# Exercise 2.14 of SICP

Exercise 2.14: Demonstrate that Lem is right. Investigate the behavior of the system on a variety of arithmetic expressions. Make some intervals A and B, and use them in computing the expressions A/A and A/B. You will get the most insight by using intervals whose width is a small percentage of the center value. Examine the results of the computation in center-percent form (see exercise 2.12).

> (print (div-interval (make-center-percent 6.8 1) (make-center-percent 6.8 1)))
[1.0002000200020003,1.9998000199979908]
> (print (div-interval (make-center-percent 6.8 .1) (make-center-percent 6.8 .1)))
[1.000002000002,.19999980000021655]
> (print (div-interval (make-center-percent 6.8 .1) (make-center-percent 3.4 .1)))
[2.000004000004,.19999980000021655]
> (print (div-interval (make-center-percent 6.8 .1) (make-center-percent 1.7 .1)))
[4.000008000008,.19999980000021655]
> (print (div-interval (make-center-percent 6.8 .1) (make-center-percent 1.7 .01)))
[4.000000440000004,.1099999890000035]
> (print (par1 (make-center-percent 6.8 .1) (make-center-percent 1.7 .01)))
[1.3600022771855311,.19199981581619266]
> (print (par2 (make-center-percent 6.8 .1) (make-center-percent 1.7 .01)))
[1.3599998237438813,.028000014256019334]
> (print (par1 (make-center-percent 6.8 .1) (make-center-percent 6.8 .1)))
[3.4000136000136,.2999992000024115]
> (print (par2 (make-center-percent 6.8 .1) (make-center-percent 6.8 .1)))
[3.4000000000000004,.10000000000000857]
> (print (par1 (make-center-percent 6.8 10) (make-center-percent 4.7 5)))
[2.844199964577264,22.613352145193346]
> (print (par2 (make-center-percent 6.8 10) (make-center-percent 4.7 5)))
[2.777440701636504,7.05260392723452]

(define (par1 r1 r2)
(div-interval (mul-interval r1 r2)
(define (par2 r1 r2)
(let ((one (make-interval 1 1)))
(div-interval one
(div-interval one r2)))))

(define (make-center-percent value p)
(make-center-width value (* (abs value) (/ p 100.0))))
(define (percent int)
(abs (* 100 (/ (width int) (center int)))))

(define (make-interval a b) (cons a b))
(define (lower-bound int) (car int))
(define (upper-bound int) (cdr int))
(define (make-center-width c w)
(make-interval (- c w) (+ c w)))
(define (center i)
(/ (+ (lower-bound i) (upper-bound i)) 2))
(define (width i)
(/ (- (upper-bound i) (lower-bound i)) 2))

(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)
(define (spans-zero? i)
(and
(not (> (lower-bound i) 0))
(not (< (upper-bound i) 0))))
(if (spans-zero? y)
(error "The dividing interval cannot span 0.")
(mul-interval x
(make-interval (/ 1.0 (upper-bound y))
(/ 1.0 (lower-bound y))))))

(define (sub-interval2 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 (sub-interval x y)
(make-interval (- (lower-bound x) (upper-bound y))
(- (upper-bound x) (lower-bound y))))

(define (print int)
(newline)
(display "[")
(display (center int))
(display ",")
(display (percent int))
(display "]")
(newline))