Computer Programming Concepts and Visual Basic David I. Schneider

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222 Computer Programming Concepts and Visual Basic Type of error Value of Err.Number Subscript out of range 9 Division by zero 11 File not found 53 File already open 55 File already exists 58 Disk full 61 Disk not ready 71 To set up error-trapping inside a procedure, do the following: 1. Make the first line of the procedure On Error GoTo ErrorHandler 2. Type in the lines to carry out the purpose of the procedure. 3. Make the last lines of the procedure Exit SubErrorHandler:error-handling routineResume The statement “On Error GoTo ErrorHandler” activates error-trapping. If an error occurs during the execution of a line of the procedure, the program will jump to the error-handling routine. The statement “Resume” causes the program to jump back to the line causing the error. The statement “Exit Sub”, which causes an early exit from the procedure, prevents the error-handling routine from being entered when no error occurs. For instance, the following procedure has an error-handling routine that is called when a file cannot be found. Private Sub OpenFile() On Error GoTo ErrorHandler Dim fileName As String fileName = InputBox(“Enter the name of the file to be opened.”) Open fileName For Input As #1 Exit Sub ErrorHandler: Select Case Err.Number Case 53 MsgBox “File not found. Try Again.” fileName = InputBox(“Enter the name of the file to be opened.”) Case 71 MsgBox “The drive door might be open - please check.” End Select Resume End Sub The word “ErrorHandler”, which is called a line label, can be replaced by any word of at most 40 letters. The line, which is placed just before the error-handling routine, must start at the left margin and must end with a colon. If “Resume” is replaced by “Resume Next”, then the program will jump to the line following the line causing the error. The line label must be in the same procedure as the On Error statement. However, the error-handling routine can call another procedure. There are two variations of the On Error statement. The statement “On Error GoTo 0” turns off error-trapping. The statement “On Error Resume Next” specifies that when a runtime error occurs, execution continues with the statement following the statement where the error occurred. COMMENTS 1. Sequential files make efficient use of disk space and are easy to create and use. Their disadvantages are as follows:
(a) Often a large portion of the file must be read in order to find one specific item. (b) An individual item of the file cannot be changed or deleted easily. Another type of file, known as a random-access file, has neither of the disadvantages of sequential files; however, random-access files typically use more disk space, require greater effort to program, and are not flexible in the variety and format of the stored data. Random-access files are discussed in Section 8. 2. Consider the sequential file shown in Figure 7-1 at the end of Example 2. This file is said to consist of three records of two fields each. A record holds all the data about a single individual. Each item of data is called a field. The three records are “Barbra”, 1942 “Ringo”, 1940 “Sylvester”, 1946 and the two fields are name field, year of birth field 7.2 USING SEQUENTIAL FILES In addition to being accessed for information, sequential files are regularly updated by modifying certain pieces of data, removing some records, and adding new records. These tasks can be performed most efficiently if the files are first sorted. ■ SORTING SEQUENTIAL FILES The records of a sequential file can be sorted on any field by first reading the data into parallel arrays and then sorting on a specific array. EXAMPLE 1 The following program sorts the sequential file YOB.TXT of the previous section by year of birth. Private Sub cmdSort_Click() Dim numPeople As Integer ‘Sort data from YOB.TXT file by year of birth numPeople = NumberOfRecords(“YOB.TXT”) ReDim nom(1 To numPeople) As String ReDim yearBorn(1 To numPeople) As Integer Call ReadData(nom(), yearBorn(), numPeople) Call SortData(nom(), yearBorn(), numPeople) Call ShowData(nom(), yearBorn(), numPeople) Call WriteData(nom(), yearBorn(), numPeople) End Sub Private Function NumberOfRecords(filespec As String) As Integer Dim nom As String, yearBorn As Integer Dim n As Integer ‘Used to count records n = 0 Open filespec For Input As #1 Do While Not EOF(1) Input #1, nom, yearBorn n = n + 1 Loop Close #1 Using Sequential Files 223
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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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Page 223: S E C T I O N 7 SEQUENTIAL FILES
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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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