Question

I have a vector like: a = c(1:10) and I need to remove multiple values, like: 2, 3, 5

How to delete those numbers (they are NOT the positions in the vector) in the vector?

at the moment i loop the vector and do something like:

a[!a=NUMBER_TO_REMOVE]

But I think there is a function that does it automatically.

Answer 1

The %in% operator tells you which elements are among the numers to remove:

> a <- sample (1 : 10)
> remove <- c (2, 3, 5)
> a
 [1] 10  5  2  7  1  6  3  4  8  9
> a %in% remove
 [1] FALSE  TRUE  TRUE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE
> a [! a %in% remove]
 [1] 10  7  1  6  4  8  9

Note that this will silently remove incomparables (stuff like NA or Inf) as well (while it will keep duplicate values in a as long as they are not listed in remove).

• If a can contain incomparables, but remove will not, we can use match, telling it to return 0 for non-matches and incomparables (%in% is a conventient shortcut for match):

  > a <- c (a, NA, Inf)
  > a
   [1]  10   5   2   7   1   6   3   4   8   9  NA Inf
  > match (a, remove, nomatch = 0L, incomparables = 0L)
   [1] 0 3 1 0 0 0 2 0 0 0 0 0
  > a [match (a, remove, nomatch = 0L, incomparables = 0L) == 0L]
  [1]  10   7   1   6   4   8   9  NA Inf
  

  incomparables = 0 is not needed as incomparables will anyways not match, but I’d include it for the sake of readability.
  This is, btw., what setdiff does internally (but without the unique to throw away duplicates in a which are not in remove).

• If remove contains incomparables, you’ll have to check for them individually, e.g.

  if (any (is.na (remove))) 
    a <- a [! is.na (a)]
  

  (This does not distinguish NA from NaN but the R manual anyways warns that one should not rely on having a difference between them)

  For Inf/ -Inf you’ll have to check both sign and is.finite