[datatable-help] variable column names

[Sam Steingold]

I am still missing something:

--8<---------------cut here---------------start------------->8---
> dt <- data.table(user=c(rep(4, 5),rep(3, 5)), behavior=c(rep(FALSE,5),rep(TRUE,5)),
                 country=c(rep(1,4),rep(2,6)), language=c(rep(6,6),rep(5,4)),
                 event=1:10, key=c("user","country","language"))
> dt
    user behavior country language event
 1:    3     TRUE       2        5     7
 2:    3     TRUE       2        5     8
 3:    3     TRUE       2        5     9
 4:    3     TRUE       2        5    10
 5:    3     TRUE       2        6     6
 6:    4    FALSE       1        6     1
 7:    4    FALSE       1        6     2
 8:    4    FALSE       1        6     3
 9:    4    FALSE       1        6     4
10:    4    FALSE       2        6     5
>   users <- dt[, sum(behavior) > 0, by=user]
Finding groups (bysameorder=TRUE) ... done in 0secs. bysameorder=TRUE and o__ is length 0
Detected that j uses these columns: behavior 
Optimization is on but j left unchanged as 'sum(behavior) > 0'
Starting dogroups ... done dogroups in 0 secs
> users
   user    V1
1:    3  TRUE
2:    4 FALSE
> setnames(users, "V1", "behavior")
--8<---------------cut here---------------end--------------->8---

Now I want to do the same thing as in
http://stackoverflow.com/questions/16200815/summarize-a-data-table-with-unreliable-data
for both fields
> fields <- c("country","language")

here is what I tried so far:

--8<---------------cut here---------------start------------->8---
dt[, .N, .SDcols=fields, by=eval(list("user",fields))]
Error in `[.data.table`(dt, , .N, .SDcols = fields, by = eval(list("user",  : 
  The items in the 'by' or 'keyby' list are length (1,2). Each must be same length as rows in x or number of rows returned by i (10).
Calls: [ -> [.data.table
--8<---------------cut here---------------end--------------->8---

the idea is to do something like

--8<---------------cut here---------------start------------->8---
> dt.out <- dt[, .N, by=list(user,country)][, list(country[which.max(N)], max(N)/sum(N)), by=user]
> setnames(dt.out, c("V1", "V2"),  paste0("country",c(".name", ".support")))
> users <- users[dt.out]
   user behavior country.name country.support
1:    3     TRUE            2             1.0
2:    4    FALSE            1             0.8
--8<---------------cut here---------------end--------------->8---

except that I do not want to have the literal "country" and "language"
and that I am sure there is a way to avoid copying users in
> users <- users[dt.out]
by a ":=" trick.

Thanks.

> * Matthew Dowle <zqbjyr at zqbjyr.cyhf.pbz> [2013-04-24 21:54:17 +0100]:
>
> where ... is eval(myid)
> iigc
>> Or:
>> DT[,lapply(.SD,sum),by=...,.SDcols=myvars]

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