[R] data.table: reference column in "i"-part by column name in string object

[Pascal A. Niklaus]

I am converting data.frame-based code to data.table, for performance 
reasons.

Is there a way to refer to columns using expressions in the "i"-part?

Here is an example:

a <- data.table(x=rep(LETTERS[1:10],each=2), y=1:20)
v <- "x"

For the j-part, I can access the column whose name is stored in v as

a[,..v]

However, for the i-part I did not find a good way to achieve the same. 
For a simple case, the following works

xx <- "B"
a[xx, on=v]

However, I can't see how this is easily expanded to more complex logical 
expressions.

Pascal