Creating a variable that appends together strings

Question

I have a data frame like so:

moves	game_id
e2e4	1
cfc8	1
g6e4	1
f6g8	2
f8g5	2
e2e4	3
d8b6	3
h7a3	3

I want to create a column which appends each previous string in 'moves' to a list that grows, but restarts for the next 'game_id'.

So that I create a table like so:

moves	game_id	move_list
e2e4	1	e2e4
cfc8	1	e2e4 cfc8
g6e4	1	e2e4 cfc8 g6e4
f6g8	2	f6g8
f8g5	2	f6g8 f8g5
e2e4	3	e2e4
d8b6	3	e2e4 d8b6
h7a3	3	e2e4 d8b6 h7a3

Thanks in advance for the help!

Answer 1

We may use accumulate after grouping

library(dplyr)
library(purrr)
library(stringr)
df1 %>% 
 group_by(game_id) %>%
  mutate(move_list  = accumulate(moves, str_c, sep = " ")) %>% 
  ungroup

-output

# A tibble: 8 × 3
  moves game_id move_list     
  <chr>   <int> <chr>         
1 e2e4        1 e2e4          
2 cfc8        1 e2e4 cfc8     
3 g6e4        1 e2e4 cfc8 g6e4
4 f6g8        2 f6g8          
5 f8g5        2 f6g8 f8g5     
6 e2e4        3 e2e4          
7 d8b6        3 e2e4 d8b6     
8 h7a3        3 e2e4 d8b6 h7a3

---

If the output should be a list column, use c/append instead of pasteing

df1 %>% 
  group_by(game_id) %>% 
  mutate(move_list  = accumulate(moves,  c)) %>% 
  ungroup
# A tibble: 8 × 3
  moves game_id move_list
  <chr>   <int> <list>   
1 e2e4        1 <chr [1]>
2 cfc8        1 <chr [2]>
3 g6e4        1 <chr [3]>
4 f6g8        2 <chr [1]>
5 f8g5        2 <chr [2]>
6 e2e4        3 <chr [1]>
7 d8b6        3 <chr [2]>
8 h7a3        3 <chr [3]>

data

df1 <- structure(list(moves = c("e2e4", "cfc8", "g6e4", "f6g8", "f8g5", 
"e2e4", "d8b6", "h7a3"), game_id = c(1L, 1L, 1L, 2L, 2L, 3L, 
3L, 3L)), class = "data.frame", row.names = c(NA, -8L))