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+;; Extended Exercise Interval arithmetic; contd...
+;; Ex. 2.9 Width of intervals
+
+;; Rp = (/ 1 (+ (/ 1 R1) (/ 1 R2)))
+
+;; 
+;; Given information, a.k.a what Alyssa did
+;;
+(define (add-interval x y) 
+  (make-interval (+ (lower-bound x) (lower-bound y))
+		 (+ (upper-bound x) (upper-bound y))))
+
+
+(define (mul-interval x y)
+  (let ((p1 (* (lower-bound x) (lower-bound y)))
+	(p2 (* (lower-bound x) (upper-bound y)))
+	(p3 (* (upper-bound x) (lower-bound y)))
+	(p4 (* (upper-bound x) (upper-bound y))))
+    (make-interval (min p1 p2 p3 p4)
+		   (max p1 p2 p3 p4))))
+
+
+(define (div-interval x y)
+  (mul-interval
+   x
+   (make-interval (/ 1.0 (upper-bound y))
+		  (/ 1.0 (lower-bound y)))))
+
+
+(define (make-interval a b) (cons a b))
+
+
+
+;;
+;; Ex. 2.7 Define lower-bound and upper-bound
+;;
+(define (lower-bound i) (car i))
+
+(define (upper-bound i) (cdr i))
+
+
+
+
+;;
+;; Ex. 2.8 sub-interval
+;; 
+;; Subtracting intervals means expressing as a pair, the difference
+;; of lower bounds and the difference of upper bounds, with the following
+;; things in mind:
+;;;; The delta of two UPPER bounds could sometimes be _smaller_ than
+;;;; the delta of two lower bounds.
+;;;; Deltas could be negative too.
+(define (sub-interval x y)
+  (let ((d1 (- (lower-bound x) (lower-bound y)))
+	(d2 (- (upper-bound x) (upper-bound y))))
+    (make-interval (min d1 d2)
+		   (max d1 d2))))
+
+
+
+
+;; 
+;; Ex. 2.9 width of interval
+;; 
+
+;; The width of an interval will always be a positive number.
+;; (Assuming, of course, resistances are intervals of real numbers.)
+(define (width-of-interval-ex2-9 x)
+  (/ (abs (- (upper-bound x)
+	     (lower-bound x)))
+     2))
+
+
+;; 
+;; Ex. 2.10 Modify width-of-interval to makes use of Ben Bitdiddle's
+;; expert remarks. (Spake he; "It's not clear what it means to divide
+;; by an interval that spans zero.")
+;; 
+
+(define (width-of-interval x)
+  (let ((up-x (upper-bound x))
+	(lo-x (lower-bound x)))
+    (if (= up-x lo-x)
+	(error "The interval for this resistor spans zero: " x)
+	(/ (abs (- up-x lo-x))
+	   2))))
+
+
+(display "****** 
+Testing Ex. 2.10 
+******")
+
+
+(define r1 (make-interval 0.5 1.0))
+(display r1)
+(width-of-interval r1)
+;Value: .25
+
+(define r2 (make-interval 0.8 1.1))
+(display r2)
+(width-of-interval r2)
+;Value: .15000000000000002
+
+(define r3 (make-interval 2.0 2.0))
+(display r3)
+(display "Call to width-of-interval with r3 is commented out, 
+as it will cause an error and prevent 
+the other tests from running. 
+Uncomment it to see what happens.")
+; (width-of-interval r3) ; uncomment this and evaluate it to see the error:
+
+;The interval for this resistor spans zero:  (2. . 2.)
+;To continue, call RESTART with an option number:
+; (RESTART 3) => Return to read-eval-print level 3.
+; (RESTART 2) => Return to read-eval-print level 2.
+; (RESTART 1) => Return to read-eval-print level 1.
+;Start debugger? (y or n): n
+
+
+(= (width-of-interval (add-interval r1 r2))
+   (width-of-interval (add-interval r2 r1)))
+
+(= (width-of-interval (sub-interval r1 r2))
+   (width-of-interval (sub-interval r2 r1)))
+
+(define r4 (make-interval 1.3 1.4))
+(define r5 (make-interval -0.9 -0.8))
+(mul-interval r4 r5)
+(mul-interval r5 r4)
+(width-of-interval (mul-interval r4 r5))
+(width-of-interval (mul-interval r5 r4))
+
+
+(= (width-of-interval (div-interval r1 r2))
+   (width-of-interval (div-interval r2 r1)))
+
+
+
+
+
+;; Ex. 2.11 Break mul-interval into nine case by testing
+;; for the signs of the endpoints of the intervals.
+
+(define (mul-interval-new x y)
+  (let ((lox (lower-bound x))
+	(upx (upper-bound x))
+	(loy (lower-bound y))
+	(upy (upper-bound y))
+	(-? (lambda (r) (negative? r)))
+	(+? (lambda (r) (positive? r))))
+    (cond ((or (< upx lox) (< upy loy)) "HAH!")
+	  ((and (+? lox) (+? upx) (+? loy) (+? upy)) (make-interval (* lox loy)
+								    (* upx upy)))	  
+	  ((and (-? lox) (-? upx) (-? loy) (-? upy)) (make-interval (* upx upy)
+								    (* lox loy)))
+	  ((and (-? lox) (+? upx) (+? loy) (+? upy)) (make-interval (* lox upy)
+								    (* upx upy)))
+	  ((and (+? lox) (+? upx) (-? loy) (+? upy)) (make-interval (* upx loy)
+								    (* upx upy)))
+	  ((and (-? lox) (-? upx) (+? loy) (+? upy)) (make-interval (* lox upy)
+								    (* upx loy)))
+	  ((and (+? lox) (+? upx) (-? loy) (-? upy)) (make-interval (* upx loy)
+								    (* lox upy)))
+	  ((and (-? lox) (-? upx) (-? loy) (+? upy)) (make-interval (* lox upy)
+								    (* upx loy)))
+	  ((and (-? lox) (+? upx) (-? loy) (-? upy)) (make-interval (* upx loy)
+								    (* loy upy)))
+	  ((and (-? lox) (+? upx) (-? loy) (+? upy)) (make-interval (min (* lox upy) (* upx loy))
+								    (max (* lox loy) (* upx upy)))))))
+
+
+(display "*****
+Testing Ex. 2.11
+*****")
+
+(define res-pairs (list (cons (make-interval 1 2) (make-interval 1 2))
+			(cons (make-interval -2 -1) (make-interval -2 -1))
+			(cons (make-interval -1 2) (make-interval 1 2))
+			(cons (make-interval 1 2) (make-interval -1 2))
+			(cons (make-interval -2 -1) (make-interval 1 2))
+			(cons (make-interval 1 2) (make-interval -2 -1))
+			(cons (make-interval -2 -1) (make-interval -1 2))
+			(cons (make-interval -2 1) (make-interval -2 -1))
+			(cons (make-interval -2 1) (make-interval -2 1))
+			; Illegal intervals follow:
+			(cons (make-interval -1 -2) (make-interval -2 1))
+			(cons (make-interval -2 1) (make-interval 2 -1))
+			(cons (make-interval 2 -1) (make-interval 2 -1))
+			(cons (make-interval -1 -2) (make-interval -1 -2))))
+
+(map (lambda (rpair) (mul-interval-new (car rpair) (cdr rpair))) res-pairs)
+
+
+
+
+