Add 1.17

sicp-ex-1.17.rkt

@@ -0,0 +1,43 @@
+(define (multiply a b)
+  (define (iter a b product)
+    (if (= b 0)
+        product
+        (iter a (- b 1) (+ product a))))
+
+  (iter a b 0))
+; multiply
+
+(define (sqr n)
+  (multiply n n))
+
+(define (divide a b)
+  (define (iter a b product)
+    (cond ((= a 0) product)
+          ((< a 0) (- product 1))
+          (else (iter (- a b) b (+ product 1)))))
+
+  (iter a b 0))
+; divide
+
+(define (halve a)
+  (divide a 2))
+
+(define (fast-ext number power)
+  (define (iter number power product)
+    (cond ((= power 0) product)
+          ((even? power) (iter (sqr number)
+                               (halve power)
+                               product))
+          (else (iter number
+                      (- power 1)
+                      (multiply number product)))))
+
+  (iter number power 1))
+; fast-ext
+
+(fast-ext 5 3)
+(fast-ext 2 2)
+(fast-ext 1 2)
+(fast-ext 9 4)
+(fast-ext 2 0)
+(fast-ext 7 1)