Count occurrences of factor in R, with zero counts reported

plyr, r

Solution

You get this for free if you define your `events` variable correctly as a factor with the desired three 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    A        1
2    A        2
3    B        1
R> table(events$type)

A B C 
2 1 0 
R>

Simply calling `table()` on the factor already does the right thing, and `ddply()` can too if you tell it not to `drop`:

R> ddply(events, .(type), summarise, quantity = sum(quantity), .drop=FALSE)
  type quantity
1    A        3
2    B        1
3    C        0
R>

Problem

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) events <- data.frame(type = c('A', 'A', 'B'), quantity = c(1, 2, 1)) ddply(events, .(type), summarise, quantity = sum(quantity)) ``` The output is the following: ``` type quantity 1 A 3 2 B 1 ``` However, what if I know that there are three types of events `A`, `B` and `C`, and I also want to see the count for `C` which is `0`? In other words, I want the output to be: ``` type quantity 1 A 3 2 B 1 3 C 0 ``` How do I do this? It feels like there should be a function defined to do this somewhere. The following are my two not-so-good ideas about how to go about this. Idea #1: I know I could do this by using a `for` loop, but I know that it is widely said that if you are using a `for` loop in `R`, then you are doing something wrong, there must be a better way to do it. Idea #2: Add dummy entries to the original data frame. This solution works but it feels like there should be a 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)) ```

Original source