solution to 2.38

diff --git a/src/sicp/ex2_38.clj b/src/sicp/ex2_38.clj
new file mode 100644 (file)
index 0000000..91f6818
--- /dev/null
+++ b/src/sicp/ex2_38.clj
@@ -0,0 +1,33 @@
+(ns sicp.ex2_38
+  (:use [clojure.test]
+        [sicp [ch2-2 :only (accumulate)]
+              [ex2_36 :only (accumulate-n)]]))
+
+;; The accumulate procedure is also known as fold-right, because it combines
+;; the first element of the sequence with the result of combining all the elements
+;; to the right. There is also a fold-left, which is similar to fold-right, except
+;; that it combines elements working in the opposite direction:
+(def fold-right accumulate)
+
+(defn fold-left [op initial sequence]
+  (if (nil? sequence)
+    initial
+    (fold-left op
+               (op initial (first sequence))
+               (next sequence))))
+
+;; What are the values of
+(comment
+  "answers inlined")
+
+(fold-right / 1 (list 1 2 3))
+;;=> 3/2
+
+(fold-left / 1 (list 1 2 3))
+;;=> 1/6
+
+(fold-right list nil (list 1 2 3))
+;;=> (1 (2 (3 nil)))
+
+(fold-left list nil (list 1 2 3))
+;;=> (((nil 1) 2) 3)
\ No newline at end of file