Matching up two vectors in R

Question

I have two vectors in R as shown below. The first one represents amino acid numbers with some positions missing while the second one represents the full list. I need to some

Answer 1

to return the indices use %in%

> (a <- letters[1:5])
[1] "a" "b" "c" "d" "e"
> b <- c("a", "b", "e", "f")
> (which(a%in%b))
[1] 1 2 5
> (which(b%in%a))
[1] 1 2 3