<div dir="ltr">Yeah, leveraging not using all columns in a full merge is a good idea - I agree that feature would be nice to have.<div><br></div><div style>I'm not using the match() function tbh, so current naming doesn't bug me, but maybe others who do use match() can weigh in if that bothers them as well.</div>
</div><div class="gmail_extra"><br><br><div class="gmail_quote">On Fri, May 3, 2013 at 10:36 AM, Gabor Grothendieck <span dir="ltr"><<a href="mailto:ggrothendieck@gmail.com" target="_blank">ggrothendieck@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Yes, except that is not really what happens since match() only matches<br>
one row whereas with mult="all", the default, all rows are matched<br>
which is not really matching in the sense of match(). The current<br>
naming confuses matching with joining and its really the latter that<br>
is being done.<br>
<br>
Regarding the existence of merge the advantage of [ is that it will<br>
automatically only take the columns needed so merge is not really<br>
equivalent to [ in all respects. Furthermore having to use different<br>
constructs for different types of merge seems awkward.<br>
<br>
<br>
On Fri, May 3, 2013 at 11:27 AM, Eduard Antonyan<br>
<div class="HOEnZb"><div class="h5"><<a href="mailto:eduard.antonyan@gmail.com">eduard.antonyan@gmail.com</a>> wrote:<br>
> Btw the way I think about the "nomatch" name is as follows - normally X[Y]<br>
> tries to match rows of Y with rows of X, and then "nomatch" tells it what to<br>
> do when there is *no match*.<br>
><br>
><br>
> On Fri, May 3, 2013 at 10:23 AM, Eduard Antonyan <<a href="mailto:eduard.antonyan@gmail.com">eduard.antonyan@gmail.com</a>><br>
> wrote:<br>
>><br>
>> To clarify - that behavior is already implemented in merge (more<br>
>> specifically merge.data.table). I don't really have a view on having it in<br>
>> X[Y] as well - I don't like all.x and all.y as the names, since there are no<br>
>> params named 'x' and 'y' in [.data.table (as opposed to merge), but some<br>
>> param that would do a full outer join could certainly be added.<br>
>><br>
>><br>
>> On Fri, May 3, 2013 at 10:09 AM, Gabor Grothendieck<br>
>> <<a href="mailto:ggrothendieck@gmail.com">ggrothendieck@gmail.com</a>> wrote:<br>
>>><br>
>>> Yes, sorry. Its nomatch= which presumably derives from the parameter<br>
>>> of the same name in the match() function. If the idea of the nomatch=<br>
>>> name was to leverage off existing argument names in R then I would<br>
>>> prefer all.y= to be consistent with merge() in place of nomatch= since<br>
>>> we are really merging/joining rather than just matching. That would<br>
>>> also allow extension to all types of join by adding <a href="http://all.an" target="_blank">all.an</a> x= argument<br>
>>> too.<br>
>>><br>
>>> On Fri, May 3, 2013 at 10:59 AM, Eduard Antonyan<br>
>>> <<a href="mailto:eduard.antonyan@gmail.com">eduard.antonyan@gmail.com</a>> wrote:<br>
>>> > I would prefer nomatch=0 as a default though, simply because that's<br>
>>> > what I<br>
>>> > do most of the time :)<br>
>>> ><br>
>>> ><br>
>>> > On Fri, May 3, 2013 at 9:57 AM, Eduard Antonyan<br>
>>> > <<a href="mailto:eduard.antonyan@gmail.com">eduard.antonyan@gmail.com</a>><br>
>>> > wrote:<br>
>>> >><br>
>>> >> A correction - the param is called "nomatch", not "match".<br>
>>> >><br>
>>> >> This use case seems like smth a user shouldn't really do - in an ideal<br>
>>> >> world you should have them both keyed by the same-name column.<br>
>>> >><br>
>>> >> As is, my view on it is that data.table is correcting the user mistake<br>
>>> >> of<br>
>>> >> naming the column in Y - y, instead of x, and so the output makes<br>
>>> >> sense and<br>
>>> >> I don't see the need of complicating the behavior by adding more cases<br>
>>> >> one<br>
>>> >> has to go through to figure out what the output columns would be.<br>
>>> >> Similar to<br>
>>> >> asking for X[J(c("b", "c", "d"))] - you wouldn't want an anonymous<br>
>>> >> column<br>
>>> >> there, would you?<br>
>>> >><br>
>>> >><br>
>>> >><br>
>>> >> On Fri, May 3, 2013 at 6:18 AM, Gabor Grothendieck<br>
>>> >> <<a href="mailto:ggrothendieck@gmail.com">ggrothendieck@gmail.com</a>> wrote:<br>
>>> >>><br>
>>> >>> I am moving this discussion which started with mdowle to the list.<br>
>>> >>><br>
>>> >>> Consider this example slightly modified from the data.table FAQ:<br>
>>> >>><br>
>>> >>> > X = data.table(x=c("a","a","b","b","b","c","c"), foo=1:7, key="x")<br>
>>> >>> > Y = data.table(y=c("b","c","d"), bar=c(4,2,3))<br>
>>> >>> > out <- X[Y]; out<br>
>>> >>> x foo bar<br>
>>> >>> 1: b 3 4<br>
>>> >>> 2: b 4 4<br>
>>> >>> 3: b 5 4<br>
>>> >>> 4: c 6 2<br>
>>> >>> 5: c 7 2<br>
>>> >>> 6: d NA 3<br>
>>> >>><br>
>>> >>> Note that the first column of the output is labelled x even though<br>
>>> >>> the<br>
>>> >>> data to produce it comes from y, e.g. "d" in out$x is not in X$x but<br>
>>> >>> does appear in Y$y so clearly the data is coming from y as opposed to<br>
>>> >>> x . In terms of SQL the above would be written:<br>
>>> >>><br>
>>> >>> select Y.y as x, ...<br>
>>> >>><br>
>>> >>> and the need to renamne the first column of out suggests that there<br>
>>> >>> may be a deeper problem here.<br>
>>> >>><br>
>>> >>> Here are some ideas to address this (they would require changes to<br>
>>> >>> data.table):<br>
>>> >>><br>
>>> >>> - the default of X[Y,, match=NA] would be changed to a default of<br>
>>> >>> X[Y,,match=0] so that it corresponds to the defaults in R's merge and<br>
>>> >>> in SQL joins.<br>
>>> >>><br>
>>> >>> - the column name of the first column in the example above would be<br>
>>> >>> changed to y if match=0 but be left at x if match=NA. In the case<br>
>>> >>> that match=0 (the proposed new default) x and y are equal so the<br>
>>> >>> first<br>
>>> >>> column can be validly labelled as x but in the case that match=NA<br>
>>> >>> they<br>
>>> >>> are not so y would be used as the column name.<br>
>>> >>><br>
>>> >>> - the name match= does seem a bit misleading since R's match only<br>
>>> >>> matches one item in the target whereas in data.table match matches<br>
>>> >>> many if mult="all" and that is the default. Perhaps some thought<br>
>>> >>> should be given to a name change here?<br>
>>> >>><br>
>>> >>> The above would seem to correspond more closely to R's merge and SQL<br>
>>> >>> join defaults. Any use cases or other comments?<br>
>>> >>><br>
>>> >>> --<br>
>>> >>> Statistics & Software Consulting<br>
>>> >>> GKX Group, GKX Associates Inc.<br>
>>> >>> tel: 1-877-GKX-GROUP<br>
>>> >>> email: ggrothendieck at <a href="http://gmail.com" target="_blank">gmail.com</a><br>
>>> >>> _______________________________________________<br>
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>>> >>> <a href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a><br>
>>> >>><br>
>>> >>><br>
>>> >>> <a href="https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help" target="_blank">https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help</a><br>
>>> >><br>
>>> >><br>
>>> ><br>
>>><br>
>>><br>
>>><br>
>>> --<br>
>>> Statistics & Software Consulting<br>
>>> GKX Group, GKX Associates Inc.<br>
>>> tel: 1-877-GKX-GROUP<br>
>>> email: ggrothendieck at <a href="http://gmail.com" target="_blank">gmail.com</a><br>
>><br>
>><br>
><br>
<br>
<br>
<br>
--<br>
Statistics & Software Consulting<br>
GKX Group, GKX Associates Inc.<br>
tel: 1-877-GKX-GROUP<br>
email: ggrothendieck at <a href="http://gmail.com" target="_blank">gmail.com</a><br>
</div></div></blockquote></div><br></div>