Irwin/McGraw-Hill Copyright© 2000 by the McGraw-Hill Companies, Inc. PowerPoint® Presentation to accompany prepared by James T. Perry University of San Diego

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Ch 10: Data Files  Create data files –Reading, writing, opening, closing  Differentiate sequential & random files  Trap user errors and handle them  Fixed length strings in user data types  Read/Write random data files  Updating a random file  Using the InputBox function

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Data Files  Data files consist of records & fields  Record key determines a record’s position in a file  Two common file organizations are sequential and random  Process a file by 1. opening the file 2. processing the data, 3. closing the file

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill FreeFile and Close Statements  FreeFile function returns the next unused file number –intFileNumber = FreeFile –In large projects, file numbers are hard to track and allocate  Close issued automatically when you end  Issue Close statement before leaving prog. –Close #1 or Close #1, #2

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Sequential File Organization  Sequential file records are stored one after another  Use Write to output data  Use Input to read data  File must be opened prior to first use  Form: Write #fn, item 1, item 2,…, item n Input #fn, item 1, item 2,…, item n  EOF function signals end of file

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Writing to a Sequential File  Write statement places data in output  Form: Write #file, o 1, o 2, …, o n  You can separate fields with commas or semicolons  Output fields are separated with commas, and strings are quoted.  / ends each record  Write #1, txtLname.Text, 54.89

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Reading Data in a Sequential File  Form: Input #fn, item 1,…,item n  Separate fields items with commas  File number must be that of an open file  When you read the last record, end-of-file signals  You detect end of file with the EOF function: EOF(FileNumber)

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Trapping Program Errors  Errors may be trapped asynchronously  Visual Basic generates an error number whenever an error occurs  To handle errors, you must –Turn on error handling feature: On Error... –Create error handling code –Determine what is to be done after processing the error (continue, end program,…)

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill The Err Object  The Err object holds information about error that just occurred  Err.Source holds the source of the error  Err.Number holds the error number  Err.Description contains error description  You can raise an error condition—turn on an error—with: Err.Raise Number:=xx

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Coding Error-Handling Routines  On Error statement designates error handler  Code to handle errors follows line label  Line label appears on line by itself and ends with a colon  A Case statement works well to sort out errors with a "switchboard"  Continue execution with Resume statement

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Exit and Exit Sub Statements  Exit Sub immediately exits current sub procedure  Exit Function immediately exits current function  Exit is used to prematurely halt execution of sub procedure or function when extraordinary conditions occur  Place Exit Sub above error handling label

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Saving Changes to a File  When data changes, ask users if they want to save the changes before program ends  Changed data is known as “dirty” data  Keep track of data changes with a global boolean flag  Any procedure that allows data to change should set the flag to True—indicating “dirty” data file  Check file just before ending program

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Sequential File Prog. Example  You can load a combo box by reading from a data file and executing the AddItem method  Reading file & list filling halts when EOF occurs on input file  (See Hands On Programming example)

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Random data files  You can read/write data in any order  Open “filename” For Random As #1 LEN=x  Get #filenumber, [record#], RecordName  RecordName is a user-defined data type: Type FullName strLastName As String * 20 strFirstName As String * 15 End Type

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Random data files  Output: Put Put #filenumber, [recordnumber], Recordname  Put #1, iRecordNo, pCustomerRecord  Get & Put statements read an entire record  You refer to fields in user-defined structure by recordname.fieldname  lstName.AddItem mRec.LastName  end of file is calculated via record lengths