Support for on-the-fly columns in on

[MichaelChirico]

The ability to create columns in on on-the-fly, like we can in j or by, would be appreciated.

As a usage example, this would have simplified my answer to the following self-join / moving-average-type answer:

http://stackoverflow.com/a/36534824/3576984

Reproduced here:

require(data.table)
date <- as.Date(c('2012-01-01','2012-01-01','2012-01-01',
                  '2012-01-02','2012-01-02','2012-01-03',
                  '2012-01-04','2012-01-05','2012-01-05',
                  '2012-01-06','2012-01-06','2012-01-06'))
email <- c('test@test.com', 'test1@test.com','test2@test.com',
           'test1@test.com', 'test2@test.com','test@test.com',
           'test@test.com','test@test.com','test@test.com',
           'test@test.com','test@test.com','test1@test.com')
dt <- data.table(date, email)

dt[dt[ , .(date3=date, date2 = date - 2, email)], 
   on = .(date >= date2, date<=date3), 
   allow.cartesian = TRUE
   ][ , .(count = uniqueN(email)), 
      by = .(date = date + 2)]

Would have been prettier / easier to parse as:

dt[dt, on = .(date >= date-2, date<=date), allow.cartesian = TRUE
   ][ , .(count = uniqueN(email)), by = date]

[jangorecki]

looks like a duplicate of #625

[aryoda]

Big vote for this enhancement since it makes the syntax of data.table shorter, more intuitive and more SQL alike.

[arunsrinivasan]

When done, update:

http://stackoverflow.com/q/37978658/559784
and
https://stackoverflow.com/q/48282827/559784

[Henrik-P]

Because Arun started a list with SO posts which may be updated, I thought it I could add some more I stumbled over:

How to merge dataframes of unequal length based on time with buffer intervals in R?

Data.Table non-equi join with arithmetic operations

subset data.frame base on a time interval + or - list of dates

[franknarf1]

To update: https://stackoverflow.com/q/52901099

[jaapwalhout]

To update: https://stackoverflow.com/q/53559119/2204410
(or close as a duplicate with one of the links above)

[UweBlock]

To update: inner_join() with range of values for one of the keys (year)

[ColeMiller1]

What would the API be? The difficult part is that users may want to use the newly created on condition. The evaluation would be easier if we could get users to use a helper function if they want to access the new column in j.

dt[dt, 
   on = .(date >= list(past_date = date-2), 
          date <= date), 
   allow.cartesian = TRUE]

The alternative would be like SQL and have the eval happen twice, once in on and then have users repeat in j

dt[dt,
   on = .(date >= date-2,
          date<=date), 
   .(x.date, past_date = i.date - 2),
   allow.cartesian = TRUE]

[UweBlock]

To update https://stackoverflow.com/a/60586692/3817004

[jangorecki]

AFAIR bmerge already do shallow copies of data, so adding new columns there would be probably easiest way to achieve that. We just need a proper handling for is.call of LHS and RHS of each element of on argument using internal .parse_on, and then using that in bmerge. The most tricky part can be handling column names well. Eventually we could allow computed columns to be named, the same as we allow in by argument: .(date2 = date >= date-2, date<=date).