Add 1.18

sicp-ex-1.18.rkt

@@ -0,0 +1,28 @@
+(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 (double a)
+  (+ a a))
+
+(define (multiply a b)
+  (define (iter a b product)
+    (cond ((= a 1) (+ product b))
+          ((even? a) (iter (halve a) (double b) product))
+          (else (iter (halve a) (double b) (+ product b)))))
+
+  (iter a b 0))
+; multiply
+
+
+(multiply 132 555)
+(multiply 32 2)
+(multiply 33 1)