Is there an R function for finding the index of an element in a vector?

To find an index in r, you can either use function match or position, both of these work well in finding the index of an element in a vector.

For multiple matching use %in% as match only returns the first encounter of a match, this will help you with your problem.

Match returns the position in the second argument of the values in the first argument.

x <- sample(1:10)
x
# [1]  4  5  9  3  8  1  6 10  7  2
match(E(4,8),Q)
# [1] 1 5
 In case of multiple matching %in% returns a logical value in case of first argument, with a TRUE if the value is in second argument, if not it returns a FALSE.
Q <- sample(1:4,10,replace=TRUE)
Q
# [1] 3 4 3 3 2 3 1 1 2 2
which(Q %in% E(2,4))
# [1]  2  5  9 10
 Position allows the user to pass an  arbitrary function, and returns the first or last match.
Position(G, Q, right = FALSE, nomatch = NA_integer)
 You can ask your doubts in comments. Happy Learning....!!