Finding elements that do not overlap between two vectors

Question

I'm trying to identify elements which are not included in the other vector. For instance in two vectors I have

list.a <- c("James", "Mary", "Jack", "Sonia", "Michelle", "Vincent")

list.b <- c("James", "Sonia", "Vincent")

is there a way to verify which people do not overlap? In the example, I would want to get the vector result that contains Mary, Jack, and Michelle.

Any suggestions will help!

Answer 1

Yes, there is a way:

setdiff(list.a, list.b)
# [1] "Mary"     "Jack"     "Michelle"

Answer 2

I think it should be mentioned that the accpeted answer is is only partially correct. The command setdiff(list.a, list.b) finds the non-overlapping elements only if these elements are contained in the object that is used as the first argument!.

If you are not aware of this behaviour and did setdiff(list.b, list.a) instead, the results would be character(0) in this case which would lead you to conclude that there are no non-overlapping elements.

Using a slightly extended example for illustration, an obvious quick fix is:

list.a <- c("James", "Mary", "Jack", "Sonia", "Michelle", "Vincent")
list.b <- c("James", "Sonia", "Vincent", "Iris")

c(setdiff(list.b, list.a), setdiff(list.a, list.b))
# [1] "Iris"     "Mary"     "Jack"     "Michelle" 

Answer 3

An extended answer based on the comments from Hadley and myself: here's how to allow for duplicates.

Final Edit: I do not recommend anyone use this, because the result may not be what you expect. If there is a repeated value in x which is not in y, you will see that value repeated in the output. But: if, say, there are four 9s in x and one 9 in y, all the 9s will be removed. One might expect to retain three of them; that takes messier code.

mysetdiff<-function (x, y, multiple=FALSE) 
{
    x <- as.vector(x)
    y <- as.vector(y)
    if (length(x) || length(y)) {
        if (!multiple) {
             unique( x[match(x, y, 0L) == 0L])  
              }else  x[match(x, y, 0L) == 0L] 
        } else x
}

Rgames> x
[1]  8  9  6 10  9
Rgames> y
[1] 5 3 8 8 1
Rgames> setdiff(x,y)
[1]  9  6 10
Rgames> mysetdiff(x,y)
[1]  9  6 10
Rgames> mysetdiff(x,y,mult=T)
[1]  9  6 10  9
Rgames> mysetdiff(y,x,mult=T)
[1] 5 3 1
Rgames> setdiff(y,x)
[1] 5 3 1