I just stumbled across a really useful infix function in R: %in%. It compares two vectors and returs a logical vector if there is a match or not for its left operand. Let us look at some examples:

> 1:10 %in% c(1,3,5,9)
   [1]  TRUE FALSE  TRUE FALSE  TRUE FALSE FALSE FALSE  TRUE FALSE

\(x\) without \(y\):

> "%w/o%" <- function(x, y) x[!x %in% y]
> (1:10) %w/o% c(3,7,12)
  [1]  1  2  4  5  6  8  9 10

In my particular use-case, I wanted to implement sampling without replacement in a loop, i.e. removing the sampled values of the previous iteration:

subsample <- sample(x=observations, size=sample_size)
...
observations <- observations[!observations %in% subsample]