Assigning values in a sequence depending on previous row in R

Question

I have a data table like this.

  ID1 ID2 member
1   a   x parent
2   a   y  child
3   a   z parent
4   a   p  child
5   a   q  child
6   b   x parent
7   b   z parent
8   b   q  child

And I want to assign a sequence like below.

  ID1 ID2 member sequence
1   a   x parent        1
2   a   y  child        2
3   a   z parent        1
4   a   p  child        2
5   a   q  child        3
6   b   x parent        1
7   b   z parent        1
8   b   q  child        2

i.e.

> dt$sequence = 1, wherever dt$member == "parent"

> dt$sequence = previous_row_value + 1, wherever dt$member=="child"

As of now I have been doing it using loops, like below.

dt_sequence <- dt[ ,sequencing(.SD), by="ID1"]

sequencing <- function(dt){
  for(i in 1:nrow(dt)){
    if(i == 1){
      dt$sequence[i] = 1
      next
    }
    if(dt[i,member] %in% "child"){
      dt$sequence[i] = as.numeric(dt$sequence[i-1]) + 1
    }
    else
      dt$sequence[i] = 1
  }
  return(dt)
}

I ran this code on a data table of 400 000 rows and it took a lot of time to complete (around 15 mins). Can anyone suggest a faster way to do it.

Answer 1

Here's one way with seq:

dt[ , sequence := seq(.N), by = cumsum(member == "parent")]

#    ID1 ID2 member sequence
# 1:   a   x parent        1
# 2:   a   y  child        2
# 3:   a   z parent        1
# 4:   a   p  child        2
# 5:   a   q  child        3
# 6:   b   x parent        1
# 7:   b   z parent        1
# 8:   b   q  child        2

How it works?

The command member == "parent" creates a logical vector. The function cumsum is used to calculate the cumulative sum. In this case, it creates vector in which a parent and the following childs have the same number. This vector is used for grouping. Finally, seq(.N) creates a sequence from 1 up to the number of elements in the group.