How to do operations on list columns in an R data.table to output another list column?

Question

I still have a difficult time thinking about how one works with R data.table columns which are lists.

Here is an R data.table:

library(data.table)
dt = data.table(
      numericcol = rep(42, 8),
      listcol = list(c(1, 22, 3), 6, 1, 12, c(5, 6, 1123), 3, 42, 1)
  )
> dt
   numericcol        listcol
1:         42        1,22, 3
2:         42              6
3:         42              1
4:         42             12
5:         42    5,   6,1123
6:         42              3
7:         42             42
8:         42              1

I would like to create a column for the absolute values between the elements of numericcol and listcol:

> dt
   numericcol        listcol    absvals 
1:         42        1,22, 3    41, 20, 39
2:         42              6    36
3:         42              1    41
4:         42             12    30
5:         42    5,   6,1123    37, 36, 1081
6:         42              3    39
7:         42             42    0
8:         42              1    41

So, my first thought would be to use sapply() as follows:

dt[, absvals := sapply(listcol, function(x) abs(x-numericcol))]

This outputs the following:

> dt
   numericcol        listcol absvals
1:         42        1,22, 3      41
2:         42              6      20
3:         42              1      39
4:         42             12      41
5:         42    5,   6,1123      20
6:         42              3      39
7:         42             42      41
8:         42              1      20

So, absvals is now a column of unlisted elements, with an individual element in each row, and is a different dimension than the data.table.

(1) How would one create absvals to retain the list structure of listcol?

(2) In cases like these, if I am only interested in a vector of the values, how do R data.table users create such a data structure?

Maybe

vec = as.vector(dt[, absvals := sapply(listcol, function(x) abs(x-numericcol))])

?

Answer 1

Another solution using mapply:

dt[, absvals := mapply(listcol, numericcol, FUN = function(x, y) abs(x-y))]

#output
dt
   numericcol        listcol        absvals
1:         42        1,22, 3       41,20,39
2:         42              6             36
3:         42              1             41
4:         42             12             30
5:         42    5,   6,1123   37,  36,1081
6:         42              3             39
7:         42             42              0
8:         42              1             41