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

An old question without a data.table solution. So here goes...

Using .N

library(data.table)
DT <- data.table(df)
DT[, .N, by = list(year, month)]

Answer 2

An alternative to the aggregate() function in this case would be table() with as.data.frame(), which would also indicate which combinations of Year and Month are associated with zero occurrences

df<-data.frame(x=rep(1:6,rep(c(1,2,3),2)),year=1993:2004,month=c(1,1:11))

myAns<-as.data.frame(table(df[,c("year","month")]))

And without the zero-occurring combinations

myAns[which(myAns$Freq>0),]