How to transpose a nested vector in clojure

Question

I have the following variable

 (def a [[1 2] [3 4] [5 6]])

and want to return

[[1 3 5][2 4 6]]

and if input is [[1 2] [3 4] [5 6] [7 8 9]] then the required result is

[[1 3 5 7] [2 4 6 8] [9]]

How to do it in clojure?

Answer 1

I like ifett's implementation, though it seems weird to use reduce-kv to build a vector that could be easily build with map/mapv.

So, here is how I would've done it:

(defn transpose [v]
  (mapv (fn [ind]
          (mapv #(get % ind)
                (filter #(contains? % ind) v)))
        (->> (map count v)
             (apply max)
             range)))