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这是一本关于VC++自学教程的书籍 对于初学者以及编程有一定基础的人均有帮助

24: 
25: double  Area (double base, double height)    // Function definition
26: {
27: area = (0.5*base*height);
28: return area;
29: }
</PRE>
<P>This program declares three variables, base, height, and area, on line 8. Variables
store values that are used by the program. The type of a variable specifies the values
to be stored in the variable. Table A.1 shows the various types supported by C++.</P>
<P>
<H4>TABLE A.1. VARIABLE DATA TYPES.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Variable Data Type</I></TD>
		<TD ALIGN="LEFT"><I>Values</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">unsigned short int</TD>
		<TD ALIGN="LEFT">0 to 65,535</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">short int</TD>
		<TD ALIGN="LEFT">-32,768 to 32,767</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">unsigned long int</TD>
		<TD ALIGN="LEFT">0 to 4,294,967,925</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">long int</TD>
		<TD ALIGN="LEFT">-2,147,483,648 to 2,147,483,647</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">int</TD>
		<TD ALIGN="LEFT">-2,147,483,648 to 2,147,483,647 (32 bit)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">unsigned int</TD>
		<TD ALIGN="LEFT">0 to 4,294,967,295 (32 bit)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">char</TD>
		<TD ALIGN="LEFT">256 character values</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">float</TD>
		<TD ALIGN="LEFT">1.2e-38 to 3.4e38</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">double</TD>
		<TD ALIGN="LEFT">2.2e-308 to 1.8e308</TD>
	</TR>
</TABLE>
</P>
<P>To define a variable, you first define its type, followed by the name. You may
also assign values to variables by using the assignment (=) operator, as in these
two examples:</P>
<P>
<PRE>double base = 5;
unsigned long int base =5;
</PRE>
<P>In C++, you may also define your own type definition. You do this by using the
keyword typedef, followed by the existing type and name:</P>
<P>
<PRE>typedef unsigned long int ULONG;
ULONG base =5;
</PRE>
<P>Defining your own type does save you the trouble of typing the entire declaration.</P>
<P>The next line of the code, line 9, defines the prototype of your function:</P>
<P>
<PRE>double Area (double,double);
</PRE>
<P>This function has a type double, a name Area, and a parameter list of two variables
of type double. When you define the prototype, it is not necessary to define the
parameters, but it is a good practice to do so. This program takes two inputs from
the user, namely base and height of the triangle, and calculates the area of the
triangle. The base, height, and area are all variables. The Helloworld.cpp example
used the insertion (&lt;&lt;) operator. In this example, you use the extraction (&gt;&gt;)
operator. The program queries the user to enter a value for the height of the triangle
on line 13. When the user enters a value for height, the data from the screen is
extracted and placed into the variable height. The process is repeated for the base
of the triangle on lines 15 and 16. After accepting the input from the user, the
function main() passes execution to the function Area(base,height) along with the
parameter values for base and height. When main()<I> </I>passes the execution to
the function Area(base, height), it expects a value of type double in return from
the function. The calculation of the area of the triangle is conducted on line 27:</P>
<P>
<PRE>area = (0.5*base*height);
</PRE>


<BLOCKQUOTE>
	<P>
<HR>
<STRONG>NOTE:</STRONG> Area is the name of a function, and area is a variable name.
	Because C++ is case sensitive, it clearly distinguishes these two names. 
<HR>


</BLOCKQUOTE>

<P>This statement uses the standard operators, the assignment operator (=), and the
multiplication operator (*). The assignment operator assigns the result of (0.5*base*height)
to the variable area. The multiplication operator (*) calculates the resulting values
of (0.5*base*height). The assignment operator (=) has an evaluation order from right-to-left.
Hence, the multiplication is carried out prior to assigning the values to area. The
five basic mathematical operators are addition (+), subtraction (-), multiplication
(*), division (/), and modulus (%).</P>
<P>Line 28 of the Area function returns the value of the variable area to the main()
function. At this point, the control of the program is returned to line 18 of the
main() function. The remainder of the program displays the result of area to the
screen.</P>
<P>
<H3><A NAME="Heading3"></A>The if Statement, Operators, and Polymorphism</H3>
<PRE>While programming large complex programs, it is often necessary to query the user and provide direction to the program based on his input. This is accomplished by using the if statement. The next example demonstrates the application of an if statement. The format of the if statement is
</PRE>
<PRE>if (<I>this expression</I>)
   <I>do this</I>;
</PRE>
<P>The if statement is often used in conjunction with relational operators. Another
format of the if statement is</P>
<P>
<PRE>if (<I>this expression</I>)
   <I>do this</I>;
else
   <I>do this</I>;
</PRE>
<P>Because if statements often use relational operators, let's review relational
operators. Relational operators are used to determine if two expressions or numbers
are equal. If the two expressions or numbers are not equal, the statement will evaluate
to either 0 or false. Table A.2 lists the six relational operators defined in C++.</P>
<P>
<H4>TABLE A.2. RELATIONAL OPERATORS.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Operator</I></TD>
		<TD ALIGN="LEFT"><I>Name</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">==</TD>
		<TD ALIGN="LEFT">Comparative</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">!=</TD>
		<TD ALIGN="LEFT">Not equal</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&gt;</TD>
		<TD ALIGN="LEFT">Greater than</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&lt;</TD>
		<TD ALIGN="LEFT">Less than</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&gt;=</TD>
		<TD ALIGN="LEFT">Greater than or equal to</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&lt;=</TD>
		<TD ALIGN="LEFT">Less than or equal to</TD>
	</TR>
</TABLE>
</P>
<P>C++ also has logical operators. The advantage of logical operators is the ability
to compare two individual expressions and conclude whether they are true or false.
Table A.3 lists the three logical operators.</P>
<P>
<H4>TABLE A.3. LOGICAL OPERATORS.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Symbol</I></TD>
		<TD ALIGN="LEFT"><I>Operator</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&amp;&amp;</TD>
		<TD ALIGN="LEFT">AND</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">||</TD>
		<TD ALIGN="LEFT">OR</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">!</TD>
		<TD ALIGN="LEFT">NOT</TD>
	</TR>
</TABLE>
</P>
<P>An important and powerful feature of C++ is function overloading, or polymorphism.
<I>Polymorphism</I> is the ability to have more than one function with the same name
that differ in their parameter lists. The next example is an extension of the previous
triangle code. In this program, you will calculate the area of a triangle and a circle.
You will be asked whether you want to calculate the area of a triangle or a circle.
Depending upon your response, 1 for triangle and 2 for circle, the program collects
your input and calculates the area. In Listing A.3, the Area function is overloaded.
The same function name is used to calculate the area of the triangle or the circle.
The functions differ only in their parameter lists.</P>
<P>
<H4>LISTING A.3. Overload.ccp.</H4>
<PRE> 1: // Workspace Name:  Overload
 2: // Program Name:  Overload.cpp
 3: 
 4: # include &lt;iostream.h&gt;
 5: 
 6: double  base,height,radius;        // Global variables
 7: double  Area_of_triangle,Area_of_circle;  // Global variables
 8: int  choice;                // Global variable
 9: 
10: double  Area (double,double);       // Function prototype
11: double  Area (double);           // Function prototype
12: 
13: const double  pi = 3.14;          // Constant variable
14: 
15: void main()                 // main function
16: 
17: {
18:   cout &lt;&lt; &quot;To find the area of a Triangle, input 1 \n&quot;;
19:   cout &lt;&lt; &quot;To find the area of a Circle, input 2 \n&quot;;
20:   cin &gt;&gt; choice;
21: 
22: if (choice == 1)
23: 
</PRE>
<PRE>24: {
</PRE>
<PRE>25:   cout &lt;&lt; &quot;Enter the base of the triangle: &quot;;
26:   cin &gt;&gt; base;
27:   cout &lt;&lt; &quot;Enter the height of the triangle: &quot;;
28:   cin &gt;&gt; height;
29: 
30: Area_of_triangle = Area(base,height);
31: 
32:   cout &lt;&lt; &quot;The Area of the Triangle is: &quot;&lt;&lt;Area_of_triangle&lt;&lt;endl;
33: }
34: 
35: if (choice == 2)
36: 
37: {
38:   cout &lt;&lt; &quot;Enter radius of the Circle: &quot;;
39:   cin &gt;&gt; radius;
40:   Area_of_circle = Area(radius);
41:   cout &lt;&lt; &quot;The area of the Circle is: &quot;&lt;&lt;Area_of_circle&lt;&lt;endl;
42: }
43: 
44: if (choice != 1 &amp;&amp; choice != 2)
45: 
46: {
47:   cout &lt;&lt; &quot;Sorry! You must enter either 1 or 2 \n&quot;;
48: }
49: }
50: 
51: double Area (double base, double height)
52: {
53:   return (0.5*base*height)
54: }
55: 
56: double Area(double radius)
57: {
58:   return (pi*radius*radius);
59: }
</PRE>
<H3><A NAME="Heading4"></A>Global and Local Variables</H3>
<P>In all of the preceding examples, the variables have been declared at the beginning
of the program, prior to defining the main() function. Declaring variables in this
fashion is more akin to C programs than C++. They are global variables and can be
accessed by all the functions. However, you may also define local variables that
have a scope only in a particular function. Local variables can have the same names
as the global variables, but they do not change the global variables. Local variables
refer only to the function in which they are defined. This difference can be confusing
and lead to erratic results.</P>
<P>The program in Listing A.4 clearly shows the difference between global and local
variables. You will calculate the area of a circle using global variables and local
variables.</P>
<P>
<H4>LISTING A.4. Global.cpp.</H4>
<PRE> 1: // Workspace:  Variable
 2: // Program name:  Global.cpp
 3: 
 4: #include &lt;iostream.h&gt;
 5: 
 6: double  area;
 7: double Area (double);
 8: const double pi = 3.14;
 9: double  radius = 5;
10: 
11: int main()
12: 
13: {
14: cout&lt;&lt;&quot;This Program Calculates The Area Of A Circle \n&quot;;
15: area = Area (radius);
16: cout &lt;&lt; &quot;The Area of the Circle is: &quot;&lt;&lt;area&lt;&lt;endl;
17: cout &lt;&lt; &quot;The Radius In the Main() Function is: &quot;&lt;&lt;radius&lt;&lt;endl;
18: return 0;
19: }
20: 
21: double  Area (double radius)
22: {
23: area = (pi*radius*radius);
24: cout&lt;&lt;&quot;The Radius In the Area() Function is: &quot;&lt;&lt;radius&lt;&lt;endl;
25: return area;
26: }
</PRE>
<P>The variable radius is accessible in the main() function and also the Area() function,
and it is the same. The result of executing this program is shown in Figure A.6.</P>
<P><A HREF="javascript:popUp('0afig06.gif')"><B>FIGURE A.6.</B></A><B> </B><I>Global.cpp--using
a global variable.</I></P>
<P>As the program executes, it shows the value of the variable radius in the different
functions. You will now modify the global variable to be a local variable. Add an
additional line to the Area function defining a local variable:</P>
<P>
<PRE>double radius = 2;
</PRE>
<P>Compile and execute this program. The results are shown in Figure A.7.</P>
<P><A HREF="javascript:popUp('0afig07.gif')"><B>FIGURE A.7.</B></A><B> </B><I>Global.cpp--global
and local variables.</I></P>
<P>You will notice that the value of the variable radius remains unchanged in the
main() function and changes locally in the Area() function. The area of the circle
is calculated based on the value of the local variable, whereas at the same time,
the value of the global variable is not changed but is hidden from the function Area().</P>


<BLOCKQUOTE>
	<P>
<HR>