Count number of rows within each group

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: ``` df2 <- aggregate(x ~ Year + Month, data = df1, sum) ``` Now, I would like to count observations but can't seem to find the proper argument for `FUN`. Intuitively, I thought it would be as follows: ``` df2 <- aggregate(x ~ Year + Month, data = df1, count) ``` But, no such luck. Any ideas? Some toy data: ``` set.seed(2) df1 <- data.frame(x = 1:20, Year = sample(2012:2014, 20, replace = TRUE), Month = sample(month.abb[1:3], 20, replace = TRUE)) ```