[datatable-help] Error in coercing matrices within j expressions

[Frank Erickson]

Maybe not ultrafast, but with nice syntax:

CJ(i=iset,j=jset)[criterion(i,j)]

I guess it should be parallelizable, but that wouldn't be with data.table,
if I understand this correctly:
http://stackoverflow.com/questions/14759905/data-table-and-parallel-computing


On Tue, Sep 17, 2013 at 5:42 PM, Nathaniel Graham <npgraham1 at gmail.com>wrote:

> Oops; I meant to reply to all, and then forgot after I discarded and
> rewrote my
> message a few times.  I suspect (although I'm not absolutely certain) that
> if
> NULL or similar did the same thing as returning a 0-row data.table with the
> appropriate number of columns, some operations could be sped up a bit.
> In those cases, the data.table code wouldn't need to check the number and
> type of the columns returned.
>
> I suspect that unless someone knows a secret, ultrafast way to iterate
> through
> a list of all combinations of a set of items and return the subset of
> those that
> match some criteria, that I'm as close to optimal as I'm likely to get
> right now.
>
>
> -------
> Nathaniel Graham
> npgraham1 at gmail.com
> npgraham1 at uky.edu
>
>
> On Tue, Sep 17, 2013 at 5:22 PM, Frank Erickson <FErickson at psu.edu> wrote:
>
>> Well, rbindlist(list()) says "Null data.table" (though it doesn't pass
>> the is.null() test). Maybe someone else has an idea how to deal with the
>> no-results case. By the way, it's best to use "reply to all" to make sure
>> you reply to the mailing list, too; they should be able to see your message
>> quoted below, though.
>>
>> --Frank
>>
>>
>> On Tue, Sep 17, 2013 at 5:03 PM, Nathaniel Graham <npgraham1 at gmail.com>wrote:
>>
>>> Frank,
>>>
>>> Thanks.  This seems to have done the trick, so long as I'm careful to
>>> check for
>>> zero-length lists and return data.table(i = integer(), j = integer()) in
>>> those
>>> cases.  Essentially, I have to test every combination of i and j to see
>>> if it's
>>> "interesting" or not, and some groups have a lot of rows.  At the moment
>>> I'm
>>> attacking some other low hanging fruit, like speeding up the comparisons
>>> I have to do.
>>>
>>> As a side note, it would be kind of nice if there was a simple way to
>>> clue
>>> data.table to the fact that there are no rows to return, like returning
>>> NULL
>>> or NA or similar.
>>>
>>> -------
>>> Nathaniel Graham
>>> npgraham1 at gmail.com
>>> npgraham1 at uky.edu
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/datatable-help/attachments/20130917/083dfc70/attachment.html>