dplyr count number of one specific value of variable

Question

Say I have a dataset like this:

id <- c(1, 1, 2, 2, 3, 3)
code <- c("a", "b", "a", "a", "b", "b")
dat <- data.frame(id, code)

I.e.,

    id  code
1   1   a
2   1   b 
3   2   a
4   2   a
5   3   b
6   3   b

Using dplyr, how would I get a count of how many a's there are for each id

i.e.,

   id  countA
1   1   1
2   2   2
3   3   0

I'm trying stuff like this which isn't working,

countA<- dat %>%
group_by(id) %>%
summarise(cip.completed= count(code == "a"))

The above gives me an error, "Error: no applicable method for 'group_by_' applied to an object of class "logical""

Thanks for your help!

Answer 1

Try the following instead:

library(dplyr)
dat %>% group_by(id) %>%
  summarise(cip.completed= sum(code == "a"))

Source: local data frame [3 x 2]
    id cip.completed
  (dbl)         (int)
1     1             1
2     2             2
3     3             0

This works because the logical condition code == a is just a series of zeros and ones, and the sum of this series is the number of occurences.

Note that you would not necessarily use dplyr::count inside summarise anyway, as it is a wrapper for summarise calling either n() or sum() itself. See ?dplyr::count. If you really want to use count, I guess you could do that by first filtering the dataset to only retain all rows in which code==a, and using count would then give you all strictly positive (i.e. non-zero) counts. For instance,

dat %>% filter(code==a) %>% count(id)

Source: local data frame [2 x 2]

     id     n
  (dbl) (int)
1     1     1
2     2     2