Select values from different columns based on a variable containing column names [duplicate]

Answer 1

An excuse to use the obscure .BY:

DT[, newval := .SD[[.BY[[1]]]], by=new]

   col1 col2 col3  new newval
1:    1    4   55 col1      1
2:    2    3   44 col2      3
3:    3   34   35 col2     34
4:    4   44   87 col3     87

How it works. This splits the data into groups based on the strings in new. The value of the string for each group is stored in newname = .BY[[1]]. We use this string to select the corresponding column of .SD via .SD[[newname]]. .SD stands for Subset of Data.

Alternatives. get(.BY[[1]]) should work just as well in place of .SD[[.BY[[1]]]]. According to a benchmark run by @David, the two ways are equally fast.