Data Table - Select Value of Column by Name From Another Column

Question

I have a data table with a number of columns containing values. I have another column which defines which one of those columns whose value I need to select. I am having tr

Answer 1

The following should be memory efficient and a little easier to read/follow.

for (i in unique(d[["name.of.col"]]))
    d[ name.of.col==i, value.of.col:=get(i) ]

d
   value.1 value.2 name.of.col value.of.col
1:     one     two     value.1          one
2:     uno     dos     value.2          dos
3:       1       2     value.1            1