Computer Programming Concepts and Visual Basic David I. Schneider

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364 Computer Programming Concepts and Visual Basic End Sub ‘Form code Private round As CCircle Private Sub Form_Load() Set round = New CCircle End Sub Private Sub cmdMove_Click() round.Move (500) End Sub Private Sub cmdQuit_Click() End End Sub [Run, and press the command button five times.] COMMENTS 1. The statement Set objectVar = Nothing dissociates the object variable from the actual object and frees up the memory used by the variable. 2. An object variable declared inside a procedure ceases to exist when the procedure is exited. (We say that the object falls out of scope.) The effect is the same as when the variable is set to Nothing. 3. The counterpart to the Initialize event is the Terminate event procedure which has the template Private Sub Class_Terminate() End Sub and is automatically invoked when all references to the object are Set to Nothing or when the object falls out of scope. This procedure is often used to set any objects you may have created inside the class module to Nothing.
4. Methods can be either Function or Sub procedures. A method that returns a value must be a Function procedure; otherwise it can be a Sub procedure. 5. A program with a class module has at least three components: a form, form code, and class module code. Although the View menu gives you access to the form (Alt/V/B) and the form code (Alt/V/C), only the Project Explorer gives access to all three. To view the code for a class module, double-click on the name of the class in the Project Explorer. 6. A class module is saved in a file whose file name has the extension .cls. Therefore, classes have both a name (Csomething) and a file name (something.cls). 7. To insert an existing (saved) class module into a program, click on Add Class Module in the Project menu, click on the Existing tab, and enter the filespec for the class module. 8. To delete a class from a program, right click on the class name in the Project Explorer and click on Remove className. 9. An object also can be declared with a statement of the form Private pupil As Object However, objects so declared require more time and effort for Visual Basic to access. 10. The set of properties, methods, and events for a class is called the class interface. The classes CStudent and CPFStudent have the same interface, even though they carry out the task of computing a grade differently. The programmer need only be aware of the SemGrade method and needn’t be concerned about its implementation. The feature that two classes can have behaviors that are named the same and have essentially the same purpose but different implementations is called polymorphism. 11. Sometimes you will have difficulty deciding whether an interface item should be a property or a method. As a rule of thumb, properties should access data and methods should perform operations. 12. The ByVal (which stands for “by value”) keyword is automatically inserted before parameters in Property Let statements invoked from the Tools menu. However, this keyword is optional. The default way of passing an argument to a parameter is ByRef (which stands for “by reference”). Usually, passing by value is more efficient than passing by reference. 13. The default parameter name in a Property Let procedure is vNewValue. Although we usually substitute a meaningful name, we retain the convention of beginning the name with the prefix v. 14. We could have preceded our member variables with the keyword Public and allowed direct access to the variables. However, this is considered poor programming practice. By using Property Let procedures to update the data, we can enforce constraints and carry out validation. 15. In a class module, a property is implemented by two procedures, one to set and the other to retrieve the property value. These procedures that access properties are sometimes referred to as accessor methods. 12.2 COLLECTIONS AND EVENTS Collections and Events 365 “An object without an event is like a telephone without a ringer.” Anonymous A collection is an entity, similar to an array, that is especially well-suited to working with sets of objects. This section discusses collections of objects and user-defined events for classes.
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Computer Programming Concepts and V
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CONTENTS SECTION 1: PROBLEM SOLVING
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13.3 WEB PAGE PROGRAMMING WITH VBSC
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S E C T I O N 1 PROBLEM SOLVING
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4 Computer Programming Concepts and
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10 Computer Programming Concepts an
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S E C T I O N 2 FUNDAMENTALS OF PRO
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3.1 SUB PROCEDURES, PART I Structur
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The second task performed by the ad
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‘The density (number of people pe
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"Hawaii", 1184000, 6471 "Alaska", 6
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dure by just using the variable in
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Call CalculateSum(x, y, s) Call Dis
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■ FORM-LEVEL VARIABLES Visual Bas
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3. In Appendix D, the section “St
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Private Sub cmdDetermine_Click() Di
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Private Sub cmdCompute_Click() Dim
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preceded with the word ByVal. Built
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It is clear from the hierarchy char
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The meaningful procedure names, alo
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FIGURE 3-9 Sample Run for Programmi
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4.1 RELATIONAL AND LOGICAL OPERATOR
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(a) (2 < n) And (n < 6) (b) (2 < n)
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SOLUTION: In the following program,
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The Else part of an If block can be
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picTax.Cls picTax.Print “Your FIC
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Case 4, 5 picOutcome.Print “You a
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EXAMPLE 3 The following program use
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Case Else picResult.Print “The st
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4. The items in the value list must
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The hierarchy chart in Figure 4-4 s
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‘Program to compute employees’
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Private Function TaxMarried(adjPay
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and tells us whether the food meets
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S E C T I O N 5 REPETITION
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132 Computer Programming Concepts a
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S E C T I O N 6 ARRAYS
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S E C T I O N 7 SEQUENTIAL FILES
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S E C T I O N 8 RANDOM-ACCESS FILES
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S E C T I O N 9 THE GRAPHICAL DISPL
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10.1 LIST BOXES AND COMBO BOXES The
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Private Sub cmdAdd_Click() Dim item
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either typing in information or jus
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Private Sub cmdDisplay_Click() picF
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clicks on the square to toggle betw
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circle and the button is said to be
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hsbBar.Value changes accordingly. T
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THE SHAPE CONTROL The shape control
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FIGURE 10-9 A Picture Created with
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Rows 9 ScrollBars 0 - flexScrollBar
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Private Sub cmdQuit_Click() End End
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Here, the menu bar contains two men
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Clipboard.Clear deletes the content
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Page 353 and 354: 9. When all fields have been specif
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Page 357 and 358: TABLE 11.5(B) TABLE 11.5(C) course
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Page 369 and 370: [Run, enter the data for a student
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Page 377 and 378: txtFinal.Text = “” picGrades.Pr
Page 379 and 380: ■ EVENTS In the previous section
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APPENDIX C VISUAL BASIC STATEMENTS,
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CANCEL The Cancel property provides
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COLS and ROWS The Cols and Rows pro
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DEFINT, DEFLNG, DEFSNG, DEFDBL, DEF
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ENVIRON Visual Basic has an environ
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FONT.ITALIC or FONTITALIC These pro
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yields “1234.”, Format(1234000,
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the length of the string currently
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ISNUMERIC The value of the function
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Unlock statement is used to remove
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Focus Default Default Number First
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OR (Logical Operator) The logical e
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the same as in the original Dimensi
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SECOND The function Second extracts
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STRETCH When the Stretch property o
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UNLOCK The Unlock command is intend
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[COLOR]: Numbers written in base 16
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Appendix D 457
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466 Technology of the Internet D Da
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468 Technology of the Internet Rela
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470 Technology of the Internet 2. D
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SYSTEM REQUIREMENTS CD-ROM INSTRUCT
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Computer Programming Concepts and Visual Basic David I. Schneider

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