solutions to 4.38, 4.39 and 4.40

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

diff --git a/src/sicp/ex2_20.clj b/src/sicp/ex2_20.clj
index 3bab563170fad1ad72261f641440d82f2a86a4b9..590ff31b8c93a3bf289c95a97a43df14e4926027 100644 (file)
--- a/src/sicp/ex2_20.clj
+++ b/src/sicp/ex2_20.clj
@@ -14,7 +14,7 @@
   (if (empty? xs)
     (reverse result)
     (let [a (first xs)
-       t (pred a)]
+          t (pred a)]
       (if t
        (calc-parity pred (rest xs) (cons a result))
        (calc-parity pred (rest xs) result)))))
@@ -28,7 +28,7 @@
   (if (empty? xs)
     nil
     (let [a (first xs)
-       t (pred a)]
+          t (pred a)]
       (if t
        (cons a (calc-parity-3 pred (rest xs)))
        (calc-parity-3 pred (rest xs))))))
@@ -38,6 +38,15 @@
     (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)
@@ -52,3 +61,13 @@
   (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)))