如何交叉引用data.table中的列

Question

I need to cross-reference from a column in a data.table to multiple other columns in it. As an example I have :

library(data.table) 

dt = data.table(mtcars)
sub = dt[1:3]
sub[, v1 := ('vs', 'am', 'gear')]
head(dt) 
mpg     cyl disp    hp  drat    wt      qsec    vs  am  gear    carb
21.0    6   160    110  3.90    2.620   16.46   0   1   4        4
21.0    6   160    110  3.90    2.875   17.02   0   1   4        4
22.8    4   108    93   3.85    2.320   18.61   1   1   4        1

So the column v1 contains the information which column the value for should be taken. So for the sub i want a new column with value : row 1: the value from the column 'vs' from the first row row 2: the value from column 'am' from the second row row 3: the value from the third row of 'gear' in short an a data.table like

mpg    cyl  disp hp     drat    wt      qsec    vs  am  gear carb   v1  v2
21.0    6   160  110    3.90    2.620   16.46   0   1   4    4      vs  0
21.0    6   160  110    3.90    2.875   17.02   0   1   4    4      am  1 
22.8    4   108  93     3.85    2.320   18.61   1   1   4    1    gear  4

unfortunately somthing like

sub[, v2 := get(value))]

or

sub[, v2 := get((sub$value))] 

doesn't lead to the solution as v2 would be equal to [0,0,1] but not [0,1,4] Thanks in advance

Answer 1

Okay, this was easier than I thought, but for anyone who wants to achieve something similar a group by for the respective column solved my problem. SO the solution would be :

sub[, v2 := get(c(value)), by = value]

Which leads to the desired result :

head(sub[, .(mpg, v1, v2 )]
mpg     v1      v2
21.0    vs      0
21.0    am      1
22.8    gear    4