Replace sets of rows in a tibble with a single row - r

Question

There is a similar question for data.table at Replace sets of rows in a data.table with a single row but I am looking for an equivalent solution in tidyverse. So, I have a tibble like:
DT <- tibble (
id=c(1,1,1,1,2,2,2,2),
location = c("a","b","c","d","a","b","d","e"),
seq = c(1,2,3,4,1,2,3,4))
For every id, I want to look for the sequence b,c,d and if there is such a thing, I want to replace the rows with b and c with a single row, let's say z. The values for the other variables should retain the values of the previous b (in this case id and seq)
So in this case, the new tibble should be
DT.Tobe <- tibble (
id=c(1,1,1,2,2,2,2),
place = c("a","z","d","a","b","d","e"),
seq = c(1,2,4,1,2,3,4))
I was not able to find even a starting point for this...

Answer 1

library(dplyr)
# library(zoo) # rollapply
DT %>%
group_by(id) %>%
mutate(
isseq = zoo::rollapply(location, 3, FUN = function(z) identical(z, c("b", "c", "d")), align = "left", partial = TRUE),
isseq = isseq | lag(isseq, default = FALSE)
) %>%
group_by(id, isseq) %>%
summarize(
across(everything(), ~ {
if (cur_group()$isseq) {
if (cur_column() == "location") "z" else first(.)
} else .
})
) %>%
ungroup() %>%
select(-isseq)
# # A tibble: 7 x 3
# id location seq
# <dbl> <chr> <dbl>
# 1 1 a 1
# 2 1 d 4
# 3 1 z 2
# 4 2 a 1
# 5 2 b 2
# 6 2 d 3
# 7 2 e 4
The order is changed because the group_by(isseq) tends to keep "like" together. This should be easy to either re-order (assuming "seq" is meaningful) or pre-add an order variable and using it later.
If it is possible for a single id to have multiple of such sequences (if so, say something), then run-length encoding will be needed here as well (to differentiate between different b-c-d sequences in the same id).

Answer 2

One possible option would be to use a for-loop. Here is my pseudocode.
for (i in nrows(DT)){ # Repeat this if statement for each row in your DT
if (place[i] == "b & place[i+1] == "c"){ # if the first item is B and the second item is C
DT <- DT %>%
dplyr::replace(place[i] == "z") # Replaces item B with the z character
DT[-(i+1)] # Deletes item C's row
}
}
The Dplyr cheat sheet has some useful functions that may help with finding the right tools for the if-statement part of this pseudocode.
What are your thoughts?