[datatable-help] datatable-help Digest, Vol 59, Issue 2

[Juan Manuel Truppia]

I don't know what you are trying to achieve, but I usually quote the list,
instead of generating a list of quotes. I think that your issue is similar
to something I've faced in the past, and I usually solve it like this

dt <- data.table(a = runif(10))
ee <- quote(list(3 * a, a +2))
dt[, c("b", "c") := eval(ee)]

I still don't know how to define the column names in the quoted expression,
instead of in the `:=` call.

Hope it helps


On Thu Jan 15 2015 at 8:00:10 AM <
datatable-help-request at lists.r-forge.r-project.org> wrote:

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>    1. best way of eval-ing a list of quoted expressions (Frank Erickson)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 14 Jan 2015 17:06:36 -0500
> From: Frank Erickson <fperickson at wisc.edu>
> To: "data.table help" <datatable-help at lists.r-forge.r-project.org>
> Subject: [datatable-help] best way of eval-ing a list of quoted
>         expressions
> Message-ID:
>         <CAJd-hdmsub_h_oh2rGrBuvbGFCk4S9ZoTMi+
> FrfEssH9mqoLiA at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi,
>
> I'm wondering what the most idiomatic or efficient approach is...? Here's
> my example:
>
> expr_nonlin = list(
>   early = quote(tt/TT*(tt/TT < .2)),
>   late  = quote(tt/TT*(tt/TT > .8))
> )
>
> # eval on a single expr works
> data.table(tt=1,TT=100)[,early:=eval(expr_nonlin$early)][]
>
> # lapply eval does not work
> data.table(tt=1,TT=100)[,names(expr_nonlin):=lapply(expr_nonlin,eval)][]
>
> # (1) envir fixes it
> DT <- data.table(tt=1,TT=100)
> DT[,names(expr_nonlin):=lapply(expr_nonlin,eval,envir=DT)][]
>
> # (2) or a for loop
> DT <- data.table(tt=1,TT=100)
> for (i in names(expr_nonlin)) DT[,(i):=eval(expr_nonlin[[i]])]
>
> (1) and (2) both work. Is either preferable?
>
> (1) calls [.data.table fewer times, but messes around with environments,
> which always seem fragile.
>
> ------------------
>
> One more quick question: In approach (1), is there a way to skip the
> names(zzz):= part? I see that this doesn't work:
>
> DT <- data.table(tt=1,TT=100)
> DT[,do.call(`:=`,lapply(expr_nonlin,eval,envir=DT))][]
>
>
> Thanks,
>
> Frank
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