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

[Matthew Dowle]

Yes, single delimiter files too. Yes it should be faster than normal speed 
tweaks
on read.table.

One (very very basic) test so far has shown 4 times faster for a 7.5MB file 
on disk (5.5s
down to 1.3s).   The code and test is already in the package (so you can run 
that test now),
see data.table:::read  (3 colons), and the 2 source files :
https://r-forge.r-project.org/scm/viewvc.php/pkg/R/read.R?view=markup&root=datatable
https://r-forge.r-project.org/scm/viewvc.php/pkg/src/readfile.c?view=markup&root=datatable

But, it doesn't look like I did the speed tweaks for read.csv in that 
comparison.  What are
they again?    Any help with this feature would be great.

Matthew

"Chris Neff" <caneff at gmail.com> wrote in message 
news:CAAuY0RVoWwFGbcPrCOd6gT4GoamozGuexqSLioS3QHuFhF0c4g at mail.gmail.com...
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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