I assumed they meant create a table :) that looks cool, what's i.top ? I can get a very similar to yours result by writing:
X[Y][, head(.SD, top[1]), by = a] and I probably would want the following to produce your result (this might depend a little on what exactly i.top is): X[Y, head(.SD, i.top), by = a] On Wed, Apr 24, 2013 at 5:28 PM, Matthew Dowle <[email protected]>wrote: > ** > > > > That sentence on that linked webpage seems incorect English, since table > is a noun not a verb. Should "table" be "join" perhaps? > > Anyway, by-without-by is often used with join inherited scope (JIS). For > example, translating their example : > > 1> X = data.table(a=1:3,b=1:15, key="a") > 1> X > a b > 1: 1 1 > 2: 1 4 > 3: 1 7 > 4: 1 10 > 5: 1 13 > 6: 2 2 > 7: 2 5 > 8: 2 8 > 9: 2 11 > 10: 2 14 > 11: 3 3 > 12: 3 6 > 13: 3 9 > 14: 3 12 > 15: 3 15 > 1> Y = data.table(a=c(1,2), top=c(3,4)) > 1> Y > a top > 1: 1 3 > 2: 2 4 > 1> X[Y, head(.SD,i.top)] > a b > 1: 1 1 > 2: 1 4 > 3: 1 7 > 4: 2 2 > 5: 2 5 > 6: 2 8 > 7: 2 11 > 1> > > > > If there was no by-without-by (analogous to CROSS BY), then how would that > be done? > > > > On 24.04.2013 22:22, Eduard Antonyan wrote: > > By that you mean current behavior? You'd get current behavior by > explicitly specifying the appropriate "by" (i.e. "by" equal to the key). > Btw, I'm trying to understand SQL CROSS APPLY vs JOIN using > http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/, and I > can't figure out how by-without-by (or with by-with-by for that matter:) ) > helps with e.g. the first example there: > "We table table1 and table2. table1 has a column called rowcount. > > For each row from table1 we need to select first rowcount rows from table2, > ordered by table2.id" > > > > > On Wed, Apr 24, 2013 at 4:01 PM, Matthew Dowle <[email protected]>wrote: > >> But then what would be analogous to CROSS APPLY in SQL? >> >> > I'd agree with Eduard, although it's probably too late to change >> behavior >> > now. Maybe for data.table.2? Eduard's proposal seems more closely >> > aligned with SQL behavior as well (SELECT/JOIN, then GROUP, but only if >> > requested). >> > >> > S. >> > >> >> Date: Mon, 22 Apr 2013 08:17:59 -0700 >> >> From: [email protected] >> >> To: [email protected] >> >> Subject: Re: [datatable-help] changing data.table by-without-by >> >> syntax to require a "by" >> >> >> >> I think you're missing the point Michael. Just because it's possible to >> >> do it >> >> the way it's done now, doesn't mean that's the best way, as I've tried >> >> to >> >> argue in the OP. I don't think you've addressed the issue of >> unnecessary >> >> complexity pointed out in OP. >> >> >> >> >> >> >> >> -- >> >> View this message in context: >> >> >> http://r.789695.n4.nabble.com/changing-data-table-by-without-by-syntax-to-require-a-by-tp4664770p4664990.html >> >> Sent from the datatable-help mailing list archive at Nabble.com. >> >> _______________________________________________ >> >> datatable-help mailing list >> >> [email protected] >> >> >> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help >> > >> _______________________________________________ >> > datatable-help mailing list >> > [email protected] >> > >> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help >> >> >> > >
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