Alternative way to dynamic symbol usage in j

[renkun-ken]

Using dynamically determined symbol in j can be quite annoying (as discussed in #633, #2589).

I'm not sure if the following idea makes sense, which seems easier to implement and nicer to look at for me.

library(data.table)
dt <- data.table(id = rep(1:2, 5), x1 = rnorm(10), x2 = rnorm(10), y1 = rnorm(10), y2 = rnorm(10))
dt[, .(xsum = sum(x), ysum = sum(y)), by = id, alias = c(x = "x1", y = "y2")]

In the above example, if a new argument like alias is introduced, a named character vector would give fixed names to dynamically determined symbols, which makes it much easier to use them in j.

Some new problems may occur such as when x or y are already defined in dt. But the syntax looks much cleaner if the dynamic symbols can be determined outside dt.

[jangorecki]

I like the idea, somehow similar to substitute() where env argument refers to alias. We could also substitute function names this way.

[jangorecki]

It seems that my PR #4304 actually re-invents and implements what @renkun-ken wrote in this issue, + maybe call arg names substitution :)

[renkun-ken]

Didn't remember this issue. Explains why your nice solution looks so great to me. 😄