new file for section 1.2

diff --git a/chapter1/ch1_2.clj b/chapter1/ch1_2.clj
new file mode 100644 (file)
index 0000000..6780735
--- /dev/null
+++ b/chapter1/ch1_2.clj
@@ -0,0 +1,54 @@
+(ns sicp.chapter1.ch1_2
+  (:use (clojure.contrib trace)))
+
+
+(defn factorial [n]
+  (if (= n 1)
+    1
+    (* n (factorial (- n 1)))))
+
+;; stack friendly version
+(defn factorial2 [n]
+  (loop [x n acc 1] 
+    (if (= x 1)
+      acc
+      (recur (- x 1) (* acc x)))))
+
+;; even better
+(defn factorial3 [n]
+  (reduce * (range 1 (inc n))))
+
+(comment
+ user> (dotrace [factorial] (factorial 3))
+ TRACE t2701: (factorial 3)
+ TRACE t2702: |    (factorial 2)
+ TRACE t2703: |    |    (factorial 1)
+ TRACE t2703: |    |    => 1
+ TRACE t2702: |    => 2
+ TRACE t2701: => 6
+ 6
+)
+
+(comment
+  sicp.chapter1.ch1_2> (dotrace [factorial] (factorial 6))
+  TRACE t2778: (factorial 6)
+  TRACE t2779: |    (factorial 5)
+  TRACE t2780: |    |    (factorial 4)
+  TRACE t2781: |    |    |    (factorial 3)
+  TRACE t2782: |    |    |    |    (factorial 2)
+  TRACE t2783: |    |    |    |    |    (factorial 1)
+  TRACE t2783: |    |    |    |    |    => 1
+  TRACE t2782: |    |    |    |    => 2
+  TRACE t2781: |    |    |    => 6
+  TRACE t2780: |    |    => 24
+  TRACE t2779: |    => 120
+  TRACE t2778: => 720
+  720
+  )
+
+(comment
+  sicp.chapter1.ch1_2> (dotrace [factorial2] (factorial2 6))
+  TRACE t2806: (factorial2 6)
+  TRACE t2806: => 720
+  720
+  )
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