Create a conditional timeline based on events in R

Question

I have data where the 'Law' variable indicates changes in legislation, in different places ('Place'):

Person  Place  Year  Law
1         A    1990   0
2         A    1991   1
3         A    1992   1
4         B    1990   0
5         B    1991   0
6         B    1992   1
7         B    1993   1
8         B    1993   1
9         B    1993   1
10        B    1992   1

Basically the law was implemented in place A in 1991 and remained in force for all subsequent time periods. It was implemented in place B in 1992 and remained in force, & so on.

I would like to create a new variable that takes on a value of 0 for the year the law was implemented, 1 for 1 year after, 2 for 2 years after, -1 for the year before, -2 for 2 years before, and so on.

I need the final dataframe to look like:

Person  Place  Year  Law  timeline 
1         A    1990   0     -1
2         A    1991   1      0
3         A    1992   1      1
4         B    1990   0     -2
5         B    1991   0     -1
6         B    1992   1      0
7         B    1993   1      1
8         B    1993   1      2
9         B    1993   1      2
10        B    1992   1      1

I have tried:

library(dplyr)
df %>% 
 group_by(Place) %>%
 arrange(Year) %>%
 mutate(timeline = rank(Law))

but it's not working like I need. What am I doing wrong? Can I do this in dplyr or do I need to create a complex for loop?

Answer 1

You can subtract the row_numer by the index where the Law is implemented:

df %>% 
    arrange(Year) %>% 
    group_by(Place) %>% 
    mutate(timeline = row_number() - which(diff(Law) == 1) - 1) %>% 
    arrange(Place)

# A tibble: 7 x 5
# Groups:   Place [2]
#  Person Place  Year   Law timeline
#   <int> <fct> <int> <int>    <dbl>
#1      1 A      1990     0      -1.
#2      2 A      1991     1       0.
#3      3 A      1992     1       1.
#4      4 B      1990     0      -2.
#5      5 B      1991     0      -1.
#6      6 B      1992     1       0.
#7      7 B      1993     1       1.