Calculate average monthly total by groups from data.table in R

Question

I have a data.table with a row for each day over a 30 year period with a number of different variable columns. The reason for using data.table is that the .csv file I\'m usi

Answer 1

If you're not looking for complicated functions and just want the mean, then the following should suffice:

DT[, sum(Runoff) / length(unique(year(Date))), list(Key, month(Date))]
#   Key month       V1
#1:   A     1 4.366667
#2:   B     1 3.266667