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

Create a new variable Count with a value of 1 for each row:

df1["Count"] <-1

Then aggregate dataframe, summing by the Count column:

df2 <- aggregate(df1[c("Count")], by=list(Year=df1$Year, Month=df1$Month), FUN=sum, na.rm=TRUE)