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

[Chris Neff]

On 6 October 2011 00:15, Matthew Dowle <mdowle at mdowle.plus.com> wrote:
> 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).
>

Is this a fast file loader for any files that could be read using
read.table, or just dual delimited files?  If you can make a way to
load things that is faster than read.table with the normal speed
tweaks that get mentioned for it, I'd be ecstatic.


> 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
>>
>>
>>
>>
>>
>
>
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