Consider the following dplyr query
> mpg %>% group_by(class) %>% summarise(n())
The output is
# A tibble: 7 x 2
class n()
<chr> <int>
1 2seater 5
2 compact 47
3 midsize 41
4 minivan 11
5 pickup 33
6 subcompact 35
7 suv 62
Now, I want to filter the result as follows:
> mpg %>% group_by(class) %>% filter(hwy==21) %>% summarise(n())
That is, I want to show the number of car classes having a highway mileage 21. Here is the result:
# A tibble: 2 x 2
class n()
<chr> <int>
1 minivan 1
2 subcompact 1
This is the expected result, but what I want to see instead is all the classes again, and in case a class does not have a car with a highway mileage of 21, then n() should be reported as 0. How can I do this?
In other words, I want the dplyr query that shows the following output:
# A tibble: 7 x 2
class n()
<chr> <int>
1 2seater 0
2 compact 0
3 midsize 0
4 minivan 1
5 pickup 0
6 subcompact 1
7 suv 0
where n() is the number car classes with a highway mileage of 21.
Is this possible?
