dplyr + group_by and avoid alphabetical sorting

Question

I have the following data:

    data <- structure(list(user = c(1234L, 1234L, 1234L, 1234L, 1234L, 1234L, 
1234L, 1234L, 1234L, 1234L, 1234L, 4758L, 4758L, 9584L, 9584L, 
9584L, 9584L, 9584L, 9584L), time = c(1L, 2L, 3L, 4L, 5L, 6L, 
7L, 8L, 9L, 10L, 11L, 5L, 6L, 1L, 2L, 3L, 4L, 5L, 6L), fruit = structure(c(1L, 
6L, 1L, 1L, 6L, 5L, 5L, 3L, 4L, 1L, 2L, 4L, 2L, 1L, 6L, 5L, 5L, 
3L, 2L), .Label = c("apple", "banana", "lemon", "lime", "orange", 
"pear"), class = "factor"), count = c(1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L), cum_sum = c(1L, 
2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L, 10L, 11L, 1L, 2L, 1L, 2L, 3L, 
4L, 5L, 6L)), .Names = c("user", "time", "fruit", "count", "cum_sum"
), row.names = c(NA, -19L), class = "data.frame")

For every user in this set, I want to look at the sequence of fruits over time. But, some fruits are listed "back to back" in time.

   user time  fruit count cum_sum
1  1234    1  apple     1  1
2  1234    2   pear     1  2
3  1234    3  apple     1  3
4  1234    4  apple     1  4
5  1234    5   pear     1  5
6  1234    6 orange     1  6
7  1234    7 orange     1  7

What I'm looking for is more of a time-series by user by unique fruit.

Problem is, if I group by user and fruit then summarise, dplyr automatically sorts fruit alphabetically:

data %>% 
    group_by(user, fruit) %>%
    summarise(temp_var=1) %>%
    mutate(cum_sum = cumsum(temp_var))

What I really want is, for user 1234 above (for example) for the fruits to be listed in order of time series, but removing any duplicates. So where we see apple > pear > apple > apple > pear > orange > orange, we'd instead only see apple > pear > apple > pear > orange

Answer 1

So using rleid function from the latest data.table version on CRAN we can simply do (though not sure regarding your exact desired output)

library(data.table) ## v >= 1.9.6
res <- setDT(data)[, .(fruit = fruit[1L]), by = .(user, indx = rleid(fruit))
                    ][, cum_sum := seq_len(.N), by = user
                      ][, indx := NULL]
res
#     user  fruit cum_sum
#  1: 1234  apple       1
#  2: 1234   pear       2
#  3: 1234  apple       3
#  4: 1234   pear       4
#  5: 1234 orange       5
#  6: 1234  lemon       6
#  7: 1234   lime       7
#  8: 1234  apple       8
#  9: 1234 banana       9
# 10: 4758   lime       1
# 11: 4758 banana       2
# 12: 9584  apple       1
# 13: 9584   pear       2
# 14: 9584 orange       3
# 15: 9584  lemon       4
# 16: 9584 banana       5