Subsetting on all but empty grep returns empty vector

Question

Suppose I have some character vector, which I'd like to subset to elements that don't match some regular expression. I might use the - operator to remove the subset that grep matches:

> vec <- letters[1:5]
> vec
[1] "a" "b" "c" "d" "e"
> vec[-grep("d", vec)]
[1] "a" "b" "c" "e"

I'm given back everything except the entries that matched "d". But if I search for a regular expression that isn't found, instead of getting everything back as I would expect, I get nothing back:

> vec[-grep("z", vec)]
character(0)

Why does this happen?

Answer 1

It's because grep returns an integer vector, and when there's no match, it returns integer(0).

> grep("d", vec)
[1] 4
> grep("z", vec)
integer(0)

and the since the - operator works elementwise, and integer(0) has no elements, the negation doesn't change the integer vector:

> -integer(0)
integer(0)

so vec[-grep("z", vec)] evaluates to vec[-integer(0)] which in turn evaluates to vec[integer(0)], which is character(0).

You will get the behavior you expect with invert = TRUE:

> vec[grep("d", vec, invert = TRUE)]
[1] "a" "b" "c" "e"
> vec[grep("z", vec, invert = TRUE)]
[1] "a" "b" "c" "d" "e"