Computer Programming Concepts and Visual Basic David I. Schneider

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28 Computer Programming Concepts and Visual Basic Code consists of statements that carry out tasks. Visual Basic has a repertoire of over 200 statements and we will use many of them in this text. In this section, we limit ourselves to statements that change properties of objects while a program is running. Properties of an object are changed in code with statements of the form objectName.property = setting where objectName is the name of the form or a control, property is one of the properties of the object, and setting is a valid setting for that object. Such statements are called assignment statements. They assign values to properties. Here are three other assignment statements. The statement txtBox.Font.Size = 12 sets the size of the characters in the text box named txtBox to 12. The statement txtBox.Font.Bold = True converts the characters in the text box to boldface. The statement txtBox.Text = “” clears the contents of the text box; that is, it invokes the blank setting. Most events are associated with objects. The event clicking cmdButton is different from the event clicking picBox. These two events are specified cmdButton_Click and picBox_Click. The statements to be executed when an event occurs are written in a block of code called an event procedure. The structure of an event procedure is Private Sub objectName_event() statements End Sub The word Sub in the first line signals the beginning of the event procedure, and the first line identifies the object and the event occurring to that object. The last line signals the termination of the event procedure. The statements to be executed appear between these two lines. (Note: The word Private indicates that the event procedure cannot be invoked by an event from another form. This will not concern us until much later in the text. The word Sub is an abbreviation of Subprogram.) For instance, the event procedure Private Sub cmdButton_Click() txtBox.Text = “” End Sub clears the contents of the text box when the command button is clicked. ■ AN EVENT PROCEDURE WALKTHROUGH The form in Figure 2-13, which contains a text box and a command button, will be used to demonstrate what event procedures are and how they are created. Three event procedures will be used to alter the appearance of a phrase that is typed into the text box. The event procedures are txtPhrase_LostFocus, txtPhrase_GotFocus, and cmdBold_Click. Object Property Setting frmWalkthrough Caption Demonstration txtPhrase Text (blank) cmdBold Caption Make Phrase Bold FIGURE 2-13 The Interface for the Event Procedure Walkthrough
Object box 1. Create the interface in Figure 2-13. The Name properties of the form, text box, and command button should be set as shown in the Object column. The Caption property of the form should be set to Demonstration, the Text property of the text box should be made blank, and the Caption property of the command button should be set to Make Phrase Bold. 2. Double-click on the text box. A window, called the Code window, appears. See Figure 2-14. Just below the title bar are two drop-down list boxes. The left box is called the Object box and the right box is called the Procedure box. (When you position the mouse pointer over one of these list boxes, its type appears.) FIGURE 2-14 A Code Window 3. Click on the down-arrow button to the right of the Procedure box. The dropdown menu that appears contains a list of all possible event procedures associated with text boxes. See Figure 2-15. FIGURE 2-15 Drop-Down Menu of Event Procedures 4. Scroll down the list of event procedures and click on LostFocus. (LostFocus is the 14th event procedure.) The lines Private Sub txtPhrase_LostFocus() Procedure box End Sub appear in the code window with a blinking cursor poised at the beginning of the blank line. Visual Basic Events 29
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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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cedures that are available to all f
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The Flags property influences certa
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13. Additional forms serve as new w
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6. Voting Machine. The members of t
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11.1 AN INTRODUCTION TO DATABASES T
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9. Click on the down arrow to the r
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clicked or a Move, Update, Delete,
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Do While (Not foundFlag) And (Not d
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mary key. Databases of student enro
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question mark stands for a single c
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1995 populations. When the second c
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If a Find method does not locate a
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place an x in the check box next to
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population (in descending order). W
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Creating and Designing Databases 34
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9. When all fields have been specif
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Creating and Designing Databases 34
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TABLE 11.5(B) TABLE 11.5(C) course
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(a) Update the quantity field of th
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12.1 CLASSES AND OBJECTS noun A wor
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Task Statement Assign a value to a
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EXAMPLE 1 and then an instance of t
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[Run, enter the data for a student
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SOLUTION: Object Property Setting f
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4. Methods can be either Function o
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‘Student Class (CStudent) Private
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txtFinal.Text = “” picGrades.Pr
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■ EVENTS In the previous section
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frmCircles.ForeColor = vbRed ‘Mak
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Public Function X() As Integer X =
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End Sub Private Sub cmdDropBomb_Cli
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Next i End Sub Public Function Read
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Remove method. The number of object
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S E C T I O N 13 COMMUNICATING WITH
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A P P E N D I C E S
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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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