99 Clojure Problems (11 – 14)

So thanks to two great comments (Jake McCrary on let/loop and Zac on the & symbol), I’ve been refactoring my solutions a little. I’m only going to update my previous posts with my first revelation on the loop, as I figure the bad example of what I was doing before probably outweighs being able to see how I’m learning clojure.

So anyway, once I grokked the (loop [] expr) syntax, things became worlds easier. I don’t know if it’s because I’m getting used to clojure, or just the idea of functional programming.

; P11 (*) Modified run-length encoding.
(comment "Modify the result of problem P10 in such a way that if an element has no  duplicates it is simply copied into the result list. Only elements with duplicates are transferred as (N, E) terms.")
(defn encodeModified [l]
    (fn [a] (if (= (first a) 1) (last a) a))
    (encode l)))

; P12 (**) Decode a run-length encoded list.
(comment "Given a run-length code list generated as specified in problem P10, construct its uncompressed version.")
(defn decode [l]
  (mapcat (fn [a]
            (let [n (first a)
                  value (last a)]
              (loop [curr 0
                     output '()]
                  (= curr n)
                  (recur (inc curr) (cons value output)))))) l))

; P13 (**) Run-length encoding of a list (direct solution).
(comment "Implement the so-called run-length encoding data compression method directly."
(comment "I.e. don't use other methods you've written (like P09's pack);  do all the work directly.")
(defn encodeDirect [l]
  (loop [output '()
         [head & tail] l
         last-seen nil
         n 0]
     (if (nil? head)
      (reverse output)
      (if (= last-seen head)
        (recur output tail last-seen (inc n))
        (recur (if (nil? last-seen) output (cons (list n last-seen) output)) tail head 1)))))

; P14 (*) Duplicate the elements of a list.
(defn duplicate [l]  (mapcat (fn [a] (list a a)) l))

Now that I think things are relatively under control, I’m steaming along nicely into what I think is the end of my “beginner” stage in clojure.

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