[datatable-help] data.table question: ave like behavior

[Ozan Bakis]

Thank you very much...
ozan

On 24 May 2010 17:38, Short, Tom <TShort at epri.com> wrote:

> > From: datatable-help-bounces at lists.r-forge.r-project.org
> [mailto:datatable-help-bounces at lists.r-forge.r-project.org] On Behalf Of
> Ozan Bakis
> > Sent: Monday, May 24, 2010 10:12
> > To: datatable-help at lists.r-forge.r-project.org
> > Subject: [datatable-help] data.table question: ave like behavior
> >
> >
> > Hi,
> >
> > I have just noticed and installed your package and am trying to
> understand how it works.
> > The commented part in the following example from the reference manual
> (p. 20) gives an error.
> >
> > dt <- data.table(a=rep(1:5, 1), b=1:10)
> > transform(dt, c = a^2)
> > within(dt, {
> > b <- rev(b)
> > c <- a^2
> > rm(a)
> > })
> > dt[, transform, c = max(b), by="a"] # like "ave
> >
>
> Ozan, we experimented with that syntax for a while, but it was
> slow, so we took it out. The new syntax is as follows:
>
> dt[, transform(.SD, c = max(b)), by = a]
>
> .SD is a datatable of the subset, so you can manipulate that, in
> this case with transform.
>
> Another way to get ave functionality is:
>
> dt$c <- dt[, rep(max(b), length(a)), by = a]$V1
>
> This may be a little faster and more memory friendly (and maybe
> harder to read).
>
> You should be able to adapt either approach to the second part of
> your email.
>
> - Tom
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/datatable-help/attachments/20100524/78c63997/attachment.htm>