Apply function on each cell in a column and add the result to a new column

Question

I have a data.table as shown below. What I want is to apply a function to each of the elements in column C. The function will take in a vector(since Col C contains vector elements) and return another vector. This resultant vector should be added to a new column.

     A   B         C        
1:  16  151 c(2579, 2659, 2752)
2:  16  152 c(2580, 2660, 2753) 
3:  16  153 c(2581, 2661, 2754)
4:  16  154 c(2582, 2662, 2755)
5:  16  155 c(2583, 2663, 2756)
6:  16  156 c(2584, 2664, 2757)

For example let us consider a function 'isOdd' that takes in a vector and returns a logical vector. The output table after applying this function should look like

     A   B           C            isOdd
1:  16  151 c(2579, 2659, 2752)  c(T,T,F)
2:  16  152 c(2580, 2660, 2753)  c(F,F,T) 
3:  16  153 c(2581, 2661, 2754)  c(T,T,F)
4:  16  154 c(2582, 2662, 2755)  c(F,F,T)
5:  16  155 c(2583, 2663, 2756)  c(T,T,F)
6:  16  156 c(2584, 2664, 2757)  c(F,F,T)

How do I achieve this?

Answer 1

Using R's apply functions, we can easily accomplish your goal. Lets say that d is your data.table you are working with. Basically lapply passes each row of column "C" to the anonymous function, which then further passes each element of the passed in row to the function, isOdd.

isOdd <- function(x) {
    if (x %% 2 == 0) return("F") 
    else return("T")
}

d$isOdd <- lapply(d$C, function(x) sapply(x, isOdd))

Answer 2

DT = data.table(A=letters[1:3], B=list(1:3,4:6,7:8))
DT
   A     B   # notice B is list column containing vectors of different lengths
1: a 1,2,3   # it isn't a character vector, although it prints like one
2: b 4,5,6
3: c   7,8

> DT[,L:=sapply(B,length)][]
   A     B L
1: a 1,2,3 3
2: b 4,5,6 3
3: c   7,8 2

> isOdd = function (x) x%%2 == 0

> DT[,odd:=lapply(B,isOdd)][]
   A     B L              odd
1: a 1,2,3 3 FALSE,TRUE,FALSE
2: b 4,5,6 3  TRUE,FALSE,TRUE
3: c   7,8 2       FALSE,TRUE