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

[Frank Erickson]

Thanks! I keep forgetting that I can eval() a larger statement. I think
I'll stick to an option that keeps the names close to the definitions for
now, though.

By the way, you replied to the r-forge forum "digest," outside of the
original thread. I might've missed the reply if there weren't so few active
conversations.

On Thu, Jan 15, 2015 at 11:07 AM, Juan Manuel Truppia <jmtruppia at gmail.com>
wrote:

> 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)
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>>
>> ----------------------------------------------------------------------
>>
>> 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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