Np. Those are documented in ?data.table and ?.SD, but it's hard to put the pieces together.
This is an "update join". The developers have a vignette on joins planned, but in the meantime, maybe my notes on it can help: http://franknarf1.github.io/r-tutorial/_book/tables.html#dt-joins On Wed, Feb 14, 2018 at 4:01 PM, Bernstein, Elliot J < ejbernst...@wellington.com> wrote: > Frank – > > > > Thank you very much for your help. > > > > That works, but unfortunately I don’t understand why. I’ve been searching > the package help and vignettes, but haven’t found an explanation. Is this > syntax documented somewhere? (Nearly everything about that line is a > mystery to me: What does it mean to use .SD in the i argument to DT? What > is “on = .(date) doing? And how is “x.y” apparently referring to column “y” > of y.dt?) > > > > Thanks. > > > > - Elliot > > > > *From:* Frank Erickson [mailto:by.hook...@gmail.com] > *Sent:* Wednesday, February 14, 2018 3:27 PM > *To:* Bernstein, Elliot J > *Cc:* firstname.lastname@example.org > *Subject:* Re: [datatable-help] Rolling Join With Groups in One Table > > > > You mean > > > > x.dt[, y := y.dt[.SD, on=.(date), roll=TRUE, x.y]] > > > > ? > > > > On Wed, Feb 14, 2018 at 3:18 PM, Bernstein, Elliot J < > ejbernst...@wellington.com> wrote: > > How do I execute a rolling join when one table has groups, and the other > does not? For example: > > > > x.dt <- as.data.table( > > expand.grid(date = seq(as.Date("2017-01-01"), as.Date("2017-12-31"), > "days"), > > group = c("A", "B") > > ) > > ) > > x.dt[, x := rnorm(.N)] > > setkey(x.dt, group, date) > > > > y.dt <- data.table(date = seq(as.Date("2017-01-01"), > as.Date("2017-12-31"), "months")) > > y.dt[, y := month(date)] > > setkey(y.dt, date) > > > > result <- y.dt[x.dt, roll = TRUE] > > > > The last line fails because “group” is part of the key in x.dt, but not > y.dt: > > > > Error in bmerge(i, x, leftcols, rightcols, io, xo, roll, rollends, > nomatch, : > > typeof x.date (double) != typeof i.group (integer) > > > > I would like the value of y in each row of the result to be the month > number (from y.dt), regardless of the value of the group column. > > > > Thanks. > > > > - Elliot > > > > > _______________________________________________ > datatable-help mailing list > email@example.com > https://lists.r-forge.r-project.org/cgi-bin/mailman/ > listinfo/datatable-help > > >
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