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<center><h2>cs150: Notes 9</h2></center>

<h4>Assignments Due</h4>


<ul>
<li> <b>Monday, 12 February</b> (beginning of class): <a
href="/cs150/ps/ps3/">Problem Set 3</a>
</ul>

<h4>find-closest</h4>

<pre>
(define (find-closest-number goal numbers)
   (if (= 1 (length numbers))
       (car numbers)
       (if (< (abs (- goal (car numbers)))
              (abs (- goal 
                      (find-closest goal (cdr numbers)))))
           (car numbers)
           (find-closest goal (cdr numbers)))))

(define (find-closest goal lst closeness)
  (if (= 1 (length lst))
      (car lst)
      (if (< (closeness goal (car lst))
             (closeness goal 
                         (find-closest goal (cdr lst) closeness)))
          (car lst)
          (find-closest goal (cdr lst) closeness))))

(define (pick-closest closeness goal num1 num2)
   (if (< (closeness goal num1)
            (closeness goal num2))
       num1
       num2))

(define (find-closest goal lst closeness)
   (if (= 1 (length lst))
 (car lst)
 (pick-closest closeness goal (car lst) 
      (find-closest goal (cdr lst) closeness))))

</pre>

<font size=+1><b>Languages</b></font>
<p>
How can we compare notations for describing languages?
<p>
<p><br><p><br><p><br><p><br>
<br>
<p>

What is the difference between RTN and BNF?
<p class="page"></p>

<font size=+1><b>Music and Recursion</b></font>
<p>

<center>
<A href="song.jpg" border=0><img border=0 src="song.jpg" width=648 height=486></a>
</center>
<blockquote>
<em>Song</em> ::= <em>Verse</em> <em>VBBD</em> <em>VBBD</em>
<em>Better</em> <em>Coda</em><br>
<em>VBBD</em> ::= <em>Verse</em> <em>Bridge</em> <em>Bridge</em>
<em>Dadada</em> (ends on C)<br>
<em>Coda</em> ::= F Eb Bb F <em>Coda</em><br>
Note: the <em>Coda</em> has no base case, and should continue forever (time permitting).
</blockquote>


<font size=+1><b>Challenge Problem</b></font><br>
Define a Scheme procedure that can produce the INT and Gplot graphs from
GEB Chapter 5.  Hint: you may need to think about curves differently
from PS3.  (A solution is worth two gold stars.)
<p>

<font size=+1><b>Links</b></font>

<ul>
<li> Listen to Bach's 
<A
href="http://www.people.vcu.edu/~elhaij/GEB/Music/BachHarmonicLabyrinth.mid"><em>Little
Harmonic Labyrinth</em></a> (MIDI file); <a href="http://www.people.vcu.edu/~elhaij/GEB/Music/index.html">More GEB Music from VCU course</a>

<li> <a href="http://www.gnu.org/">GNU's Not Unix</a> (<em>GOD Over Djinn</em>-like acronym)
<li><a
href="http://www.fractalwisdom.com/FractalWisdom/fractal.html">
<em>The Story of Benoit B. Mandelbrot
and the Geometry of Chaos</em>
</a>

</ul>

<div class="quote">
Hofstadter's Law: <em>It always takes longer than you expect,<br>even when you
take Hofstadter's Law into account.</em>
</quote>
</font>

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