There are two kinds of set difference: the usual set difference
<https://en.wikipedia.org/wiki/Set_difference#Relative_complement>, also
called the asymmetric difference or relative complement, and the symmetric
difference <https://en.wikipedia.org/wiki/Symmetric_difference>.

In R, setdiff(a,b) is the usual asymmetric set difference, that is, all the
elements of a which are not elements of b. It is the set equivalent of the
logical expression (a and not b).

The symmetric difference consists of all the elements which appear in
either a or b, but not both. It is the set equivalent of logical xor.

I don't believe there is a standard R function for the symmetric
difference, but you can calculate it as

         symdiff <- function (a,b) setdiff(union(a,b),intersect(a,b))
or
         symdiff <- function(a,b) unique( c( a[is.na(match(a,b))],
b[is.na(match(b,a))]
) )

Challenge: is there a shorter or more efficient way to calculate symdiff
using R primitives?

           -s


On Mon, Jun 2, 2014 at 2:57 AM, Raphael Päbst <raphael.paebst@gmail.com>
wrote:

> Hello everyone, I have a question which is probably rooted in my lack
> of understanding when it comes to math.
>
> I just did the following:
>
> v <- c(1:20)
> w <- c(11:30)
> setdiff(v, w)
>
> and got:
> 1 2 3 4 5 6 7 8 9 10
>
> Then I did the following:
> setdiff(w, v)
> and got, not surprisingly:
> 21 22 23 24 25 26 27 28 29 30
>
> Now I was originally expecting to get bot with the first call of
> setdiff(v, w) and couldn't find any reason not to expect this from
> ?setdiff()
>
> Am I missing somethin vital here or does setdiff() always give me the
> elements of the first set that are not in the second one and not those
> which are exclusive to either one, just dropping the ones in the
> intersection of both sets?
>
> Many Thanks in advance
>
> Raphael
>
> ______________________________________________
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> PLEASE do read the posting guide
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>

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