[datatable-help] merge/join/match

[Eduard Antonyan]

Arun,

I think Gabor understands that. If I understand him correctly he simply
wants an option for X[Y, some.option = TRUE] to return merge(X,Y, all =
TRUE). Also, do note that merge(X, Y, all.x = TRUE) and merge(Y, X, all.x =
TRUE) don't have to return the same answer.


On Fri, May 3, 2013 at 10:46 AM, Arunkumar Srinivasan <aragorn168b at gmail.com
> wrote:

>  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> 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>
> 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> 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> 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>
> 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> 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
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>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com
>
>
>
>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com
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