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

data.table, r

Solution

Another option:

d[ , value.of.col := diag(as.matrix(.SD)), .SDcols = d[ , name.of.col]]
> 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

EDIT add a faster solution:

d[ , value.of.col :=
      melt(d,id.vars='name.of.col')[name.of.col==variable, value]]

Problem

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 trouble finding a way to do this. Here is a simple example. ``` > d <- data.table( value.1 = c("one", "uno", "1"), value.2 = c("two", "dos", "2"), name.of.col = c("value.1","value.2","value.1")) > d value.1 value.2 name.of.col 1: one two value.1 2: uno dos value.2 3: 1 2 value.1 ``` I would like to add a column 'value.of.col' which contains the value of the column specified by 'name.of.col'. ``` > 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 ```

Original source