Flatten list column in data frame with ID column

Question

My data frame contains the output of a survey with a select multiple question type. Some cells have multiple values.

df <- data.frame(a=1:3,b=I(list(1,1:2,1:3)))
df
  a       b
1 1       1
2 2    1, 2
3 3 1, 2, 3

I would like to flatten out the list to obtain the following output:

df
  a       b
1 1       1
2 2       1
3 2       2
4 3       1
5 3       2
6 3       3

should be easy but somehow I can't find the search terms. thanks.

Answer 1

You can just use unnest from "tidyr":

library(tidyr)
unnest(df, b)
#   a b
# 1 1 1
# 2 2 1
# 3 2 2
# 4 3 1
# 5 3 2
# 6 3 3

Answer 2

Using base R, one option is stack after naming the list elements of 'b' column with that of the elements of 'a'. We can use setNames to change the names.

stack(setNames(df$b, df$a))

Or another option would be to use unstack to automatically name the list element of 'b' with 'a' elements and then do the stack to get a data.frame output.

stack(unstack(df, b~a))

---

Or we can use a convenient function listCol_l from splitstackshape to convert the list to data.frame.

library(splitstackshape)
listCol_l(df, 'b')