Construct a variable that conditionally takes a certain value until another condition is met - r

Question

I have a panel dataset with data on conflicts for which I want to identify the post-conflict years.
So I constructed a variable myself, which codes a transition from conflict to peace with "3". Whenever the values for a new country begin, I coded that same variable with NA. S
What I want to do now is to create a new binary variable which identifies post-conflict years with a 1 and conflict years and never conflict with 0. For that I would have to assign every year, following a 3 in the transition variable with a 1 until there is an NA in the same column. As follows:
Country Year transition post-conflict
Afghanistan 1994 0 0
Afghanistan 1995 0 0
Afghanistan 1996 3 1
Afghanistan 1997 2 1
Afghanistan 1998 2 1
Albania 1994 NA 0
Albania 1994 2 0
How could I go about this?

Answer 1

You probably shouldn't use NA like that. It prevents functions like which, sum, and cumsum from working as you may want them to. You likely don't need to mark the first row of a new country anyway, since most R functions you would use for your analysis can group by Country without needing a special marker showing where each group starts.
Below I change NA to something different, and make transition a factor. Then you can use cumsum to create your new column.
library(data.table)
setDT(df) # assuming your data is called df
# fix transition column
df[is.na(transition), transition := 90]
df[, transition := as.factor(transition)]
# create post_conflict column
df[, post_conflict := cumsum(transition == 3), by = Country]
# Country Year transition post_conflict
# 1: Afghanistan 1994 0 0
# 2: Afghanistan 1995 0 0
# 3: Afghanistan 1996 3 1
# 4: Afghanistan 1997 2 1
# 5: Afghanistan 1998 2 1
# 6: Albania 1994 90 0
# 7: Albania 1994 2 0