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

Considering @Ben answer, R would throw an error if df1 does not contain x column. But it can be solved elegantly with paste:

aggregate(paste(Year, Month) ~ Year + Month, data = df1, FUN = NROW)

Similarly, it can be generalized if more than two variables are used in grouping:

aggregate(paste(Year, Month, Day) ~ Year + Month + Day, data = df1, FUN = NROW)