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<CENTER><B><FONT SIZE=6>CS100<B>B</B></FONT></B></CENTER>
<P>
<CENTER><B><FONT SIZE=6>Lecture 9<BR>
</FONT></B></CENTER>
<P>
<FONT SIZE=6>CONCEPTS --- last lecture</FONT>
<P>
<FONT SIZE=6>Simple expressions and programs; call/assignment
statements and their evaluation; examples</FONT>
<P>
<FONT SIZE=6>CONCEPTS --- this lecture</FONT>
<P>
<FONT SIZE=6>Compound statements and scoping rules; blocks and
declarations; fun with </FONT><TT><FONT SIZE=6 FACE="Lucida Console">else</FONT></TT><FONT SIZE=6>;
iteration with </FONT><TT><FONT SIZE=6 FACE="Lucida Console">while</FONT></TT><FONT SIZE=6>
and </FONT><TT><FONT SIZE=6 FACE="Lucida Console">break; </FONT></TT><FONT SIZE=6>fencepost
errors<BR>
</FONT>
<P>
<FONT SIZE=6>HANDOUT: Sample Prelim #1 questions<BR>
</FONT>
<P>
<CENTER><B><FONT SIZE=6>Prelim #1 notes</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>Review session is Sunday at 3:00</FONT>
<LI><FONT SIZE=6>Prelim is Monday 7:30-9:00 PM</FONT>
<UL>
<LI><FONT SIZE=6>Coverage is: everything (but there will be no
detailed questions on the stack model)</FONT>
<LI><FONT SIZE=6>Closed book, closed notes, mostly for your benefit.
Bring a writing implement.</FONT>
<LI><FONT SIZE=6>We don't stress memorization</FONT>
<LI><FONT SIZE=6>We will forgive minor syntactic errors</FONT>
</UL>
<LI><FONT SIZE=6>Room assignments: A-L Upson B17, M-Z Phillips
101</FONT>
<LI><FONT SIZE=6>Don't miss the prelim!</FONT>
<UL>
<LI><FONT SIZE=6>Make up exams are oral, and guaranteed to be
no easier than the real one</FONT>
</UL>
</UL>
<P>
<CENTER><B><FONT SIZE=6>Compound statements</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>Another kind of C statement (we've actually seen
this already, but hadn't given it a name)</FONT>
<LI><FONT SIZE=6>This is what the curly braces </FONT><TT><FONT SIZE=6 FACE="Lucida Console">{}</FONT></TT><FONT SIZE=6>
do, at least so far</FONT>
<UL>
<LI><FONT SIZE=6>Sort of like begin and end in PASCAL</FONT>
</UL>
<LI><FONT SIZE=6>Syntax: an open curly brace </FONT><TT><FONT SIZE=6 FACE="Lucida Console">{</FONT></TT><FONT SIZE=6>,
then 0 or more statements, then a closed curly brace </FONT><TT><FONT SIZE=6 FACE="Lucida Console">}</FONT><FONT SIZE=5 FACE="Lucida Console">{</FONT><FONT SIZE=6>x
= 3;y = square(x);}</FONT></TT>
<LI><FONT SIZE=6>A compound expression can include declaration
statements, but they must all appear at the beginning (i.e.,
before any statement which is not a declaration statement).</FONT>
<LI><FONT SIZE=6>Example with declaration:</FONT><FONT SIZE=5 FACE="Lucida Console">{</FONT><FONT SIZE=6>int
y;y = square(x);}</FONT>
</UL>
<P>
<FONT SIZE=6></FONT>
<P>
<CENTER><B><FONT SIZE=6>Scoping rules</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>Nasty example (don't write code like this)</FONT><FONT SIZE=5 FACE="Lucida Console">void
main(){</FONT><FONT SIZE=6>int x;x = 3;{int x;x =4;printf("%d",x);}}</FONT>
</UL>
<P>
<FONT SIZE=6></FONT>
<UL>
<LI><FONT SIZE=6>The rule is simple: to figure out which </FONT><FONT SIZE=6 FACE="Lucida Console">x</FONT><FONT SIZE=6>
is intended, look outwards from the line you are on until you
find </FONT><FONT SIZE=6 FACE="Lucida Console">x</FONT><FONT SIZE=6>.</FONT>
<UL>
<LI><FONT SIZE=6>You can add this to the stack model, but it's
a little ugly and we won't do it</FONT>
<LI><FONT SIZE=6>We won't use this feature in anything very complicated
in CS100B</FONT>
</UL>
</UL>
<P>
<CENTER><B><FONT SIZE=6>Where have we seen compound statements?</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>Conditional statements often involve compound
statements</FONT><FONT SIZE=5 FACE="Lucida Console">if (test)
thenstatement else elsestatement</FONT>
<LI><FONT SIZE=6 FACE="Lucida Console">test</FONT><FONT SIZE=6>
is an expression, but </FONT><FONT SIZE=6 FACE="Lucida Console">thenstatement</FONT><FONT SIZE=6>
and </FONT><FONT SIZE=6 FACE="Lucida Console">elsestatement</FONT><FONT SIZE=6>
can be any statement</FONT>
<LI><FONT SIZE=6>It is good practice to use compound statements
with conditional statements - it makes your code much easier to
read</FONT>
<LI><FONT SIZE=6>Where else have we seen compound statements?</FONT>
<UL>
<LI><FONT SIZE=6>Hint: in this case, they usually involve declarations</FONT>
</UL>
</UL>
<P>
<CENTER><B><FONT SIZE=6>Fun with else</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>You don't actually need to use </FONT><FONT SIZE=6 FACE="Lucida Console">else</FONT><FONT SIZE=6>
with </FONT><FONT SIZE=6 FACE="Lucida Console">if</FONT><FONT SIZE=5 FACE="Lucida Console">if
(x != 0) y = 1/x;printf("%d",42);</FONT>
</UL>
<P>
<FONT SIZE=5 FACE="Lucida Console"></FONT>
<UL>
<LI><FONT SIZE=6>This can lead to totally unreadable code, especially
if you nest your </FONT><FONT SIZE=6 FACE="Lucida Console">if</FONT><FONT SIZE=6>
statements. Don't do this!</FONT><FONT SIZE=5 FACE="Lucida Console">if
(x != 0) if (y != 0) z = x/y;else z = -1;</FONT>
</UL>
<P>
<FONT SIZE=5 FACE="Lucida Console"></FONT>
<UL>
<LI><FONT SIZE=6>There is a reasonable way of doing multiway conditional
statements</FONT><FONT SIZE=5 FACE="Lucida Console">if (test1)
statement1else if (test2) statement2else if (test3) statement3…else
defaultstatement</FONT>
<LI><FONT SIZE=6>I don't personally find this incredibly readable,
but sometimes it's necessary</FONT>
</UL>
<P>
<CENTER><B><FONT SIZE=6>Iteration</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>Today we'll mostly cover the syntax of iteration
in C</FONT>
<UL>
<LI><FONT SIZE=6>We'll do some examples of its use in section</FONT>
</UL>
<LI><FONT SIZE=6>Iteration involves executing some part of your
program repeatedly (as does recursion!)</FONT>
<LI><FONT SIZE=6>The easiest way to do iteration is with </FONT><FONT SIZE=6 FACE="Lucida Console">while</FONT><FONT SIZE=5 FACE="Lucida Console">while
(expression) statement</FONT>
<LI><FONT SIZE=6>Typically, </FONT><FONT SIZE=6 FACE="Lucida Console">statement</FONT><FONT SIZE=6>
is a compound statement</FONT>
<LI><FONT SIZE=6>semantics of </FONT><FONT SIZE=6 FACE="Lucida Console">while</FONT><FONT SIZE=6>:</FONT>
<UL>
<LI><FONT SIZE=6>evaluate expression. </FONT>
<UL>
<LI><FONT SIZE=6>If it is false (zero), then we're done with the
while statement.</FONT>
<LI><FONT SIZE=6>If it is true (non-zero), then evaluate statement,
<I>then evaluate the while statement again</I></FONT>
</UL>
</UL>
</UL>
<P>
<FONT SIZE=6></FONT>
<P>
<CENTER><B><FONT SIZE=6>Example</FONT></B></CENTER><FONT SIZE=5 FACE="Lucida Console">void
countdown(int count){ while (count > 0) {</FONT><FONT SIZE=6>printf("%d…",count);
count = count - 1;} printf("Blastoff!");}<BR>
void main(){ start = 10; countdown(start); printf("%d",start);}
<BR>
</FONT>
<UL>
<LI><FONT SIZE=6>This example also shows off a subtle but important
property of the stack model</FONT>
<LI><FONT SIZE=6>Note that if the test expression is never 0,
your while statement will run forever</FONT>
<LI><FONT SIZE=6>How do we get out of a </FONT><FONT SIZE=6 FACE="Lucida Console">while</FONT><FONT SIZE=6>
statement early?</FONT><FONT SIZE=5 FACE="Lucida Console"> while
(count > 0) {</FONT><FONT SIZE=6>printf("%d…",count);if
(NASA_goofed()) return; count = count - 1;}</FONT>
</UL>
<P>
<CENTER><B><FONT SIZE=6>Break statements</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>Suppose that instead of getting out of the entire
countdown function we merely want to get out of the while statement</FONT>
<UL>
<LI><FONT SIZE=6>NASA didn't goof, the astronauts got impatient
and decided to blast off before the countdown finished</FONT>
</UL>
<LI><FONT SIZE=6>There is a good way and a bad way. We're only
going to teach you the good way.</FONT>
<LI><FONT SIZE=6>The </FONT><FONT SIZE=6 FACE="Lucida Console">break</FONT><FONT SIZE=6>
statement terminates a </FONT><FONT SIZE=6 FACE="Lucida Console">while</FONT><FONT SIZE=6>
statement</FONT><FONT SIZE=5 FACE="Lucida Console">void countdown(int
count){ while (count > 0) {</FONT><FONT SIZE=6>printf("%d…",count);
count = count - 1;if (astronauts_bored(count)) break;} printf("Blastoff!");}</FONT>
</UL>
<P>
<CENTER><B><FONT SIZE=6>What about nested while statements?</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>Again, this allows unreadable code to be created</FONT>
<UL>
<LI><FONT SIZE=6>The world has enough of this without you folks
contributing!</FONT>
</UL>
<LI><FONT SIZE=6>Small example:</FONT><FONT SIZE=5 FACE="Lucida Console">while(x
> 0)</FONT><FONT SIZE=6>{ x = x - 1; while (y < 0) {
if (abs(x-y) < 10) break; } if (test(z)) break; }</FONT>
<LI><FONT SIZE=6>The rule is that a </FONT><FONT SIZE=6 FACE="Lucida Console">break</FONT><FONT SIZE=6>
statement terminates the smallest </FONT><FONT SIZE=6 FACE="Lucida Console">while</FONT><FONT SIZE=6>
statement that encloses it</FONT>
</UL>
<P>
<CENTER><B><FONT SIZE=6>Iteration and the stack model</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>As far as the stack model is concerned, iteration
primitives aren't much different than conditional statements.</FONT>
<UL>
<LI><FONT SIZE=6>They simply manipulate the program counter</FONT>
<LI><FONT SIZE=6>These primitives are sometimes called Control
Flow primitives</FONT>
</UL>
</UL>
<P>
<CENTER><B><FONT SIZE=6>Fencepost errors</FONT></B></CENTER>
<UL>
<LI><FONT SIZE=6>With iteration (and also, with recursion) it
becomes very easy to make fencepost errors</FONT>
<LI><FONT SIZE=6>Suppose you want to build a 10' fence, and you
need to install posts which are 1' apart</FONT>
<UL>
<LI><FONT SIZE=6>How many fenceposts do you need?</FONT><FONT SIZE=5 FACE="Lucida Console">void
nastybug(){</FONT><FONT SIZE=6>int count;count = 10;while (count
> 0) { count = count - 1;something(1/count); }}</FONT>
</UL>
</UL>
<P>
<FONT SIZE=6></FONT>
<UL>
<LI><FONT SIZE=6>This kind of bug is hard to find, in general</FONT>
</UL>
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