[datatable-help] column of named vectors in data.table and possible bug

[Thell Fowler]

Perhaps a 'too late' reply, but have you thought about bringing the names
into the DT, using them, then dropping them?

For example:

> DT[, n:=names(DT$B)]
> DT[,list(B=list(B),Names=list(n)),by=A]
   A     B Names
1: 1 6,7,8 a,b,c
2: 2  9,10   d,e
> DT$n<-NULL



On Sat, Aug 24, 2013 at 2:57 AM, Arunkumar Srinivasan <aragorn168b at gmail.com
> wrote:

>  Dear all,
>
> Suppose we've construct a data.table in this manner:
>
> x <- c(1,1,1,2,2)
> y <- 6:10
> setattr(y, 'names', letters[1:5])
> DT<- data.table(A = x, B = y)
>
> DT$B
>  a  b  c  d  e
>  6  7  8  9 10
>
> You see that DT maintains the name of vector B. But if we do:
>
> DT[, names(B), by=A]
>    A V1
> 1: 1  a
> 2: 1  b
> 3: 1  c
> 4: 2  a
> 5: 2  b
> 6: 2  c
>
> There are two things here: First, you see that only the names of the first
> grouping is correct (A = 1). Second, the rest of the result has the same
> names, and the result is also recycled to fit the length. Instead of 5
> rows, we get 6 rows.
>
> A way to get around it would be:
>
> DT[, names(DT$B)[.I], by=A]
>    A V1
> 1: 1  a
> 2: 1  b
> 3: 1  c
> 4: 2  d
> 5: 2  e
>
> However, if one wants to do:
>
> DT[, list(list(B)), by=A]$V1
> [[1]]
> a b c
> 6 7 8
>
> [[2]]
>  a  b
>  9 10
>
> You see that the names are once again wrong (for A = 2). Just the first
> one remains right.
>
> My question is, is it allowed usage of having names for column vectors? If
> so, then this should be a bug. If not, it'd be a great feature to have.
>
> Arun
>
>
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>



-- 
Sincerely,
Thell
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