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

[Nathaniel Graham]

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
>>
>
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