Count occurrences of factor in R, with zero counts reported

Question

I want to count the number of occurrences of a factor in a data frame. For example, to count the number of events of a given type in the code below:

library(plyr         


        

Answer 1

Quite similar to @DWin's answer:

> aggregate(quantity~type, events, FUN=sum)
  type quantity
1    A        3
2    B        1
3    C        0