Convert list to string with conditions - r

Question

I have a dataframe that looks like:
x <- tibble(
experiment_id = rep(c('1a','1b'),each=5),
keystroke = rep(c('a','SHIFT','b','SPACE','e'),2)
)
I know I can concatenate a list into a string using str_c or str_flatten and only keep certain values like below:
> y <- c('b','a','SPACE','d')
> y[y %in% letters]
[1] "b" "a" "d"
But when I try the same thing in a grouped pipe:
x_out <- x %>%
group_by(experiment_id) %>%
mutate(
grp = cumsum(lag(keystroke=='SPACE',default=0))) %>%
group_by(grp, .add=TRUE) %>%
mutate(within_keystrokes = list(keystroke),
within_word = within_keystrokes[within_keystrokes %in% letters]
) %>%
ungroup()
I get the error:
Error: Problem with `mutate()` input `within_word`.
x Input `within_word` can't be recycled to size 2.
ℹ Input `within_word` is `within_keystrokes[within_keystrokes %in% letters]`.
ℹ Input `within_word` must be size 2 or 1, not 0.
ℹ The error occurred in group 1: experiment_id = "1a", grp = 0.
I read this answer and tried using ifelse but still ran into errors.
Any insight into what I'm doing wrong?
EDIT: EXPECTED OUTPUT Sorry for not including this. I would expect the final df to look like:
x <- tibble(
experiment_id = rep(c('1a','1b'),each=5),
keystroke = rep(c('a','SHIFT','b','SPACE','e'),2),
within_keystrokes = list(list('a','SHIFT','b','SPACE'),
list('a','SHIFT','b','SPACE'),
list('a','SHIFT','b','SPACE'),
list('a','SHIFT','b','SPACE'),
'e',
list('a','SHIFT','b','SPACE'),
list('a','SHIFT','b','SPACE'),
list('a','SHIFT','b','SPACE'),
list('a','SHIFT','b','SPACE'),
'e'),
within_word = rep(list('ab','ab','ab','ab','e'),2)
)

Answer 1

You almost solved your issue. You could use
library(dplyr)
library(stringr)
x %>%
group_by(experiment_id, grp = cumsum(lag(keystroke == "SPACE", default = 0))) %>%
mutate(
within_keystrokes = list(keystroke),
within_word = list(str_c(keystroke[keystroke %in% letters], collapse = ""))
)
to get
# A tibble: 10 x 4
experiment_id keystroke within_keystrokes within_word
<chr> <chr> <list> <list>
1 1a a <list [4]> <chr [1]>
2 1a SHIFT <list [4]> <chr [1]>
3 1a b <list [4]> <chr [1]>
4 1a SPACE <list [4]> <chr [1]>
5 1a e <chr [1]> <chr [1]>
6 1b a <list [4]> <chr [1]>
7 1b SHIFT <list [4]> <chr [1]>
8 1b b <list [4]> <chr [1]>
9 1b SPACE <list [4]> <chr [1]>
10 1b e <chr [1]> <chr [1]>
If you don't want within_word to be a list, just remove the list() function.