Computer Programming Concepts and Visual Basic David I. Schneider

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254 Computer Programming Concepts and Visual Basic For recordNum = 1 To LOF(1) / Len(college) Also, the pair of statements lastRecord = LOF(1) / Len(college) Put #1, lastRecord + 1, college can be used to add a record to the end of the file. COMMENTS 1. Random-access files are also known as direct-access or relative files. Because each record has the same number of characters, the computer can calculate where to find a specified record and, therefore, does not have to search for it sequentially. 2. Unlike sequential files, random-access files needn’t be closed between placing information into them and reading from them. 3. Random-access files do not have to be filled in order. For instance, a file can be opened and the first Put statement can be Put #n, 9, recVar. In this case, space is allocated for the preceding eight records. 4. If the record number r is omitted from a Put or Get statement, then the record number used will be the one following the number most recently used in a Put or Get statement. For instance, if the line Put #1, , college is added to the program in Example 1 after the existing Put statement, then the information on Virginia Tech will occupy records 3 and 4 of the file COL- LEGES.TXT. 5. Users often enter records into a random-access file without keeping track of the record numbers. If file #n is open, then the value of the function Loc(n) is the number of the record most recently written to or read from file n with a Put or Get statement. 6. Each record in a random-access file has the same length. This length can be any number from 1 to 32767. 7. When the statement Open “COLLEGES.TXT” For Random As #1 Len = Len(college) is typed, the words “For Random” can be omitted. The smart editor will insert them automatically. 8. The decision of whether to store data in a sequential file or in a random-access file depends on how the data are to be processed. If processing requires a pass through all the data in the file, then sequential files are probably desirable. If processing involves seeking out one item of data, however, random-access files are the better choice.
SUMMARY 1. A fixed-length string is a variable declared with a statement of the form Dim var As String * n. The value of var is always a string of n characters. 2. A record is a composite user-defined data type with a fixed number of fields each of which can be of most data types. Type statements [in this text appearing in the (Declarations) section of a BAS Module] define record types and Dim statements are used to declare a variable to be of that type. 3. After a record type has been specified, the associated random-access file is an ordered collection of record values numbered 1, 2, 3, and so on. Record values are placed into the file with Put statements and read from the file with Get statements. At any time, the value of LOF(n) / Len(recordVar) is the number of the highest record value in the file and the value of Loc is the number of the record value most recently accessed by a Put or Get statement. PROGRAMMING PROJECTS 1. Balance a Checkbook. Write an interactive program to request information (payee, check number, amount, and whether or not the check has cleared) for each check written during a month and store this information in a random file. The program should then request the balance at the beginning of the month and display the current balance and the payee and amount for every check still outstanding. 2. A teacher maintains a random-access file containing the following information for each student: name, social security number, grades on each of two hourly exams, and the final exam grade. Assume the random-access file GRADES.TXT has been created with string fields of lengths 25 and 11 and three numeric fields, and all the names and social security numbers have been entered. The numeric fields have been initialized with zeros. Write a program with the five command buttons “Display First Student,” “Record Grade(s) & Display Next Student,” “Locate Student,” “Print Grade List,” and “Done” to allow the teacher to do the following. (a) Enter all the grades for a specific exam. (b) Locate and display the record for a specific student so that one or more grades may be changed. (c) Print a list of final grades that can be posted. The list should show the last four digits of the social security number, the grade on the final exam, and the semester average of each student. The semester average is determined by the formula (exam1 + exam2 + 2 * finalExam) / 4. Summary 255
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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 212 and 213: 204 Computer Programming Concepts a
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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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