It is highly unlikely that there is a bug here. With 2 variables it works all right: 
> data.table(with(mydf, table(grp, categ)))
   categ grp N
1:     A   X 2
2:     B   X 2
3:     A   Y 2
4:     B   Y 2
> 
> 
> data.frame(with(mydf, table(grp, categ)))
  grp categ Freq
1   A     X    2
2   B     X    2
3   A     Y    2
4   B     Y    2

Thanks for your help.

It seems like as.data.table(with(mydf, table(grp, categ, condition))) works, though I'm not sure why just data.table isn't. My guess there is some method withing data.frame for arrays which isn't present in data.table because in your first example it's an array compared to the second one. You could also do something like Res <- setDT(data.frame(with(mydf, table(grp, categ, condition)))), though it still doesn't answer the question.

data.table invokes as.data.table only for matrix or data.frame input. data.frame invokes as.data.frame also for other input.

you should file a bug report on github - there exists as.data.table.table but data.table doesn't delegate to it; fwiw I'd almost always use as.data.table to convert between types - it's usually faster

Fixed in commit #1760 of data.table v1.9.5. Closes #1043.
