Computer Programming Concepts and Visual Basic David I. Schneider

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APPENDIX D VISUAL BASIC DEBUGGING TOOLS Errors in programs are called bugs and the process of finding and correcting them is called debugging. Since Visual Basic does not discover errors due to faulty logic, they present the most difficulties in debugging. One method of discovering a logical error is by deskchecking, that is, tracing the values of variables on paper by writing down their expected value after “mentally executing” each line in the program. Desk checking is rudimentary and highly impractical except for small programs. Another method of debugging involves placing Print methods at strategic points in the program and displaying the values of selected variables or expressions until the error is detected. After correcting the error, the Print methods are removed. For many programming environments, desk checking and Print methods are the only debugging methods available to the programmer. The Visual Basic debugger offers an alternative to desk checking and Print methods. It allows you to pause during the execution of your program in order to view and alter values of variables. These values can be accessed through the Immediate, Watch, and Locals windows, known collectively as the three Debug windows. THE THREE PROGRAM MODES At any time, a program is in one of three modes[\m]design mode, run mode, or break mode. The current mode is displayed in the Visual Basic title bar. Title bar during design time. Title bar during run time. Title bar during break mode. At design time you place controls on a form, set their initial properties, and write code. Run time is initiated by pressing the Start button. Break mode is invoked automatically when a run-time error occurs. While a program is running, you can manually invoke Break mode by pressing Ctrl+Break, clicking on Break in the Run menu, or clicking on the Break icon (located between the Start and Stop icons). While the program is in break mode, you can use the Immediate window to examine and change values of variables and object settings. When you enter Break mode, the Start button on the Toolbar changes to a Continue button. You can click on it to proceed with the execution of the program. Appendix D 457
458 Computer Programming Concepts and Visual Basic THE IMMEDIATE WINDOW You can set the focus to the Immediate window by clicking on it (if visible), by pressing Ctrl+G, or by choosing “Immediate Window” from the View menu. Although the Immediate window can be used during design time, it is primarily used in Break mode. When you type a statement into the Immediate window and press the Enter key, the statement is executed at once. A statement of the form Print expression displays the value of the expression on the next line of the Immediate window. In Figure D.1, three statements have been executed. (When the program was interrupted, the variable numVar had the value 10.) In addition to displaying values of expressions, the Immediate window also is commonly used to change the value of a variable with an assignment statement before continuing to run the program. Note 1: Any statement in the Immediate window can be executed again by placing the cursor anywhere on the statement and pressing the Enter key. Note 2: In earlier versions of Visual Basic the Immediate window was called the Debug window. FIGURE D.1 Three Print Statements Executed in the Immediate Window THE WATCH WINDOW You can designate an expression as a watch expression or a break expression. Break expressions are of two varieties: those that cause a break when they become true and those that cause a break when they change value. At any time, the Watch window shows the current values of all watch and break expressions. In the Watch window of Figure D.2, the type of each expression is specified by an icon as shown in Table 1. FIGURE D.2 The Watch Window TABLE D.1 Watch Type Icons Icon Type of expression Watch expression Break when expression is true Break when expression has changed The easiest way to add an expression to the Watch window is to right-click on a variable in the code window and then click on “Add Watch” to call up an Add Watch dialog box. You
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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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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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Page 431 and 432: APPENDIX C VISUAL BASIC STATEMENTS,
Page 433 and 434: CANCEL The Cancel property provides
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Page 437 and 438: DEFINT, DEFLNG, DEFSNG, DEFDBL, DEF
Page 439 and 440: ENVIRON Visual Basic has an environ
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Page 447 and 448: ISNUMERIC The value of the function
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Page 457 and 458: SECOND The function Second extracts
Page 459 and 460: STRETCH When the Stretch property o
Page 461 and 462: UNLOCK The Unlock command is intend
Page 463 and 464: [COLOR]: Numbers written in base 16
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Page 475 and 476: 466 Technology of the Internet D Da
Page 477 and 478: 468 Technology of the Internet Rela
Page 479 and 480: 470 Technology of the Internet 2. D
Page 481: SYSTEM REQUIREMENTS CD-ROM INSTRUCT
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Computer Programming Concepts and Visual Basic David I. Schneider

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