plyr - Count occurrences of factor in R, with zero counts reported -

i want count number of occurrences of factor in data frame. example, count number of events of given type in code below:

library(plyr) events <- data.frame(type = c('a', 'a', 'b'),                        quantity = c(1, 2, 1)) ddply(events, .(type), summarise, quantity = sum(quantity)) 

the output following:

     type quantity 1           3 2    b        1 

however, if know there 3 types of events a, b , c, , want see count c 0? in other words, want output be:

     type quantity 1           3 2    b        1 3    c        0 

how do this? feels there should function defined somewhere.

the following 2 not-so-good ideas how go this.

idea #1: know using for loop, know said if using for loop in r, doing wrong, there must better way it.

idea #2: add dummy entries original data frame. solution works feels there should more elegant solution.

events <- data.frame(type = c('a', 'a', 'b'),                        quantity = c(1, 2, 1)) events <- rbind(events, data.frame(type = 'c', quantity = 0)) ddply(events, .(type), summarise, quantity = sum(quantity)) 

you free if define events variable correctly factor desired 3 levels:

r> events <- data.frame(type = factor(c('a', 'a', 'b'), c('a','b','c')),  +                       quantity = c(1, 2, 1)) r> events   type quantity 1           1 2           2 3    b        1 r> table(events$type)  b c  2 1 0  r>  

simply calling table() on factor right thing, , ddply() can if tell not drop:

r> ddply(events, .(type), summarise, quantity = sum(quantity), .drop=false)   type quantity 1           3 2    b        1 3    c        0 r>