Basic question about minus sign for negative operator

Question

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)?

Answer 1

So the - sign in x[-y] is understood as items of x that are not y.

No, y is used as indices of x here. You can think of it like this:

setNames(x, paste0('x', seq_along(x)))
#  x1  x2  x3  x4  x5  x6  x7  x8  x9 x10 
# 10   9   8   7   6   5   4   3   2   1 

x[-c(4, 7)]  ## delete x4 and x7
# [1] 10  9  8  6  5  3  2  1

If you want elements in x that are not in y you need to say:

x[!x %in% y]
# [1] 10  9  8  6  5  3  2  1

y1 <- NULL
x[!x %in% y1]
# [1] 10  9  8  7  6  5  4  3  2  1

Or:

setdiff(x, y)
# [1] 10  9  8  6  5  3  2  1

setdiff(x, y1)
# [1] 10  9  8  7  6  5  4  3  2  1

Accordingly, in this one we delete all element indices from 1:10, and since we have only 10 elements in x we get the empty vector:

x[-(1:10)]
# numeric(0)

For the last one, we may read in documetation help("Extract"), that "an empty index selects all values: this is most often used to replace all the entries but keep the attributes."

x[-integer()]
# integer(0)

How to delete nothing

If we want to delete nothing, we need an index that is higher than the number of elements in x,

x[-11]
# [1] 10  9  8  7  6  5  4  3  2  1

where we could do dynamically:

x[-.Machine$integer.max]
# [1] 10  9  8  7  6  5  4  3  2  1