[datatable-help] best way of eval-ing a list of quoted expressions (Frank Erickson)

[Juan Manuel Truppia]

Forgot to change the second line. Here it goes

dt <- data.table(a = runif(10))
ee <- quote(list(b = 3 * a, c =a +2))
dt[, names(ee)[-1] := eval(ee)]

On Fri Jan 16 2015 at 1:35:09 PM Frank Erickson <fperickson at wisc.edu> wrote:

> Ah, good idea. That's what I'll do next time.
>
> I hadn't realized that "names" could look inside a quoted/language object.
>
> Thanks!
>
> On Fri, Jan 16, 2015 at 9:56 AM, Juan Manuel Truppia <jmtruppia at gmail.com>
> wrote:
>
>> With a little more work you could keep the definitions with the names.
>> Here is how
>>
>> dt <- data.table(a = runif(10))
>> ee <- quote(list(3 * a, a +2))
>> dt[, names(ee)[-1] := eval(ee)]
>>
>> Sorry, hope this now sticks to the thread
>>
>> On Fri Jan 16 2015 at 11:19:57 AM Frank Erickson <fperickson at wisc.edu>
>> wrote:
>>
>>> 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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>>>>> Today's Topics:
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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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