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 <[email protected]>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 < > [email protected]> 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 >> <[email protected]> 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 < >> [email protected]> >> > 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 >> >> <[email protected]> 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 >> >>> _______________________________________________ >> >>> datatable-help mailing list >> >>> [email protected] >> >>> >> >>> >> 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 >> > >
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