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

Pascal A. Niklaus P@@c@|@N|k|@u@ @end|ng |rom |eu@uzh@ch
Thu Feb 21 15:10:04 CET 2019

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

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


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 


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