How to assign list as observation in data table?

Question

I have a data table that contains character observations:

library(data.table)
library(stringr)

DT = data.table(strings = c('AAABD', 'BBDA', 'AACBDAA', 'ABACD'))

I would like to create a variable that contains counts of 'A', 'AA', and 'AAA' in each observation as a list. To do this I have created a function foo:

foo <- function(str) {
    n <- str_count(str, 'A')
    n2 <- str_count(str, 'AA')
    n3 <- str_count(str, 'AAA')
    df <- list('n' = n, 'n2' = n2, 'n3' = n3)

    return(df)
}

I apply this function to DT to create a new variable for count observation as a list:

DT[, count := foo(strings)]

When I do this, I receive this error:

Warning message:
In `[.data.table`(DT, , `:=`(counts, foo(strings))) :
  Supplied 3 items to be assigned to 4 items of column 'counts' (recycled leaving remainder of 1 items).

The data table returned has count variable lists of size 4 instead of size 3 and does not represent the amount of 'A', 'AA', and 'AAA' accurately for each string observation in variable strings. How can I assign a list as an observation in a data table?

Answer 1

You need to transpose the list:

foo <- function(str) {
  n <- str_count(str, 'A')
  n2 <- str_count(str, 'AA')
  n3 <- str_count(str, 'AAA')
  df <- transpose(list('n' = n, 'n2' = n2, 'n3' = n3)) # <- add transpose 
  return(df)
}

DT[, count := foo(strings)]
DT
#     strings count
#  1:   AAABD 3,1,1
#  2:    BBDA 1,0,0
#  3: AACBDAA 4,2,0
#  4:   ABACD 2,0,0

str(DT)
# Classes ‘data.table’ and 'data.frame':    4 obs. of  2 variables:
# $ strings: chr  "AAABD" "BBDA" "AACBDAA" "ABACD"
# $ count  :List of 4
#  ..$ : int  3 1 1
#  ..$ : int  1 0 0
#  ..$ : int  4 2 0
#  ..$ : int  2 0 0