Question

Here is the function I’m trying to run…

(defn mongean [cards times]
  (let [_cards (transient cards)]
    (loop [i 0 c (get cards i) _count (count cards) _current (/ _count 2)]
      (assoc! _cards _current c)
      (if ((rem i 2) = 0)
        (def _newcur (- _current (inc i)))
        (def _newcur (+ _current (inc i))))
      (if (<= i _count)
        (recur (inc i) (get cards i) _count _newcur )))
    (persistent! _cards)))

It’s resulting in this Exception…

Exception in thread "main" java.lang.ClassCastException: clojure.lang.PersistentHashSet$TransientHashSet cannot be cast to clojure.lang.ITransientAssociative

Being new to clojure, I’d also appreciate any constructive criticism of my approach above. The goal is to take a List, and return a re-ordered list.

Answer 1

I assume that you are trying to implement the Mongean shuffle. Your approach is very imperative and you should try to use a more functional approach.

This would be a possible implementation, were we calculate the final order of the cards (as per Wikipedia formula) and then we use the built-in replace function to do the mapping:

 (defn mongean [cards]
   (let [num-cards (count cards)
         final-order (concat (reverse (range 1 num-cards 2)) (range 0 num-cards 2))]
      (replace cards final-order)))

  user> (mongean [1 2 3 4 5 6 7 8])
  (8 6 4 2 1 3 5 7)