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<center><h2>cs150: Notes 18</h2></center>

<h4>Assignments Due</h4>

<ul>
<li> <b>Now:</b> (12:02pm) Exam 1
<li> Tuesday, 27 February: If you want to be assigned a partner for PS5,
 email me before midnight Tuesday.
<li> Before <b>Friday, 30 February</b> (postponed from 21 February): Read 
<a href="http://research.amnh.org/~tyson/index.php">Neil DeGrasse
Tyson</a>'s <a href="/cs150/readings/tyson.pdf"><em>Science's Endless
Golden Age</em></a> (watching <a href="http://www.comedycentral.com/motherload/index.jhtml?ml_video=81508">Tyson's explanation of the
history of the universe</a> on <em>The Daily Show</em> is not required)
<li> <b>Wednesday, 14 March</b>: <a href="ps/ps5/">Problem Set 5</a>
<li> <b>Before Monday, 19 March</b>: Read rest of GEB part I (Chapters 2-4 and 6-9, in
addition to Chapters 1 and 5 you have already read). 
</ul>

<h4>Notes</h4>
<p>
Assignment Expression: <tt>(<b>set!</b> <em>Name</em>
 <em>Expression</em>)</tt>
<blockquote>
Evaluate <em>Expression</em> and replace the value associated with <em>Name</em>
with the value of <em>Expression</em>.
</blockquote>
Pair mutators:
<ul>
<li><tt>(<b>set-car!</b> <em>Expression</em><sub>0</sub>
<em>Expression</em><sub>1</sub>)</tt> &#151; evaluate
<em>Expression</em><sub>0</sub> and <em>Expression</em><sub>1</sub> and
store the value of <em>Expression</em><sub>1</sub> in the car part of
the value of <em>Expression</em><sub>0</sub>.
<em>Expression</em><sub>0</sub> must evaluate to a cons pair.

<li><tt>(<b>set-cdr!</b> <em>Expression</em><sub>0</sub>
<em>Expression</em><sub>1</sub>)</tt> &#151; evaluate
<em>Expression</em><sub>0</sub> and <em>Expression</em><sub>1</sub> and
store the value of <em>Expression</em><sub>1</sub> in the cdr part of
the value of
<em>Expression</em><sub>0</sub>. <em>Expression</em><sub>0</sub> must
evaluate to a cons pair.
</ul>

Why is mutation scary?
<p><br><p><br><p><br>

Why is mutation useful?
<p><br><p><br><p>

Why do our substitution evaluation rules fail when we allow mutation?
<p><br><p><br><p><br>


Would it be possible to create a circular list without using <tt>set-cdr!</tt>?
<p>
<p><br><p>

<pre>
(define (map! f lst)
   (if (null? lst) (void)
       (begin
           (set-car! lst (f (car lst)))
           (map! f (cdr lst)))))
</pre>



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