recursion done, except bonus exercises

src/recursion.clj

@@ -1,85 +1,169 @@
 (ns recursion)
 
 (defn product [coll]
-  :-)
+  (if (empty? coll)
+    1
+    (* (first coll)
+       (product (rest coll)))))
 
 (defn singleton? [coll]
-  :-)
+  (and (not (empty? coll))
+       (empty? (rest coll))))
+
+(defn my-last [a-seq]
+  (cond
+    (empty? a-seq) nil
+    (singleton? a-seq) (first a-seq)
+    :else (my-last (rest a-seq))))
 
-(defn my-last [coll]
-  :-)
 
 (defn max-element [a-seq]
-  :-)
+  (cond
+    (empty? a-seq) nil
+    (singleton? a-seq) (first a-seq)
+    :else (max (first a-seq)
+               (max-element (rest a-seq)))))
 
 (defn seq-max [seq-1 seq-2]
-  [:-])
+  (if (> (count seq-1)
+         (count seq-2))
+    seq-1
+    seq-2))
 
 (defn longest-sequence [a-seq]
-  [:-])
+  (cond
+    (empty? a-seq) nil
+    (singleton? a-seq) (first a-seq)
+    :else (seq-max (first a-seq)
+                   (longest-sequence (rest a-seq)))))
 
 (defn my-filter [pred? a-seq]
-  [:-])
+  (if (empty? a-seq)
+    a-seq
+    (if (pred? (first a-seq))
+      (cons (first a-seq)
+            (my-filter pred? (rest a-seq)))
+      (my-filter pred? (rest a-seq)))))
 
 (defn sequence-contains? [elem a-seq]
-  :-)
+  (cond
+    (empty? a-seq) false
+    ((complement =) elem (first a-seq)) (sequence-contains? elem (rest a-seq))
+    :else true))
 
 (defn my-take-while [pred? a-seq]
-  [:-])
+  (if (or (empty? a-seq)
+          ((complement pred?) (first a-seq)))
+    '()
+    (cons (first a-seq)
+          (my-take-while pred? (rest a-seq)))))
 
 (defn my-drop-while [pred? a-seq]
-  [:-])
+  (cond
+    (empty? a-seq) '()
+    (pred? (first a-seq)) (my-drop-while pred? (rest a-seq))
+    :else a-seq))
 
 (defn seq= [a-seq b-seq]
-  :-)
+  (cond
+    (and (empty? a-seq) (empty? b-seq)) true
+    (or (empty? a-seq) (empty? b-seq)) false
+    (= (first a-seq) (first b-seq)) (seq= (rest a-seq) (rest b-seq))
+    :else false))
 
 (defn my-map [f seq-1 seq-2]
-  [:-])
+  (if (or (empty? seq-1)
+          (empty? seq-2))
+    '()
+    (cons (f (first seq-1) (first seq-2))
+          (my-map f (rest seq-1) (rest seq-2)))))
 
 (defn power [n k]
-  :-)
+  (if (zero? k)
+    1
+    (* n (power n (dec k)))))
 
 (defn fib [n]
-  :-)
+  (cond
+    (zero? n) n
+    (== 1 n) n
+    :else (+ (fib (- n 1))
+             (fib (- n 2)))))
 
 (defn my-repeat [how-many-times what-to-repeat]
-  [:-])
+  (if (< how-many-times 1)
+    '()
+    (cons what-to-repeat
+          (my-repeat (dec how-many-times) what-to-repeat))))
 
 (defn my-range [up-to]
-  [:-])
+  (let [n (dec up-to)]
+    (if (< n 0)
+      '()
+      (cons n (my-range n)))))
 
 (defn tails [a-seq]
-  [:-])
+  (if (empty? a-seq)
+    '(())
+    (cons (seq a-seq)
+          (tails (rest a-seq)))))
 
 (defn inits [a-seq]
-  [:-])
+  (reverse (map reverse (tails (reverse a-seq)))))
 
 (defn rotations [a-seq]
-  [:-])
+  (if (empty? a-seq)
+    '(())
+    (rest (map concat (tails a-seq)
+                      (inits a-seq)))))
 
 (defn my-frequencies-helper [freqs a-seq]
-  [:-])
+  (if (empty? a-seq)
+    freqs
+    (let [first-e (first a-seq)
+          new-freqs (if (contains? freqs first-e)
+                      (assoc freqs first-e (inc (get freqs first-e)))
+                      (assoc freqs first-e 1))]
+      (my-frequencies-helper new-freqs (rest a-seq)))))
 
 (defn my-frequencies [a-seq]
-  [:-])
+  (my-frequencies-helper {} a-seq))
 
 (defn un-frequencies [a-map]
-  [:-])
+  (apply concat (map (fn [[e n]] (repeat n e)) a-map)))
 
 (defn my-take [n coll]
-  [:-])
+  (if (or (empty? coll)
+          (< n 1))
+    '()
+    (cons (first coll)
+          (my-take (dec n)
+                   (rest coll)))))
 
 (defn my-drop [n coll]
-  [:-])
+  (if (> n 0)
+    (my-drop (dec n) (rest coll))
+    coll))
 
 (defn halve [a-seq]
-  [:-])
+  (let [div (int (/ (count a-seq) 2))]
+    (vector (my-take div a-seq)
+            (my-drop div a-seq))))
 
 (defn seq-merge [a-seq b-seq]
-  [:-])
+  (cond
+    (empty? a-seq) b-seq
+    (empty? b-seq) a-seq
+    (< (first a-seq) (first b-seq))
+      (cons (first a-seq) (seq-merge (rest a-seq) b-seq))
+    :else
+      (cons (first b-seq) (seq-merge a-seq (rest b-seq)))))
 
 (defn merge-sort [a-seq]
-  [:-])
+  (if (or (empty? a-seq)
+          (singleton? a-seq))
+    a-seq
+    (apply seq-merge (map merge-sort (halve a-seq)))))
 
 (defn split-into-monotonics [a-seq]
   [:-])