[datatable-help] merge/join/match

Arunkumar Srinivasan aragorn168b at gmail.com
Fri May 3 17:46:53 CEST 2013 

Gabor, 

X[Y] and Y[X] are not necessarily the same operations (meaning, they don't *have* to give the same output). However, merge(X,Y) and merge(Y,X) *have* to provide the same output (except for the column order and names). In that sense, a join is a bit different from a merge, no? 

Arun


On Friday, May 3, 2013 at 5:36 PM, Gabor Grothendieck wrote:

> Yes, except that is not really what happens since match() only matches
> one row whereas with mult="all", the default, all rows are matched
> which is not really matching in the sense of match(). The current
> naming confuses matching with joining and its really the latter that
> is being done.
> 
> Regarding the existence of merge the advantage of [ is that it will
> automatically only take the columns needed so merge is not really
> equivalent to [ in all respects. Furthermore having to use different
> constructs for different types of merge seems awkward.
> 
> 
> On Fri, May 3, 2013 at 11:27 AM, Eduard Antonyan
> <eduard.antonyan at gmail.com (mailto:eduard.antonyan at gmail.com)> wrote:
> > Btw the way I think about the "nomatch" name is as follows - normally X[Y]
> > tries to match rows of Y with rows of X, and then "nomatch" tells it what to
> > do when there is *no match*.
> > 
> > 
> > On Fri, May 3, 2013 at 10:23 AM, Eduard Antonyan <eduard.antonyan at gmail.com (mailto:eduard.antonyan at gmail.com)>
> > wrote:
> > > 
> > > To clarify - that behavior is already implemented in merge (more
> > > specifically merge.data.table). I don't really have a view on having it in
> > > X[Y] as well - I don't like all.x and all.y as the names, since there are no
> > > params named 'x' and 'y' in [.data.table (as opposed to merge), but some
> > > param that would do a full outer join could certainly be added.
> > > 
> > > 
> > > On Fri, May 3, 2013 at 10:09 AM, Gabor Grothendieck
> > > <ggrothendieck at gmail.com (mailto:ggrothendieck at gmail.com)> wrote:
> > > > 
> > > > Yes, sorry. Its nomatch= which presumably derives from the parameter
> > > > of the same name in the match() function. If the idea of the nomatch=
> > > > name was to leverage off existing argument names in R then I would
> > > > prefer all.y= to be consistent with merge() in place of nomatch= since
> > > > we are really merging/joining rather than just matching. That would
> > > > also allow extension to all types of join by adding all.an x= argument
> > > > too.
> > > > 
> > > > On Fri, May 3, 2013 at 10:59 AM, Eduard Antonyan
> > > > <eduard.antonyan at gmail.com (mailto:eduard.antonyan at gmail.com)> wrote:
> > > > > I would prefer nomatch=0 as a default though, simply because that's
> > > > > what I
> > > > > do most of the time :)
> > > > > 
> > > > > 
> > > > > On Fri, May 3, 2013 at 9:57 AM, Eduard Antonyan
> > > > > <eduard.antonyan at gmail.com (mailto:eduard.antonyan at gmail.com)>
> > > > > wrote:
> > > > > > 
> > > > > > A correction - the param is called "nomatch", not "match".
> > > > > > 
> > > > > > This use case seems like smth a user shouldn't really do - in an ideal
> > > > > > world you should have them both keyed by the same-name column.
> > > > > > 
> > > > > > As is, my view on it is that data.table is correcting the user mistake
> > > > > > of
> > > > > > naming the column in Y - y, instead of x, and so the output makes
> > > > > > sense and
> > > > > > I don't see the need of complicating the behavior by adding more cases
> > > > > > one
> > > > > > has to go through to figure out what the output columns would be.
> > > > > > Similar to
> > > > > > asking for X[J(c("b", "c", "d"))] - you wouldn't want an anonymous
> > > > > > column
> > > > > > there, would you?
> > > > > > 
> > > > > > 
> > > > > > 
> > > > > > On Fri, May 3, 2013 at 6:18 AM, Gabor Grothendieck
> > > > > > <ggrothendieck at gmail.com (mailto:ggrothendieck at gmail.com)> wrote:
> > > > > > > 
> > > > > > > I am moving this discussion which started with mdowle to the list.
> > > > > > > 
> > > > > > > Consider this example slightly modified from the data.table FAQ:
> > > > > > > 
> > > > > > > > X = data.table(x=c("a","a","b","b","b","c","c"), foo=1:7, key="x")
> > > > > > > > Y = data.table(y=c("b","c","d"), bar=c(4,2,3))
> > > > > > > > out <- X[Y]; out
> > > > > > > > 
> > > > > > > 
> > > > > > > x foo bar
> > > > > > > 1: b 3 4
> > > > > > > 2: b 4 4
> > > > > > > 3: b 5 4
> > > > > > > 4: c 6 2
> > > > > > > 5: c 7 2
> > > > > > > 6: d NA 3
> > > > > > > 
> > > > > > > Note that the first column of the output is labelled x even though
> > > > > > > the
> > > > > > > data to produce it comes from y, e.g. "d" in out$x is not in X$x but
> > > > > > > does appear in Y$y so clearly the data is coming from y as opposed to
> > > > > > > x . In terms of SQL the above would be written:
> > > > > > > 
> > > > > > > select Y.y as x, ...
> > > > > > > 
> > > > > > > and the need to renamne the first column of out suggests that there
> > > > > > > may be a deeper problem here.
> > > > > > > 
> > > > > > > Here are some ideas to address this (they would require changes to
> > > > > > > data.table):
> > > > > > > 
> > > > > > > - the default of X[Y,, match=NA] would be changed to a default of
> > > > > > > X[Y,,match=0] so that it corresponds to the defaults in R's merge and
> > > > > > > in SQL joins.
> > > > > > > 
> > > > > > > - the column name of the first column in the example above would be
> > > > > > > changed to y if match=0 but be left at x if match=NA. In the case
> > > > > > > that match=0 (the proposed new default) x and y are equal so the
> > > > > > > first
> > > > > > > column can be validly labelled as x but in the case that match=NA
> > > > > > > they
> > > > > > > are not so y would be used as the column name.
> > > > > > > 
> > > > > > > - the name match= does seem a bit misleading since R's match only
> > > > > > > matches one item in the target whereas in data.table match matches
> > > > > > > many if mult="all" and that is the default. Perhaps some thought
> > > > > > > should be given to a name change here?
> > > > > > > 
> > > > > > > The above would seem to correspond more closely to R's merge and SQL
> > > > > > > join defaults. Any use cases or other comments?
> > > > > > > 
> > > > > > > --
> > > > > > > Statistics & Software Consulting
> > > > > > > GKX Group, GKX Associates Inc.
> > > > > > > tel: 1-877-GKX-GROUP
> > > > > > > email: ggrothendieck at gmail.com (http://gmail.com)
> > > > > > > _______________________________________________
> > > > > > > datatable-help mailing list
> > > > > > > datatable-help at lists.r-forge.r-project.org (mailto:datatable-help at lists.r-forge.r-project.org)
> > > > > > > 
> > > > > > > 
> > > > > > > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
> > > > 
> > > > 
> > > > 
> > > > --
> > > > Statistics & Software Consulting
> > > > GKX Group, GKX Associates Inc.
> > > > tel: 1-877-GKX-GROUP
> > > > email: ggrothendieck at gmail.com (http://gmail.com)
> > > > 
> > > 
> > > 
> > 
> > 
> 
> 
> 
> 
> -- 
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com (http://gmail.com)
> _______________________________________________
> datatable-help mailing list
> datatable-help at lists.r-forge.r-project.org (mailto: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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