R - cumulative sum by condition

Question

So I have a dataset which simplified looks something like this:

Year    ID     Sum
2009    999    100
2009    123     85
2009    666    100
2009    999    10         


        

Answer 1

You can use dplyr, and the base function cumsum:

require(dplyr)    

dataset %>% 
  group_by(Year, ID) %>%
  mutate(cumsum = cumsum(Sum)) %>%
  ungroup()