[plt-scheme] HtDP 27.3.2 and 27.3.4

With TOLERANCE 1/100 I find that poly is called with the following arguments:
9/2
6
15/4
9/2
33/8
9/2
63/16
33/8
129/32
33/8
255/64
129/32
513/128
129/32
1023/256
513/128
2049/512
513/128

For (poly 3 4) I find arguments:
7/2
4
15/4
4
31/8
4
63/16
4
127/32
4
255/64
4
511/128
4
 Poly is called 7 times with the same argument 4.
Can you now find out why f is called with the same argument more than once (how to avoid that?
Jos

----- Original Message ----- 
From: "David Yrueta" <dyrueta at gmail.com>
To: "PLT-Scheme Mailing List" <plt-scheme at list.cs.brown.edu>
Sent: Saturday, July 04, 2009 7:34 AM
Subject: [plt-scheme] HtDP 27.3.2 and 27.3.4


> Hi All --
> 
> Questions for both exercises refer to the function "find-root" below:
> 
> ;; find-root : (number  ->  number) number number  ->  number
> ;; to determine a number R such that f has a
> ;; root between R and (+ R TOLERANCE)
> ;;
> ;; ASSUMPTION: f is continuous and monotonic
> (define (find-root f left right)
>  (cond
>    [(<= (- right left) TOLERANCE) left]
>    [else
>      (local ((define mid (/ (+ left right) 2)))
> (cond
>   [(<= (f mid) 0 (f right))
>           (find-root f mid right)]
>   [else
>           (find-root f left mid)]))]))
> 
> ;; poly : number  ->  number
> (define (poly x)
> (* (- x 2) (- x 4)))
> 
> HtDP exercise 27.3.2:
> 
> "Use poly from 27.3.1 to test find-root. Experiment with different
> values for TOLERANCE. Use the strategy of section 17.8 to formulate
> the tests as boolean-valued expressions."
> 
> Are these tests meant to take place inside or outside the body of
> "find-root." In other words, are they conditional expressions inside
> "find-root"?  If not, how are these tests different from a typical
> "check-expect?"
> 
> (check-expect (find-root poly 3 6) 3.75)
> 
> HtDP exercise 27.3.4
> 
> "For every midpoint m, except for the last one, the function find-root
> needs to determine the value of (f m) twice. Validate this claim for
> one example with a hand-evaluation."
> 
> For the life of me, I don't see (f m) computed twice for every
> midpoint.  "Mid" is obviously computed twice, but I only see "(f mid)"
> computed once per call.  Am I missing something totally obvious?
> 
> Thanks!
> Dave Yrueta
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