[datatable-help] add a column specifying new column name via a variable

[Kaupas, George]

I'm trying to add empty columns to data.tables using a variable containing the name of the desired column, but I'm unable to figure out how to dereference the variable value to satisfy the := operator.

Here's a simple example:

require(data.table);
dt <- data.table(read.table(text="N1 N2\nA B\nC D\n", header=TRUE));
new_col_name = "N3";
dt[, new_col_name := NA];

That creates a column literally named "new_col_name", rather than "N3" as desired.

I can work around it this way:

dt[, workaround := NA];
setnames(dt, "workaround", new_col_name);

I have tried wrapping the new_col_name variable in all sorts of functions such as eval(), c(), list(), quote(), etc; all of these generate an error such as:

Error in `[.data.table`(dt, , `:=`(quote(new_col_name), NA)) : 
  LHS of := must be a single column name when with=TRUE. When with=FALSE the LHS may be a vector of column names or positions.

Surely I am overlooking something trivial; please advise.

Thanks
George
http://stackoverflow.com/users/1313052/gkaupas