loop in a data.table

Hi everyone,

I have a data.table called "data" with many columns which I want to group by
column1 using data.table, given how fast it is.

The problem with looping a data.table is that data.table does not like
quotations  to define the column names (e.g. "col2" instead of col2). I
found a way around which is to use get("col2"), which works fine but the
processing time multiples by 20.

So if I use:

data[,sum(col2),by=(key)]

entering the column names by hand, the operation is done in 1 sec. but if in
the contrary I use:

data[,sum(get("col2")),by=(key)]

using a loop to put the column names, the same operation takes 20 sec. I
cannot use the former code because I have 100000 files to process but the
later will simply take months to complete. Is there any alternative to the
function "get" or any other way in which data.table con recognize the names
of the columns?.