How to 'unlist' a column in a data.table

Question

in my table, some cells are vectors instead of single value, i.e. the column is a list instead of vector:

dt1 <- data.table(
  colA=   c(\'A1\',\'A2\',\'A         


        

Answer 1

I think @Jaap's is easiest, but here's another alternative to chew over:

#create ID column
dt1[ , ID := .I]

#unnest colB, keep ID column
dt_unnest = dt1[ , .(ID = rep(ID, lengths(colB)),
                     colB = unlist(colB))]
#merge
dt_unnest = dt_unnest[dt1[ , !'colB'], on = 'ID']
#    ID colB colA colC colD
# 1:  1   B1   A1   C1   D1
# 2:  2  B2a   A2   C2   D2
# 3:  2  B2b   A2   C2   D2
# 4:  3   B3   A3   C3   D3

Answer 2

Promoting my comment to an answer. Using:

dt1[,.(colB = unlist(colB)), by = setdiff(names(dt1), 'colB')]

gives:

   colA colC colD colB
1:   A1   C1   D1   B1
2:   A2   C2   D2  B2a
3:   A2   C2   D2  B2b
4:   A3   C3   D3   B3

Or as an alternative (a slight variation of @Frank's proposal):

dt1[rep(dt1[,.I], lengths(colB))][, colB := unlist(dt1$colB)][]