[datatable-help] merge/join/match

[Eduard Antonyan]

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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>>
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>>
>
>
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