Count number of rows within each group

Question

I have a dataframe and I would like to count the number of rows within each group. I reguarly use the aggregate function to sum data as follows:

Answer 1

An old question without a data.table solution. So here goes...

Using .N

library(data.table)
DT <- data.table(df)
DT[, .N, by = list(year, month)]