ch03.htm

Why C++ is the emerging standard in software development. The steps to develop a C++ program. How

	<P>
<HR>
<B>DON'T </B>use the term <TT>int</TT>. Use <TT>short</TT> and <TT>long</TT> to make
	it clear which size number you intended. <B>DO</B> watch for numbers overrunning
	the size of the integer and wrapping around incorrect values. <B>DO</B> give your
	variables meaningful names that reflect their use. <B>DON'T </B>use keywords as variable
	names. 
<HR>


</BLOCKQUOTE>

<CENTER>
<H3><A NAME="Heading37"></A><FONT COLOR="#000077">Enumerated Constants</FONT></H3>
</CENTER>
<P>Enumerated constants enable you to create new types and then to define variables
of those types whose values are restricted to a set of possible values. For example,
you can declare <TT>COLOR</TT> to be an enumeration, and you can define that there
are five values for <TT>COLOR</TT>: <TT>RED</TT>, <TT>BLUE</TT>, <TT>GREEN</TT>,
<TT>WHITE</TT>, and <TT>BLACK</TT>.</P>
<P>The syntax for enumerated constants is to write the keyword <TT>enum</TT>, followed
by the type name, an open brace, each of the legal values separated by a comma, and
finally a closing brace and a semicolon. Here's an example:</P>
<PRE><FONT COLOR="#0066FF">enum COLOR { RED, BLUE, GREEN, WHITE, BLACK };
</FONT></PRE>
<P>This statement performs two tasks:

<DL>
	<DD><B>1.</B> It makes <TT>COLOR</TT> the name of an enumeration, that is, a new
	type.<BR>
	<BR>
	<B>2.</B> It makes <TT>RED</TT> a symbolic constant with the value <TT>0</TT>, <TT>BLUE</TT>
	a symbolic constant with the value <TT>1</TT>, <TT>GREEN</TT> a symbolic constant
	with the value <TT>2</TT>, and so forth.
</DL>

<P>Every enumerated constant has an integer value. If you don't specify otherwise,
the first constant will have the value <TT>0</TT>, and the rest will count up from
there. Any one of the constants can be initialized with a particular value, however,
and those that are not initialized will count upward from the ones before them. Thus,
if you write</P>
<PRE><FONT COLOR="#0066FF">enum Color { RED=100, BLUE, GREEN=500, WHITE, BLACK=700 };
</FONT></PRE>
<P>then <TT>RED</TT> will have the value <TT>100</TT>; <TT>BLUE</TT>, the value <TT>101</TT>;
<TT>GREEN</TT>, the value <TT>500</TT>; <TT>WHITE</TT>, the value <TT>501</TT>; and
<TT>BLACK</TT>, the value <TT>700</TT>.</P>
<P>You can define variables of type <TT>COLOR</TT>, but they can be assigned only
one of the enumerated values (in this case, <TT>RED</TT>, <TT>BLUE</TT>, <TT>GREEN</TT>,
<TT>WHITE</TT>, or <TT>BLACK</TT>, or else <TT>100</TT>, <TT>101</TT>, <TT>500</TT>,
<TT>501</TT>, or <TT>700</TT>). You can assign any color value to your <TT>COLOR</TT>
variable. In fact, you can assign any integer value, even if it is not a legal color,
although a good compiler will issue a warning if you do. It is important to realize
that enumerator variables actually are of type <TT>unsigned int</TT>, and that the
enumerated constants equate to integer variables. It is, however, very convenient
to be able to name these values when working with colors, days of the week, or similar
sets of values. Listing 3.7 presents a program that uses an enumerated type.</P>
<P><A NAME="Heading38"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 3.7. A demonstration
of enumerated constants</B></FONT><FONT SIZE="2" COLOR="#000077"><B>.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1:  #include &lt;iostream.h&gt;
2:  int main()
3:  {
4:       enum Days { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday,                     &#194;_Saturday };
5:
6:       Days DayOff;
7:       int x;
8:
9:       cout &lt;&lt; &quot;What day would you like off (0-6)? &quot;;
10:      cin  &gt;&gt; x;
11:      DayOff = Days(x);
12:
13:      if (DayOff == Sunday || DayOff == Saturday)
14:            cout &lt;&lt; &quot;\nYou're already off on weekends!\n&quot;;
15:      else
16:            cout &lt;&lt; &quot;\nOkay, I'll put in the vacation day.\n&quot;;
17:       return 0;
<TT>18: }</TT>
Output: What day would you like off (0-6)?  1

Okay, I'll put in the vacation day.

What day would you like off (0-6)?  0

You're already off on weekends!
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On line 4, the enumerated
constant <TT>DAYS</TT> is defined, with seven values counting upward from 0. The
user is prompted for a day on line 9. The chosen value, a number between 0 and 6,
is compared on line 13 to the enumerated values for Sunday and Saturday, and action
is taken accordingly. <BR>
<BR>
The <TT>if</TT> statement will be covered in more detail on Day 4, &quot;Expressions
and Statements.&quot;</P>
<P>You cannot type the word &quot;Sunday&quot; when prompted for a day; the program
does not know how to translate the characters in <TT>Sunday</TT> into one of the
enumerated values.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>For this and all the small programs
	in this book, I've left out all the code you would normally write to deal with what
	happens when the user types inappropriate data. For example, this program doesn't
	check, as it would in a real program, to make sure that the user types a number between
	0 and 6. This detail has been left out to keep these programs small and simple, and
	to focus on the issue at hand. 
<HR>


</BLOCKQUOTE>

<CENTER>
<H3><A NAME="Heading40"></A><FONT COLOR="#000077">Summary</FONT></H3>
</CENTER>
<P>This chapter has discussed numeric and character variables and constants, which
are used by C++ to store data during the execution of your program. Numeric variables
are either integral (<TT>char</TT>, <TT>short</TT>, and <TT>long int</TT>) or they
are floating point (<TT>float</TT> and <TT>double</TT>). Numeric variables can also
be <TT>signed</TT> or <TT>unsigned</TT>. Although all the types can be of various
sizes among different computers, the type specifies an exact size on any given computer.</P>
<P>You must declare a variable before it can be used, and then you must store the
type of data that you've declared as correct for that variable. If you put too large
a number into an integral variable, it wraps around and produces an incorrect result.</P>
<P>This chapter also reviewed literal and symbolic constants, as well as enumerated
constants, and showed two ways to declare a symbolic constant: using <TT>#define</TT>
and using the keyword <TT>const</TT>.
<CENTER>
<H3><A NAME="Heading41"></A><FONT COLOR="#000077">Q&amp;A</FONT></H3>
</CENTER>

<DL>
	<DD><B>Q. If a short int can run out of room and wrap around, why not always use
	long integers?<BR>
	</B><BR>
	<B>A .</B>Both <TT>short</TT> integers and <TT>long</TT> integers will run out of
	room and wrap around, but a <TT>long</TT> integer will do so with a much larger number.
	For example, an <TT>unsigned short int</TT> will wrap around after 65,535, whereas
	an <TT>unsigned long int</TT> will not wrap around until 4,294,967,295. However,
	on most machines, a <TT>long</TT> integer takes up twice as much memory every time
	you declare one (4 bytes versus 2 bytes), and a program with 100 such variables will
	consume an extra 200 bytes of RAM. Frankly, this is less of a problem than it used
	to be, because most personal computers now come with many thousands (if not millions)
	of bytes of memory.<BR>
	<BR>
	<B>Q. What happens if I assign a number with a decimal point to an integer rather
	than to a float? Consider the following line of code:</B>
</DL>

<PRE><FONT COLOR="#0066FF">int aNumber = 5.4;
</FONT></PRE>

<DL>
	<DD><B>A.</B> A good compiler will issue a warning, but the assignment is completely
	legal. The number you've assigned will be truncated into an integer. Thus, if you
	assign <TT>5.4</TT> to an integer variable, that variable will have the value <TT>5</TT>.
	Information will be lost, however, and if you then try to assign the value in that
	integer variable to a <TT>float</TT> variable, the <TT>float</TT> variable will have
	only <TT>5</TT>.<BR>
	<BR>
	<B>Q. Why not use literal constants; why go to the bother of using symbolic constants?<BR>
	</B><BR>
	<B>A.</B> If you use the value in many places throughout your program, a symbolic
	constant allows all the values to change just by changing the one definition of the
	constant. Symbolic constants also speak for themselves. It might be hard to understand
	why a number is being multiplied by 360, but it's much easier to understand what's
	going on if the number is being multiplied by <TT>degreesInACircle</TT>.<BR>
	<BR>
	<B>Q. What happens if I assign a negative number to an unsigned variable? Consider
	the following line of code:</B>
</DL>

<PRE><FONT COLOR="#0066FF">unsigned int aPositiveNumber = -1;
</FONT></PRE>

<DL>
	<DD><B>A.</B> A good compiler will warn, but the assignment is legal. The negative
	number will be assessed as a bit pattern and assigned to the variable. The value
	of that variable will then be interpreted as an <TT>unsigned</TT> number. Thus, -1,
	whose bit pattern is <TT>11111111 11111111</TT> (<TT>0xFF</TT> in hex), will be assessed
	as the <TT>unsigned</TT> value <TT>65,535</TT>. If this information confuses you,
	refer to Appendix C.<BR>
	<BR>
	<B>Q. Can I work with C++ without understanding bit patterns, binary arithmetic,
	and hexadecimal?</B><BR>
	<BR>
	<B>A.</B> Yes, but not as effectively as if you do understand these topics. C++ does
	not do as good a job as some languages at &quot;protecting&quot; you from what the
	computer is really doing. This is actually a benefit, because it provides you with
	tremendous power that other languages don't. As with any power tool, however, to
	get the most out of C++ you must understand how it works. Programmers who try to
	program in C++ without understanding the fundamentals of the binary system often
	are confused by their results.
</DL>

<CENTER>
<H3><A NAME="Heading42"></A><FONT COLOR="#000077">Workshop</FONT></H3>
</CENTER>
<P>The Workshop provides quiz questions to help you solidify your understanding of
the material covered, and exercises to provide you with experience in using what
you've learned. Try to answer the quiz and exercise questions before checking the
answers in Appendix D, and make sure that you understand the answers before continuing
to the next chapter.
<CENTER>
<H4><A NAME="Heading43"></A><FONT COLOR="#000077">Quiz</FONT></H4>
</CENTER>

<DL>
	<DD><B>1.</B> What is the difference between an integral variable and a floating-point
	variable?<BR>
	<BR>
	<B>2.</B> What are the differences between an <TT>unsigned short int</TT> and a <TT>long
	int</TT>?<BR>
	<BR>
	<B>3.</B> What are the advantages of using a symbolic constant rather than a literal
	constant?<BR>
	<BR>
	<B>4.</B> What are the advantages of using the <TT>const</TT> keyword rather than
	<TT>#define</TT>?<BR>
	<BR>
	<B>5.</B> What makes for a good or bad variable name?<BR>
	<BR>
	<B>6.</B> Given this <TT>enum</TT>, what is the value of <TT>BLUE</TT>?
</DL>

<PRE><FONT COLOR="#0066FF">
enum COLOR { WHITE, BLACK = 100, RED, BLUE, GREEN = 300 };
</FONT></PRE>

<DL>
	<DD><B>7.</B> Which of the following variable names are good, which are bad, and
	which are invalid?
	<DL>
		<DD><B><BR>
		a.</B> <TT>Age</TT><BR>
		<B><BR>
		b.</B> <TT>!ex</TT><BR>
		<B><BR>
		c.</B><TT> R79J</TT><BR>
		<B><BR>
		d.</B><TT> TotalIncome<BR>
		</TT><B><BR>
		e.</B> <TT>__Invalid</TT>
	</DL>
</DL>

<CENTER>
<H4><A NAME="Heading44"></A><FONT COLOR="#000077">Exercises</FONT></H4>
</CENTER>

<DL>
	<DD><B>1.</B> What would be the correct variable type in which to store the following
	information?
	<DL>
		<DD><BR>
		<B>a.</B> Your age.<BR>
		<BR>
		<B>b.</B> The area of your backyard.<BR>
		<BR>
		<B>c.</B> The number of stars in the galaxy.<BR>
		<BR>
		<B>d.</B> The average rainfall for the month of January.
	</DL>
	<DD><BR>
	<B>2.</B> Create good variable names for this information.<BR>
	<BR>
	<B>3.</B> Declare a constant for pi as 3.14159.<BR>
	<BR>
	<B>4.</B> Declare a <TT>float</TT> variable and initialize it using your pi constant.<BR>
	
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