ch07.htm

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<TT>small</TT> is not bigger than <TT>large</TT>.<BR>
<BR>
<TT>large</TT> isn't negative.<BR>
<BR>
<TT>small</TT> doesn't overrun the size of a small integer (<TT>MAXSMALL</TT>).<BR>
<BR>
On line 23, the value in <TT>small</TT> is calculated modulo 5,000. This does not
change the value in <TT>small</TT>; however, it only returns the value <TT>0</TT>
when <TT>small</TT> is an exact multiple of 5,000. Each time it is, a dot (<TT>.</TT>)
is printed to the screen to show progress. On line 26, <TT>small</TT> is incremented,
and on line 28, <TT>large</TT> is decremented by 2.<BR>
When any of the three conditions in the <TT>while</TT> loop fails, the loop ends
and execution of the program continues after the <TT>while</TT> loop's closing brace
on line 29.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>The modulus operator (%) and compound
	conditions are covered on Day 3, &quot;Variables and Constants.&quot; 
<HR>


</BLOCKQUOTE>

<H4 ALIGN="CENTER"><A NAME="Heading16"></A><FONT COLOR="#000077">continue and break</FONT></H4>
<P>At times you'll want to return to the top of a <TT>while</TT> loop before the
entire set of statements in the <TT>while</TT> loop is executed. The <TT>continue</TT>
statement jumps back to the top of the loop.</P>
<P>At other times, you may want to exit the loop before the exit conditions are met.
The <TT>break</TT> statement immediately exits the <TT>while</TT> loop, and program
execution resumes after the closing brace.</P>
<P>Listing 7.4 demonstrates the use of these statements. This time the game has become
more complicated. The user is invited to enter a <TT>small</TT> number and a <TT>large</TT>
number, a <TT>skip</TT> number, and a <TT>target</TT> number. The <TT>small</TT>
number will be incremented by one, and the <TT>large</TT> number will be decremented
by 2. The decrement will be skipped each time the <TT>small</TT> number is a multiple
of the <TT>skip</TT>. The game ends if <TT>small</TT> becomes larger than <TT>large</TT>.
If the <TT>large</TT> number reaches the <TT>target</TT> exactly, a statement is
printed and the game stops.</P>
<P>The user's goal is to put in a target number for the <TT>large</TT> number that
will stop the game.</P>
<P><A NAME="Heading17"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 7.4. break and
continue.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1:    // Listing 7.4
2:    // Demonstrates break and continue
3:
4:    #include &lt;iostream.h&gt;
5:
6:    int main()
7:    {
8:      unsigned short small;
9:      unsigned long  large;
10:      unsigned long  skip;
11:      unsigned long target;
12:      const unsigned short MAXSMALL=65535;
13:
14:      cout &lt;&lt; &quot;Enter a small number: &quot;;
15:      cin &gt;&gt; small;
16:      cout &lt;&lt; &quot;Enter a large number: &quot;;
17:      cin &gt;&gt; large;
18:      cout &lt;&lt; &quot;Enter a skip number: &quot;;
19:      cin &gt;&gt; skip;
20:      cout &lt;&lt; &quot;Enter a target number: &quot;;
21:      cin &gt;&gt; target;
22:
23:    cout &lt;&lt; &quot;\n&quot;;
24:
25:     // set up 3 stop conditions for the loop
26:      while (small &lt; large &amp;&amp; large &gt; 0 &amp;&amp; small &lt; 65535)
27:
28:      {
29:
30:        small++;
31:
32:         if (small % skip == 0)  // skip the decrement?
33:         {
34:           cout &lt;&lt; &quot;skipping on &quot; &lt;&lt; small &lt;&lt; endl;
35:           continue;
36:         }
37:
38:         if (large == target)    // exact match for the target?
39:         {
40:           cout &lt;&lt; &quot;Target reached!&quot;;
41:           break;
42:         }
43:
44:         large-=2;
45:      }                   // end of while loop
46:
47:      cout &lt;&lt; &quot;\nSmall: &quot; &lt;&lt; small &lt;&lt; &quot; Large: &quot; &lt;&lt; large &lt;&lt; endl;
48:    return 0;
<TT>49: }</TT>
Output: Enter a small number: 2
Enter a large number: 20
Enter a skip number: 4
Enter a target number: 6
skipping on 4
skipping on 8

Small: 10 Large: 8
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis: </B></FONT>In this play, the user lost; <TT>small</TT>
became larger than <TT>large</TT> before the <TT>target</TT> number of 6 was reached.<BR>
On line 26, the <TT>while</TT> conditions are tested. If <TT>small</TT> continues
to be smaller than <TT>large</TT>, <TT>large</TT> is larger than 0, and <TT>small</TT>
hasn't overrun the maximum value for a small <TT>int</TT>, the body of the <TT>while</TT>
loop is entered.</P>
<P>On line 32, the <TT>small</TT> value is taken modulo the <TT>skip</TT> value.
If <TT>small</TT> is a multiple of <TT>skip</TT>, the <TT>continue</TT> statement
is reached and program execution jumps to the top of the loop at line 26. This effectively
skips over the test for the <TT>target</TT> and the decrement of <TT>large</TT>.</P>
<P>On line 38, <TT>target</TT> is tested against the value for <TT>large</TT>. If
they are the same, the user has won. A message is printed and the <TT>break</TT>
statement is reached. This causes an immediate break out of the <TT>while</TT> loop,
and program execution resumes on line 46.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>Both <TT>continue</TT> and <TT>break</TT>
	should be used with caution. They are the next most dangerous commands after <TT>goto</TT>,
	for much the same reason. Programs that suddenly change direction are harder to understand,
	and liberal use of <TT>continue</TT> and <TT>break</TT> can render even a small <TT>while</TT>
	loop unreadable. 
<HR>


</BLOCKQUOTE>

<H3 ALIGN="CENTER"><A NAME="Heading19"></A><FONT COLOR="#000077">The continue Statement</FONT></H3>
<P><TT>continue;</TT> causes a <TT>while</TT> or <TT>for</TT> loop to begin again
at the top of the loop. Example</P>
<PRE><FONT COLOR="#0066FF">if (value &gt; 10)
     goto end;

if (value &lt; 10)
     goto end;

cout &lt;&lt; &quot;value is 10!&quot;;

end:

cout &lt;&lt; &quot;done&quot;;
</FONT></PRE>
<H3 ALIGN="CENTER"><A NAME="Heading20"></A><FONT COLOR="#000077">The break Statement</FONT></H3>
<P><TT>break;</TT> causes the immediate end of a <TT>while</TT> or <TT>for</TT> loop.
Execution jumps to the closing brace. Example</P>
<PRE><FONT COLOR="#0066FF">while (condition)
{
    if (condition2)
        break;
    // statements;
}
</FONT></PRE>
<H4 ALIGN="CENTER"><A NAME="Heading21"></A><FONT COLOR="#000077">while (1) Loops</FONT></H4>
<P>The condition tested in a <TT>while</TT> loop can be any valid C++ expression.
As long as that condition remains true, the <TT>while</TT> loop will continue. You
can create a loop that will never end by using the number 1 for the condition to
be tested. Since 1 is always true, the loop will never end, unless a <TT>break</TT>
statement is reached. Listing 7.5 demonstrates counting to 10 using this construct.</P>
<P><A NAME="Heading22"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 7.5. while (1)
loops.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1:    // Listing 7.5
2:    // Demonstrates a while true loop
3:
4:    #include &lt;iostream.h&gt;
5:
6:    int main()
7:    {
8:      int counter = 0;
9:
10:      while (1)
11:      {
12:         counter ++;
13:         if (counter &gt; 10)
14:             break;
15:      }
16:      cout &lt;&lt; &quot;Counter: &quot; &lt;&lt; counter &lt;&lt; &quot;\n&quot;;
17:       return 0;
<TT>18: </TT>
Output: Counter: 11
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On line 10, a <TT>while</TT>
loop is set up with a condition that can never be false. The loop increments the
<TT>counter</TT> variable on line 12 and then on line 13 tests to see whether <TT>counter</TT>
has gone past 10. If it hasn't, the <TT>while</TT> loop iterates. If <TT>counter</TT>
is greater than 10, the <TT>break</TT> on line 14 ends the <TT>while</TT> loop, and
program execution falls through to line 16, where the results are printed.<BR>
This program works, but it isn't pretty. This is a good example of using the wrong
tool for the job. The same thing can be accomplished by putting the test of <TT>counter</TT>'s
value where it belongs--in the <TT>while</TT> condition.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>WARNING:</B></FONT><B> </B>Eternal loops such as <TT>while
	(1)</TT> can cause your computer to hang if the exit condition is never reached.
	Use these with caution and test them thoroughly. 
<HR>


</BLOCKQUOTE>

<P>C++ gives you many different ways to accomplish the same task. The real trick
is picking the right tool for the particular job.


<BLOCKQUOTE>
	<P>
<HR>
<B>DON'T</B> use the <TT>goto</TT> statement. <B>DO</B> use <TT>while</TT> loops
	to iterate while a condition is true. <B>DO</B> exercise caution when using <TT>continue</TT>
	and <TT>break </TT>statements. <B>DO</B> make sure your loop will eventually end.
	
<HR>


</BLOCKQUOTE>

<H3 ALIGN="CENTER"><A NAME="Heading24"></A><FONT COLOR="#000077">do...while Loops</FONT></H3>
<P>It is possible that the body of a <TT>while</TT> loop will never execute. The
<TT>while</TT> statement checks its condition before executing any of its statements,
and if the condition evaluates <TT>false</TT>, the entire body of the <TT>while</TT>
loop is skipped. Listing 7.6 illustrates this.</P>
<P><A NAME="Heading25"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 7.6. Skipping
the body of the while Loop</B></FONT><FONT SIZE="2" COLOR="#000077"><B>.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1:     // Listing 7.6
2:      // Demonstrates skipping the body of
3:      // the while loop when the condition is false.
4:
5:      #include &lt;iostream.h&gt;
6:
7:      int main()
8:      {
9:         int counter;
10:        cout &lt;&lt; &quot;How many hellos?: &quot;;
11:        cin &gt;&gt; counter;
12:        while (counter &gt; 0)
13:        {
14:           cout &lt;&lt; &quot;Hello!\n&quot;;
15:           counter--;
16:        }
17:        cout &lt;&lt; &quot;Counter is OutPut: &quot; &lt;&lt; counter;
18:         return 0;
<TT>19: }</TT>
Output: How many hellos?: 2
Hello!
Hello!
Counter is OutPut: 0

How many hellos?: 0
Counter is OutPut: 0
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>The user is prompted for
a starting value on line 10. This starting value is stored in the integer variable
<TT>counter</TT>. The value of <TT>counter</TT> is tested on line 12, and decremented
in the body of the <TT>while</TT> loop. The first time through <TT>counter</TT> was
set to <TT>2</TT>, and so the body of the <TT>while</TT> loop ran twice. The second
time through, however, the user typed in <TT>0</TT>. The value of <TT>counter</TT>
was tested on line 12 and the condition was false; <TT>counter</TT> was not greater
than <TT>0</TT>. The entire body of the <TT>while</TT> loop was skipped, and <TT>Hello</TT>
was never printed.<BR>
What if you want to ensure that <TT>Hello</TT> is always printed at least once? The
<TT>while</TT> loop can't accomplish this, because the <TT>if</TT> condition is tested
before any printing is done. You can force the issue with an <TT>if</TT> statement
just before entering the <TT>while</TT>:</P>
<PRE><FONT COLOR="#0066FF">if (counter &lt; 1)  // force a minimum value
counter = 1;</FONT></PRE>
<P>but that is what programmers call a &quot;kludge,&quot; an ugly and inelegant
solution.
<H3 ALIGN="CENTER"><A NAME="Heading27"></A><FONT COLOR="#000077">do...while</FONT></H3>
<P>The <TT>do...while</TT> loop executes the body of the loop before its condition
is tested and ensures that the body always executes at least one time. Listing 7.7
rewrites Listing 7.6, this time using a <TT>do...while</TT> loop.</P>
<P><A NAME="Heading28"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 7.7. Demonstrates
do...while loop.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1:      // Listing 7.7
2:      // Demonstrates do while
3:
4:      #include &lt;iostream.h&gt;
5:
6:      int main()
7:      {
8:         int counter;
9:         cout &lt;&lt; &quot;How many hellos? &quot;;
10:        cin &gt;&gt; counter;
11:        do
12:        {
13:           cout &lt;&lt; &quot;Hello\n&quot;;
14:           counter--;
15:        }  while (counter &gt;0 );
16:        cout &lt;&lt; &quot;Counter is: &quot; &lt;&lt; counter &lt;&lt; endl;
17:         return 0;
<TT>18: }</TT></FONT>
<FONT COLOR="#0066FF">
Output: How many hellos? 2
Hello
Hello
Counter is: 0
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>The user is prompted for
a starting value on line 9, which is stored in the integer variable <TT>counter</TT>.