How to find out elements in one vector that are not in another vector
Say you have 2 vectors,
> all_cities = c("hyderabad","delhi","mumbai","koklata","chennai")
> south_cities = c("hyderabad","chennai")
How do you find out the cities that are in the north ?
> north_cities = setdiff(all_cities,south_cities)
> north_cities
[1] "delhi" "mumbai" "koklata"
The setdiff() function is part of the set operations like union(), intersect() and setequal()
In this case, all_cities is a superset and south_cities is a subset. These could be disjoint sets as well. For example, think of all cities with
- a population > 10M
- a metro rail transport
> cities_10m = c("hyderabad","delhi","mumbai","koklata","chennai")
> cities_metro = c("hyderabad","delhi","chennai","bangalore")
# cities with metro but do not have a population of 10 M
> setdiff(cities_metro,cities_10m)
[1] "bangalore
# cities that have a population of 10M without a metro
> setdiff(all_cities,cities_metro)
[1] "mumbai" "koklata"
Now, you should be able to understand why setdiff() is a asymmetrical function
# cities that have a population of 10M with a Metro
> intersect(all_cities,cities_metro)
[1] "hyderabad" "delhi" "chennai"
# cities with either a population of 10M or with a Metro
> union(cities_metro,all_cities)
[1] "hyderabad" "delhi" "chennai"
[4] "bangalore" "mumbai" "koklata"