[datatable-help] aggregate data.table by group by reference

[Gerhard Nachtmann]

Hi!

You didn't provide an reproducable example (repex), so I created one
for you including the solution.

Consider you want to add the median values for each column of the iris
dataset by species.

iris <- as.data.table(iris)
iris[iris[, lapply(.SD, median), keyby = "Species"],
paste0(names(iris), ".med") := get(paste0("i.", names(iris))), on =
"Species"]

Cheers,
Gerhard

2018-03-07 11:04 GMT+01:00 Matthew Forrest <matthew.forrest at senckenberg.de>:
> Hi all,
>
> A real data.table question here ;-)
>
> I am currently doing this:
>
> output.dt <- input.dt[,lapply(.SD, method.function), by=by.dims]
>
> where method.function is an arbitrary function and by.dims is a list of
> columns (although perhaps I could remove this is the data.table was keyed by
> those columns?)
>
> That doesn't look very data.table-y to me, potentially it would be great to
> do it with := in the proper data.table way.  Is that possible?  I have tried
> a couple of things but haven't managed. The problem is that I need to be
> flexible to deal with arbitrary columns in my data.table.
>
> Any suggestions?
>
> Many thanks,
>
> Matt
>
>
> --
> Dr Matthew Forrest
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