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

There are plenty of wonderful answers here already, but I wanted to throw in 1 more option for those wanting to add a new column to the original dataset that contains the number of times that row is repeated.

df1$counts <- sapply(X = paste(df1$Year, df1$Month), 
                     FUN = function(x) { sum(paste(df1$Year, df1$Month) == x) })

The same could be accomplished by combining any of the above answers with the merge() function.