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 <[email protected]> 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 <[email protected]> > > 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 > >> [email protected] > >> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help > > _______________________________________________ > > datatable-help mailing list > > [email protected] > > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help > > > _______________________________________________ datatable-help mailing list [email protected] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
