# outersect(): The opposite of R’s intersect() function

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**The Objective**

To find the non-duplicated elements between two or more vectors (i.e. the ‘yellow sections of the diagram above)

**The Problem**

I needed the opposite of R’s *intersect()* function, an “*outersect()*“. The closest I found was *setdiff()* but the order of the input vectors produces different results, e.g.

x = letters[1:3] #[1] "a" "b" "c" y = letters[2:4] #[1] "b" "c" "d" # The desired result is # [1] "a" "d" setdiff(x, y) #[1] "a" setdiff(y, x) #[1] "d"

*setdiff() *produces all elements of the first input vector without any matching elements from the second input vector (i.e. is asymmetric). Not quite what I’m after. I’m looking for the ‘yellow’ set of elements as in the picture at the top of the page.

**The Solution**

Concatenating the results of *setdiff()* with input vectors in both combinations works a treat:

outersect <- function(x, y) { sort(c(setdiff(x, y), setdiff(y, x))) } x = letters[1:3] #[1] "a" "b" "c" y = letters[2:4] #[1] "b" "c" "d" outersect(x, y) #[1] "a" "d" outersect(y, x) #[1] "a" "d"

**Alternative solution**

An equivalent alternative would be to use

outersect <- function(x, y) { sort(c(x[!x%in%y], y[!y%in%x])) }

but by using *setdiff()* in the first solution it makes it easier to read I think.

**Further Development**

It would be nice to extend this to a variable number of input vectors. This final task turns out to be rather simple:

outersect <- function(x, y, ...) { big.vec <- c(x, y, ...) duplicates <- big.vec[duplicated(big.vec)] setdiff(big.vec, unique(duplicates)) } # desired result is c(1, 2, 3, 6, 9, 10) outersect(1:5, 4:8, 7:10) #[1] 1 2 3 6 9 10

Awesome.

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