how to "spread" a list-column? - r

Question

Consider this simple example
mydf <- data_frame(regular_col = c(1,2),
normal_col = c('a','b'),
weird_col = list(list('hakuna', 'matata'),
list('squash', 'banana')))
> mydf
# A tibble: 2 x 3
regular_col normal_col weird_col
<dbl> <chr> <list>
1 1 a <list [2]>
2 2 b <list [2]>
I would like to extract the elements of weird_col (programmatically, the number of elements may change) so that each element is placed on a different column. That is, I expect the following output
> data_frame(regular_col = c(1,2),
+ normal_col = c('a','b'),
+ weirdo_one = c('hakuna', 'squash'),
+ weirdo_two = c('matata', 'banana'))
# A tibble: 2 x 4
regular_col normal_col weirdo_one weirdo_two
<dbl> <chr> <chr> <chr>
1 1 a hakuna matata
2 2 b squash banana
However, I am unable to do so in simple terms. For instance, using the classic unnest fails here, as it expands the dataframe instead of placing each element of the list in a different column.
> mydf %>% unnest(weird_col)
# A tibble: 4 x 3
regular_col normal_col weird_col
<dbl> <chr> <list>
1 1 a <chr [1]>
2 1 a <chr [1]>
3 2 b <chr [1]>
4 2 b <chr [1]>
Is there any solution in the tidyverse for that?

Answer 1

You can extract the values from the output of unnest, process a little to make your column names, and then spread back out. Note that I use flatten_chr because of your depth-one list-column, but if it is nested you can use flatten and spread works just as well on list-cols.
library(tidyverse)
#> Warning: package 'dplyr' was built under R version 3.5.1
mydf <- data_frame(
regular_col = c(1, 2),
normal_col = c("a", "b"),
weird_col = list(
list("hakuna", "matata"),
list("squash", "banana")
)
)
mydf %>%
unnest(weird_col) %>%
group_by(regular_col, normal_col) %>%
mutate(
weird_col = flatten_chr(weird_col),
weird_colname = str_c("weirdo_", row_number())
) %>% # or just as.character
spread(weird_colname, weird_col)
#> # A tibble: 2 x 4
#> # Groups: regular_col, normal_col [2]
#> regular_col normal_col weirdo_1 weirdo_2
#> <dbl> <chr> <chr> <chr>
#> 1 1 a hakuna matata
#> 2 2 b squash banana
Created on 2018-08-12 by the reprex package (v0.2.0).

Answer 2

unnest develops lists and vectors vertically, and one row data frames horizontally. So what we can do is change your lists into data frames (with adequate column names) and unnest afterwards.
mydf %>% mutate(weird_col = map(weird_col,~ as_data_frame(
setNames(.,paste0("weirdo_",1:length(.)))
))) %>%
unnest
# # A tibble: 2 x 4
# regular_col normal_col weirdo_1 weirdo_2
# <dbl> <chr> <chr> <chr>
# 1 1 a hakuna matata
# 2 2 b squash banana