「ClojureでNinety-Nine Lisp Problems(P16～20)」ブログ用

p16-1.clj

(defn my-drop
  "Drop every N'th element from a list."
  [xs n]
  (map #(second %) (filter #(not= (mod (first %) n) (dec n)) (map-indexed vector xs)))
)

p16-2.clj

(defn my-drop
  "Drop every N'th element from a list."
  [xs n]
  (let [elems (map-indexed vector xs), m (dec n)]
    (for [[idx elem] elems :when (not= (mod idx n) m)] elem)
  )
)

p16-3.clj

(defn my-drop
  "Drop every N'th element from a list."
  [xs n]
  (let [elems (partition n n nil xs)]
    (reduce #(concat %1 (take (dec n) %2)) nil elems)
  )
)

p16-4.clj

(defn my-drop
  "Drop every N'th element from a list."
  [xs n]
  (keep-indexed #(when-not (zero? (mod (inc %1) n)) %2) xs)
)

p16-5.clj

(defn my-drop
  "Drop every N'th element from a list."
  [xs n]
  (mapcat butlast (partition-all n (concat xs [nil])))
)

p17.clj

(defn my-split
  "Split a list into two parts; the length of the first part is given."
  [xs n]
  (let [[f s] (last (take (inc n) (iterate #(let [[xs [y & ys]] %] [(cons y xs), ys]) [nil, xs])))]
    (list (filter #(not (nil? %)) (reverse f)) (if (nil? s) '() s))
  )
)

p18.clj

(defn my-slice
  "Extract a slice from a list."
  [xs i k]
  (for [[idx elem] (map-indexed vector xs) :when (and (< (- i 2) idx) (> k idx))] elem)
)

p19.clj

(defn my-rotate
  "Rotate a list N places to the left."
  [xs n]
  (let [idx (if (pos? n) n (+ (count xs) n)), [f s] (split-at idx xs)]
    (concat s f)
  )
)

p20.clj

(defn my-remove-at
  "Remove the K'th element from a list."
  [xs n]
  (reverse (first (reduce (fn [[elems cnt] elem]
    [(if (not= cnt n) (cons elem elems) elems), (inc cnt)]
  ) [nil, 0] xs)))
)