solution to 2.11.

diff --git a/src/sicp/ex2_11.clj b/src/sicp/ex2_11.clj
index 03fbfd098031e30963b5c856a250019fcebbbeb0..290902083db4b193957a5d3d378d54c83129f983 100644 (file)
--- a/src/sicp/ex2_11.clj
+++ b/src/sicp/ex2_11.clj
@@ -44,5 +44,26 @@
 ;; -   -   -   +
 ;; -   -   -   -
 
-
-;; too boring a problem. Skipping the rest of it.
+(defn new-mul-interval [x y]
+  (let [l1 (lower-bound x)
+       u1 (upper-bound x)
+       l2 (lower-bound y)
+       u2 (upper-bound y)
+       sl1? (pos? l1)
+       su1? (pos? u1)
+       sl2? (pos? l2)
+       su2? (pos? u2)]
+    (cond (and sl1? su1? sl2? su2?)             (make-interval (* l1 l2) (* u1 u2))
+         (and sl1? su1? (not sl2?) su2?)       (make-interval (* u1 l2) (* u1 u2))
+         (and sl1? su1? (not sl2?) (not su2?)) (make-interval (* u1 l2) (* u2 l1))
+         (and (not sl1?) su1? sl2? su2?)       (make-interval (* l1 u2) (* u1 u2))
+         (and (not sl1?) su1? (not sl2?) (not su2?)) (make-interval (* u1 l2) (* l1 l2))
+         (and (not sl1?) (not su1?) sl2? su2?) (make-interval (* l1 u2) (* u1 l2))
+         (and (not sl1?) (not su1?) (not sl2?) su2?) (make-interval (* l1 u2) (* l1 l2))
+         (and (not sl1?) (not su1?) (not sl2?) (not su2?)) (make-interval (* u1 u2) (* l1 l2))
+         (and (not sl1?) su1? (not sl2?) su2?) (let [p1 (* l1 l2)
+                                                     p2 (* l1 u2)
+                                                     p3 (* u1 l2)
+                                                     p4 (* u1 u2)]
+                                                 (make-interval (min p1 p2 p3 p4)
+                                                                (max p1 p2 p3 p4))))))
\ No newline at end of file