added another solution and test for 2.20

diff --git a/src/sicp/ex2_20.clj b/src/sicp/ex2_20.clj
index 24b5f5e774fb26d2f6aaff19364f11d1f4942d2e..582330968a957aaeba1b5bcb7f19e024a64943a6 100644 (file)
--- a/src/sicp/ex2_20.clj
+++ b/src/sicp/ex2_20.clj
@@ -9,8 +9,27 @@
                            (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 '()))))
+
 (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)))