[an error occurred while processing this directive]

<center><h2>cs150: Notes 6</h2></center>

<h4>Assignments Due</h4>


<ul>
<li> (extended from 29 January) Before Monday, 5 February: Read GEB, <em>Little Harmonic Labyrinth</em> and GEB, Chapter 5
<li> Friday, 2 February (beginning of class): Problem Set 2.  (Note: Problem
Set 2 will be accepted without penalty or any permission required until
the beginning of class on Monday, 5 February, as long as you promise to
still finish reading GEB Chapter 5 before Monday's class.)
</ul>

</ul>

<h4>Notes and Questions</h4>

Is it better to solve problems by thinking about what we need to do to
solve the problem (<em>procedures</em>) or by thinking about what we
need to represent to solve the problem (<em>data</em>)?
<p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br>

What does <b>cons</b> do?<br>
<p><br><p><br><p>

What do <b>car</b> and <b>cdr</b> do?<br>
<p><br><p><br>

What is the value of <tt>(car (cdr (cons 1 (cons 2 (cons 3 null)))))</tt>?
<p><br><p>

Why do we need the special list <b><tt>null</tt></b>?<br>

<p class="page"></p>

Are there any data structures that cannot be built using just <tt>cons</tt>?
<p><br><p><br><p><br><p><br>
<p><br><p><br><p><br><p><br>


How could we define <b>cons</b>, <b>car</b> and <b>cdr</b> if Scheme did
not have them as primitives?<br>
<p>
<p><br><p><br>
<p><br><p><br>


A <em>list</em> is either (1) <b>null</b> or (2) a pair where the second
part of the pair is a list. To define procedures on lists, we need to
handle both cases: what do we do when the list is <b>null</b> and what
do we do when the list is a pair.

<p>
Define <tt>length</tt> a procedure that takes a list as its
operand and evaluates to the number of elements in that list.
<p><p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br><p><br>
[an error occurred while processing this directive]