Split a vector into vector of vectors in clojure instead of vector of lists

Question

The clojure documentation of split-at states that it takes a collection of elements and returns a vector of two lists, each containing elements greater or smaller than a given index:

(split-at 2 [1 2 3 4 5])
[(1 2) (3 4 5)]

What I want is this:

(split-at' 2 [1 2 3 4 5])
[[1 2] [3 4 5]]

This is a collection cut into two collections that keep the order of the elements (like vectors), preferably without performance penalties.

What is the usual way to do this and are there any performance optimized ways to do it?

Answer 1

If you're working exclusively with vectors, one option would be to use subvec.

(defn split-at' [idx v]
    [(subvec v 0 idx) (subvec v idx)])

(split-at' 2 [1 2 3 4 5])
;; => [[1 2] [3 4 5]]

As regards to performance, the docs on subvec state:

This operation is O(1) and very fast, as the resulting vector shares structure with the original and no trimming is done.