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

Following @Joshua's suggestion, here's one way you might count the number of observations in your df dataframe where Year = 2007 and Month = Nov (assuming they are columns):

nrow(df[,df$YEAR == 2007 & df$Month == "Nov"])

and with aggregate, following @GregSnow:

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