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

November 29, 2011
By

(This article was first published on Consistently Infrequent » R, and kindly contributed to R-bloggers)

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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