I am not sure I understand the behaviour of the minus sign.
x <- 10:1
y <- c(4, 7)
x[y]
# [1] 7 4
x[-y]
# [1] 10 9 8 6 5 3 2 1
So the - sign in x[-y] is understood as items of x that are not y.
x <- 10:1
y <- 1:10
x[-y]
# integer(0)
Again logical, if y has all the element of x, x[-y] should be empty.
Now if y is itself empty, you'd expect that x[-y] should list all the elements of x.
However:
x <- 10:1
y <- integer()
x[-y]
# integer(0)
What is it that I am not understanding?
Is there any other operator to do what I want (i.e. list the elements of x that are not in y, and thus list all elements of x is y is empty)?
@jay.sf's answer explains how to get what you want. I just wanted to add a bit more explanation, and give a suggestion for a different approach.
When you say x[-y], the minus sign doesn't have any special meaning: it just changes the sign of the elements of y. It will give the same result as
z <- -y
x[z]
When y is empty, z will be empty as well, so x[y] is the same as x[-y].
My recommendation is to use logical indices when you want to do logical operations on them. So you might have been doing this:
y <- which(x %% 50 == 11)
to choose the entries which meet the condition x %% 50 == 11. Then if you
happen to have a vector where nothing meets the condition, you get an empty y. A better approach is
y <- x %% 50 == 11
so that y is a vector of all FALSE values. If you want the complement, use
x[!y]
and you'll get all of x.