vel01.htm

简单的说明如何使用VB,非常适合初学使用者,而且是用图表来解说的

<P>Large-scale programming tool improvements did not occur until a hardware 
breakthrough occurred: NASA's space efforts developed the microchip, and microcomputers were born. Now, instead of new and potential programmers, desktop computers opened the door 
for millions of would-be programmers. These programmers demanded easier 
programming tools.

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<NOTE><I>Definition: QBasic</I> is a recent BASIC language clone.</NOTE>

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<P>Efforts were made to improve the friendliness 
of programming languages like BASIC. New versions of BASIC, such as QBasic, enabled programmers to write more powerful programs with less effort. Programmers could also write programs that had a less rigid 
style than those before. Listing 1.3 shows a 
QBasic version of the payroll calculation shown in the previous two listings. As you can see, the language is getting less strict and more free-form.

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<FONT COLOR="#000080"><B>Listing 1.3. A QBasic program.</B></FONT>

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<FONT 
COLOR="#000080">TaxRate = .25

PRINT &quot;** Payroll Calculations **&quot;

PRINT &quot;** Enter the employee's ID, hours, and pay rate&quot;

INPUT IdOfEmp$, HrsWorked, Rate

DO UNTIL (IdOfEmp$ = &quot;END&quot;)

 IF (HrsWorked &lt;= 40) THEN

 ' 
*******Compute regular pay

 GrossPay = HrsWorked * Rate

 ELSE

 ' *******Compute overtime pay

 OverTimeHours = HrsWorked - 40

 OverTimeGross = OverTimeHours * Rate * 1.5

 GrossPay = 40 * Rate + OverTimeGross

 END IF

 Taxes = GrossPay * TaxRate

 
PayNet = GrossPay - Taxes

 PRINT &quot;Emp: &quot;; IdOfEmp$; &quot;Net Pay:&quot;; PayNet

 INPUT IdOfEmp$, HrsWorked, Rate

LOOP

END</FONT></PRE>

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<FONT SIZE=4 COLOR="#FF0000"><B>You Run 
Programs to Produce Output</B></FONT></CENTER></H4>

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<NOTE><I>Definition: </I>Computers generate <I>output</I> on screens and printers.</NOTE>

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<P>Just as a recipe has a result, the cooked meal, instructions in a program produce an important result. The result of the programming effort is the finished program that makes the computer perform a directed task, such as payroll processing. After 

entering a program or loading a program from the disk, you must tell the computer to run, or execute, the program. The computer then follows the program's instructions and produces the output.

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<P>If you entered the code in Listing 1.3 into the 
QBasic programming language, the following output might result:

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<PRE>

<FONT COLOR="#000080">** Payroll Calculations **

** Enter the employee's ID, hours, and pay rate

? <B>KL823, 40, 9.25</B>

Emp: KL823 Net Pay: 277.5

? <B>PO341, 35, 10</B>


Emp: PO341 Net Pay: 262.5

? <B>LP543, 40, 10</B>

Emp: LP543 Net Pay: 300

? <B>END, 0, 0</B></FONT></PRE>

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<NOTE><B>Note: </B>The previous QBasic code requires that the user tell the program that there 
are no more employees to process by typing END as an employee ID and zero amounts for the hours and pay rate.</NOTE>

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<NOTE><B>Tip: </B>This output 
would be different if the user entered different data, because the program would calculate different payroll results.</NOTE>

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<NOTE><I>Definition: 
Input</I> is the data that the user enters.</NOTE>

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<P>As you can see from the execution of this program, the program takes the user's input and calculates net pay amounts. Input can come from sources 
other than the user. Programs might get input from a disk file, a modem connected to the telephone, or from 
other programs running along with yours. Users do not always see the output, either. Programs often perform calculations and data manipulation and 
send the output of that processing to disk files or to other computers over modem lines.

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<P>Figure 1.3 illustrates the program flow from writing the program to generating output, such as the payroll output you see here.

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<P><B> <A 
HREF="01vel03.gif">Figure 1.3. The program's instructions produce </B><B>the output.</A></B>

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<NOTE><B>Warning: </B>The output you get after typing with a word processor would be the text on paper 
or on the screen. The output from other programs might be as diverse as payroll reports, calculations, or graphics. The program's instructions determines 
what the program produces.</NOTE>

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<FONT SIZE=4 COLOR="#FF0000"><B>Exterminating Bugs Is Not Trivial</B></FONT></CENTER></H4>

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<NOTE><I>Syntax</I> is the grammar of a language.</NOTE>


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<P>Bugs are as frustrating in computer programs as they are in computer programmer's houses. Rarely will a program that you write work the first time you run it. There are two kinds of errors: <I>Syntax 
errors</I>&#151;sounds like &quot;sin tax&quot; and 
is worse in many ways&#151;and <I>logic errors</I>. Since you do not know the Visual Basic programming language yet, this simple phrase demonstrates the two kinds of errors that can appear in your code:


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<P>There are two errrors in this sentence.

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<P>What are the two errors? I'll wait...

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<P>The first error, a syntax error, is easy to spot. errrors is misspelled. The logic error takes more time to find. The logic error is that the entire 
sentence is logically wrong&#151;there is only <I>one</I> error, the syntax error errrors.

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<P>When you begin to write programs, this example sentence may come back to haunt you a bit. Syntax errors are easy to catch because Visual Basic catches all 
of them for you. In other words, if you type Tezt when Visual Basic expects you to type Text, 
Visual Basic immediately displays an error message box in the center of your screen. Visual Basic is extremely stubborn when it comes to syntax errors because it 
refuses to execute your program until you remove all the syntax errors in the code.

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<P>When you make a logic error, however, Visual Basic cannot help you out. For example, if you were writing a payroll calculation routine and accidentally subtracted 
$50 from everybody's paycheck, Visual Basic has no way of knowing that you should not 
subtract that amount. If you tell Visual Basic to subtract the amount&#151;using correct syntax, of course&#151;Visual Basic subtracts the amount. Only until your 
employees beat your door down will you know that a logic error took place.

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<NOTE><B>Tip: </B>Thoroughly test your programs before you distribute them or use them for important work. Run the 
program using all combinations of input to find and correct as many logic errors as possible.</NOTE>

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<FONT SIZE=4 COLOR="#FF0000"><B>Graphical 
User Interfaces Changed Everything</B></FONT></CENTER></H4>

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<NOTE><I>Definition: GUI</I> stands for <I>G</I>raphical <I>U</I>ser <I>I</I>nterface.</NOTE>

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<P>A challenging programming problem arose when advanced GUI visual environments, such as Microsoft Windows, appeared in the 1980s. GUI environments require more advanced and difficult programming efforts than text-based 
computing environments. Although 
users and programmers wanted easier tools, GUI environments demanded more complex programming tools.

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<NOTE><I>Definition: </I>An <I>event</I> can be a Windows 
mouse click, a key press, a menu selection, or an internal Windows activity.</NOTE>

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<P>It is relatively easy to produce the text-based output shown earlier. It is much more difficult to produce 
Windows-like screens such as the one shown in Figure 1.4. The user does not have to enter the ID, hours worked, and amounts in any preset order. 
Neither does the user have to press the three command buttons at the right of the screen in any order. In 
Windows programs, a specific event determines the next course of action. A Windows program is called an event-driven<I> </I>program as opposed to a 
text-based procedural program. In Windows, the user determines what happens next by triggering an event to 
which the Visual Basic program responds.

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<P><B> <A HREF="01vel04.gif">Figure 1.4. Users randomly control event-driven </B><B>programs.</A></B>

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<P><FONT COLOR="#FF8000"><B><I>Review: </I></B></FONT>Computer programming has progressed a long 
way from the wiring panels of the early days. Once keyboards were added, what programmers could do grew by leaps and bounds. Actually, it was not until the 
development of Visual Basic that another great leap in programming tools arrived. The power of 
programs that you can create with Visual Basic, given the simplicity of Visual Basic's environment, produces as much computer programming advantage today as the 
keyboard provided over the switch panel. As Yogi Berra once said, &quot;It ain't bragging if 
it's true!&quot;

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<FONT SIZE=5 COLOR="#FF0000"><B>From BASIC to Visual Basic</B></FONT></CENTER></H3>

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<P><FONT COLOR="#FF8000"><B><I>Concept: </I></B></FONT>As Windows programming tools 
are developed, their complexity seems to increase. The Microsoft Windows environment requires much more complicated programming efforts than DOS-based non-Windows programs 
require. Visual Basic bucks the trend by providing a simple approach to writing 
Windows programs while retaining all the beginning and advanced programming language instructions found in QBasic.

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<NOTE><I>Definition: </I>Text-based programs are often called <I>procedural 
programs</I>.</NOTE>

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<P>Procedural languages such as FORTRAN and QBasic simply don't work well for a GUI environment. The problem&#151;actually, the primary advantage for users&#151;of GUIs is the 
event-driven processing explained in the previous section. The user does not 
always do the same thing in the same order. That is, a user might want to select from a pull-down menu, click a mouse button, type text, or compute a mathematical answer, and he 
might do those tasks in virtually any order. Text-based procedural programming 
languages essentially required that the program dictate the order of the user's actions. Windows requires a different approach. The traditional programming languages cannot 
handle the GUI approach well at all.

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<P>It is possible to write event-driven programs in procedural languages. The majority of Windows programs in use today were written in the C language, a procedural language more similar in style to FORTRAN than