i. prefix is just a robust way to reference join inherited columns: the 'top' column in the i table. Like table aliases in SQL.
What about this? : 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(1,2,1), top=c(3,4,2)) 1> Y a top 1: 1 3 2: 2 4 3: 1 2 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 8: 1 1 9: 1 4 1> On 24.04.2013 23:43, Eduard Antonyan wrote: > 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] [11]> 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/ [8], 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 [9]" >>> >>> On Wed, Apr 24, 2013 at 4:01 PM, Matthew Dowle <[email protected] [10]> 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] [1] >>>> >> To: [email protected] [2] >>>> >>>>>> 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 [3] >>>> >> Sent from the datatable-help mailing list archive at Nabble.com. >>>> >> _______________________________________________ >>>> >> datatable-help mailing list >> [email protected] [4] >>>> >>>>>> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help [5] >>>> > _______________________________________________ >>>> > datatable-help mailing list > [email protected] [6] >>>> > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help [7] Links: ------ [1] mailto:[email protected] [2] mailto:[email protected] [3] http://r.789695.n4.nabble.com/changing-data-table-by-without-by-syntax-to-require-a-by-tp4664770p4664990.html [4] mailto:[email protected] [5] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help [6] mailto:[email protected] [7] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help [8] http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/ [9] http://table2.id [10] mailto:[email protected] [11] mailto:[email protected]
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