[datatable-help] Change in return type of 'by' when DT (or DT[i]) is empty

[Matthew Dowle]

Now fixed in v1.8.1.

o 'by' on an empty table now returns an empty table (#1945) and .N, .SD
  and .BY are now available in the empty case (also #1945). The column
  names and types of the returned empty table are consistent with the
  non empty case. Thanks to Malcolm Cook for reporting. Tests added.

o An empty data.table (0 rows, 1+ cols) now print as "Empty data.table"
  rather than "NULL data.table". A NULL data.table, returned by
  data.table(NULL), has 0 rows and 0 cols. DT[0] returns an empty
  data.table.


On Wed, 2012-04-18 at 09:05 -0400, Steve Lianoglou wrote:
> Hi,
> 
> On Wed, Apr 18, 2012 at 6:26 AM, Chris Neff <caneff at gmail.com> wrote:
> > I would agree that a empty data table is the expected result I would
> > have had.
> [snip]
> 
> Completely agree w/ Chris' synopsis (below) and +1 on making the change.
> 
> -steve
> 
> >  If I do
> >
> > DT[, sum(v), by=x]
> >
> > I always expect a two column data.table with columns "x" and "V1"
> > (which is sum(x)). In SQL this would read
> >
> > SELECT
> >  x,
> >  SUM(v)
> > FROM DT
> > GROUP BY x
> >
> >
> > So with DT[y<1,  sum(v), by=x] we have
> >
> > SELECT
> >  x,
> >  SUM(v)
> > FROM DT
> > WHERE y < 1
> > GROUP BY x
> >
> > Now in any SQL dialect I work with, if WHERE y < 1 didn't return any
> > rows, then the result of this would be a 0 row selection.  I believe
> > that is what it should be with data.table as well: a 0-row data.table
> > with columns "x" and "V1".
> >
> > This is a valid bug IMO, and should definitely be fixed.
> > On Wed, Apr 18, 2012 at 3:32 AM, Matthew Dowle <mdowle at mdowle.plus.com> wrote:
> >> Dear datatablers,
> >>
> >> This seems like a valid bug report. Any reasons why we shouldn't make
> >> this change?
> >>
> >> https://r-forge.r-project.org/tracker/?func=detail&atid=975&aid=1945&group_id=240
> >>
> >> Matthew
> >>
> >>
> >>
> >> _______________________________________________
> >> datatable-help mailing list
> >> datatable-help at lists.r-forge.r-project.org
> >> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
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> 
> 
>