[datatable-help] there is no package called ‘xts’

[Matthew Dowle]

Good. Well, all correct, known and expected then.  There is a feature 
request to optimize .SD[i] in DT[,.SD[i],by=...] to not actually create 
the whole .SD just to get the first or last row (or indeed any subset).  
Since that's the most natural syntax.  I often would like that myself.
In the meatime the other suggestions from Michael should be fast.  As 
he said: the one using .I[.N] should be fast.
Matthew

On 24.04.2013 20:26, Sam Steingold wrote:
>> * Michael Nelson <zvpunry.aryfba at flqarl.rqh.nh> [2013-04-24 00:41:33 
>> +0000]:
>>
>> frame[, .SD[.N], by = id]
>
> I tried
> --8<---------------cut here---------------start------------->8---
> dt <- frame[, .SD[1] ,by=id]
> --8<---------------cut here---------------end--------------->8---
> (I don't care whether I take first or last, see another message).
> and I got the note
> --8<---------------cut here---------------start------------->8---
> Finding groups (bysameorder=TRUE) ... done in 0.121secs.
> bysameorder=TRUE and o__ is length 0
> Optimization is on but j left unchanged as '.SD[1]'
> Starting dogroups ... The result of j is a named list. It's very
> inefficient to create the same names over and over again for each
> group. When j=list(...), any names are detected, removed and put back
> after grouping has completed, for efficiency. Using j=transform(), 
> for
> example, prevents that speedup (consider changing to :=).
> --8<---------------cut here---------------end--------------->8---
> and indeed it runs unbelievably slow (as if I were using data.table)
>
> thanks a lot for your detailed reply!