R | Adding index numbers - r

Question

I have two dataset which look like below
Sales
Region ReviewYear Sales Index
South Asia 2006 1.5 NA
South Asia 2009 4.5 NA
South Asia 2011 11 0
South Asia 2014 16.7 NA
Africa 2008 0.4 NA
Africa 2013 3.5 0
Africa 2017 9.7 NA
Strategy
Region StrategyYear
South Asia 2011
Africa 2013
Japan 2007
SE Asia 2009
There are multiple regions and many review years which are not periodic and not even same for all regions. I have added a column 'Index' to dataframe 'Sales' such that for a strategy year from second dataframe, the index value is zero. I now want to change NA to a series of numbers that tell how many rows before or after that particular row is to 0 row, grouped by 'Region'.
I can do this using a for loop but that is just tedious and checking if there is a cleaner way to do this. Final output should look like
Sales
Region ReviewYear Sales Index
South Asia 2006 1.5 -2
South Asia 2009 4.5 -1
South Asia 2011 11 0
South Asia 2014 16.7 1
Africa 2008 0.4 -1
Africa 2013 3.5 0
Africa 2017 9.7 1

Answer 1

Join the two datasets by Region and for each Region create an Index column by subtracting the row number with the index where StrategyYear matches the ReviewYear.
library(dplyr)
left_join(Sales, Strategy, by = 'Region') %>%
arrange(Region, StrategyYear) %>%
group_by(Region) %>%
mutate(Index = row_number() - match(first(StrategyYear), ReviewYear))
# Region ReviewYear Sales Index StrategyYear
# <chr> <int> <dbl> <int> <int>
#1 Africa 2008 0.4 -1 2013
#2 Africa 2013 3.5 0 2013
#3 Africa 2017 9.7 1 2013
#4 SouthAsia 2006 1.5 -2 2011
#5 SouthAsia 2009 4.5 -1 2011
#6 SouthAsia 2011 11 0 2011
#7 SouthAsia 2014 16.7 1 2011
data
Sales <- structure(list(Region = c("SouthAsia", "SouthAsia", "SouthAsia",
"SouthAsia", "Africa", "Africa", "Africa"), ReviewYear = c(2006L,
2009L, 2011L, 2014L, 2008L, 2013L, 2017L), Sales = c(1.5, 4.5,
11, 16.7, 0.4, 3.5, 9.7), Index = c(NA, NA, 0L, NA, NA, 0L, NA
)), class = "data.frame", row.names = c(NA, -7L))
Strategy <- structure(list(Region = c("SouthAsia", "Africa", "Japan", "SEAsia"
), StrategyYear = c(2011L, 2013L, 2007L, 2009L)), class = "data.frame",
row.names = c(NA, -4L))