R: how do I remove from a vector terms that are in another vector?

Question

In a toy problem of mine, I have a vector a made of integers and I want to efficiently remove from a terms that are also in a vector b . I wrote the code

newa=NULL
for (j in 1:length(a))
   if (min(abs(a[i]-b))>0) newa=c(newa,a[i])

but this is terrible...

Answer 1

You could just use intersect , setdiff , etc (see ?setdiff ):

a <- 1:10
b <- c(2, 3, 5, 7)

setdiff(a, b)
# [1]  1  4  6  8  9 10

Or even just use %in% :

a[!(a %in% b)] # (a %in% b) is TRUE in index i if a[i] is in b.

Answer 2

It's lightening fingers like mathematical.coffee that make it so I never get to answer questions :P

I do this a lot using %in%. And I nabbed a great little bit of code from Stephen Turner that makes it even easier!

## Returns a logical vector TRUE for elements of X not in Y
"%nin%" <- function(x, y) !(x %in% y)