solutions to 4.32 and 4.34

[sicp.git] / src / sicp / ex2_20.clj

diff --git a/src/sicp/ex2_20.clj b/src/sicp/ex2_20.clj
index 24b5f5e774fb26d2f6aaff19364f11d1f4942d2e..590ff31b8c93a3bf289c95a97a43df14e4926027 100644 (file)
--- a/src/sicp/ex2_20.clj
+++ b/src/sicp/ex2_20.clj
@@ -9,8 +9,65 @@
                            (odd? y)))
                  xs)))
 
+;; another implementation
+(defn- calc-parity [pred xs result]
+  (if (empty? xs)
+    (reverse result)
+    (let [a (first xs)
+          t (pred a)]
+      (if t
+       (calc-parity pred (rest xs) (cons a result))
+       (calc-parity pred (rest xs) result)))))
+
+(defn same-parity-2 [x & xs]
+  (if (even? x)
+    (cons x (calc-parity even? xs '()))
+    (cons x (calc-parity odd? xs '()))))
+
+(defn- calc-parity-3 [pred xs]
+  (if (empty? xs)
+    nil
+    (let [a (first xs)
+          t (pred a)]
+      (if t
+       (cons a (calc-parity-3 pred (rest xs)))
+       (calc-parity-3 pred (rest xs))))))
+
+(defn same-parity-3 [x & xs]
+  (if (even? x)
+    (cons x (calc-parity-3 even? xs))
+    (cons x (calc-parity-3 odd? xs))))
+
+(defn same-parity-4 [x & xs]
+  (conj (filter #(if (even? x) (even? %) (odd? %)) xs) x))
+
+;; result of the discussion in bang-fp
+(defn same-parity-5 [x & xs]
+  (cond (nil? xs) (list x)
+       (= (even? x) (even? (first xs))) (cons x (apply same-parity-5 xs))
+       :else (apply same-parity-5 (cons x (rest xs)))))
+
 (deftest test-same-parity
   (are [x y] [= x y]
        (same-parity 1 2 3 4 5 6 7) (list 1 3 5 7)
        (same-parity 2 3 4 5 6 7) (list 2 4 6)))
 
+(deftest test-same-parity-2
+  (are [x y] [= x y]
+       (same-parity-2 1 2 3 4 5 6 7) (list 1 3 5 7)
+       (same-parity-2 2 3 4 5 6 7) (list 2 4 6)))
+
+(deftest test-same-parity-3
+  (are [x y] [= x y]
+       (same-parity-3 1 2 3 4 5 6 7) (list 1 3 5 7)
+       (same-parity-3 2 3 4 5 6 7) (list 2 4 6)))
+
+(deftest test-same-parity-4
+  (are [x y] [= x y]
+       (same-parity-4 1 2 3 4 5 6 7) (list 1 3 5 7)
+       (same-parity-4 2 3 4 5 6 7) (list 2 4 6)))
+
+(deftest test-same-parity-5
+  (are [x y] [= x y]
+       (same-parity-5 1 2 3 4 5 6 7) (list 1 3 5 7)
+       (same-parity-5 2 3 4 5 6 7) (list 2 4 6)))