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           


        

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.