[datatable-help] variable column names

[Matthew Dowle]

Or:
DT[,lapply(.SD,sum),by=...,.SDcols=myvars]

> with = FALSE will let you use literal column names
>
>
> On Wed, Apr 24, 2013 at 3:29 PM, Sam Steingold <sds at gnu.org> wrote:
>
>> What do I do if I want to operate on several columns?
>> E.g.,
>> --8<---------------cut here---------------start------------->8---
>> length(names(dt)) = 25
>> myvars = c("col1","col4","col7")
>> myid = "user"
>> setkeyv(dt,myid)
>> --8<---------------cut here---------------end--------------->8---
>> and I want to summarize columns in myvars by myid.
>> the point is that nowhere in the code the literal "col4" or "user" may
>> appear.
>>
>> Thanks.
>>
>> --
>> Sam Steingold (http://sds.podval.org/) on Ubuntu 12.10 (quantal) X
>> 11.0.11300000
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>> limits.
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