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

If your trying the aggregate solutions above and you get the error:

invalid type (list) for variable

Because you're using date or datetime stamps, try using as.character on the variables:

aggregate(x ~ as.character(Year) + Month, data = df, FUN = length)

On one or both of the variables.