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

Another way

1) use ddply to sum a variable by group (similar to SQL group by)

X <- ddply ( dataset, .(Year,ID), sum)

2) merge the result with dataset

Y <- merge( dataset, X, by=('Year','ID')