[datatable-help] A new package multitable (data.list) remind me of a long existing feature request #202 and discussion thread

[Matthew Dowle]

Indeed. Or columns 11 and 12 of BED files (genomics). Near on the agenda
is a fast file loader straight into data.table and list columns
(dual-delimited files such as BED).

I don't believe SQL has an analogous concept to list columns? To achieve
that people may be using comma delimited strings in varchar columns, I
guess.

On Wed, 2011-10-05 at 16:19 -0500, Branson Owen wrote:
> Thank you very, very much Matthew. I think this is a very valuable (at
> least to me), and unique feature for more powerful calculation. A very
> useful application I can immediately think of is for options chains
> and order book modeling. It's much easier to track and model the whole
> option chains or order book for each time stamp or symbol, and also
> save a lot of replicating time stamps and symbols. 
> 
> 2011/10/4 Matthew Dowle <mdowle at mdowle.plus.com>
>         On Sun, 2011-10-02 at 15:14 +0800, Branson Owen wrote:
>         
>         > Oh, sorry, I was testing the syntax like:
>         >
>         > DT = data.table(A = 1:2, B = list('a', 2i))
>         >
>         > It didn't work, and I though this feature has not been
>         implemented.
>         > Thank you for pointing it out with a good example.
>         
>         
>         Natural to assume that should work. Now in 1.6.7 :
>         
>         o   data.table() now accepts list columns directly rather than
>            needing to add list columns to an existing data.table;
>         e.g.,
>         
>                DT = data.table(x=1:3,y=list(4:6,3.14,matrix(1:12,3)))
>         
>            Thanks to Branson Owen for reminding.
>         
>            Accordingly, one item has been added to FAQ 2.17
>         (differences
>            between data.frame and data.table) :
>         "data.frame(list(1:2,"k",1:4))
>            creates 3 columns, data.table creates one list column"
>         
>            As before, list columns can be created via grouping; e.g.,
>         
>                DT = data.table(x=c(1,1,2,2,2,3,3),y=1:7)
>                DT2 = DT[,list(list(unique(y))),by=x]
>                DT2
>                     x      V1
>                [1,] 1    1, 2
>                [2,] 2 3, 4, 5
>                [3,] 3    6, 7
>         
>            and list columns can be grouped; e.g.,
>         
>                DT2[,sum(unlist(V1)),by=list(x%%2)]
>                     x V1
>                [1,] 1 16
>                [2,] 0 12
>         
>         
>         
> 
>