irwin/mcgraw-hill copyright© 2000 by the mcgraw-hill companies, inc. powerpoint® presentation to...

Irwin/McGraw-HillIrwin/McGraw-Hill Copyright© 2000 by the McGraw-Hill Companies, Inc.

PowerPoint® Presentationto accompany

prepared byJames 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


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 file2. processing the data,3. closing the file


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


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, item1, item2,…, itemn

Input #fn, item1, item2,…, itemn

EOF function signals end of file


Writing to a Sequential File

Write statement places data in output Form: Write #file, o1, o2, …, on

You can separate fields with commas or semicolons

Output fields are separated with commas, and strings are quoted.

<Cr>/<Lf> ends each record Write #1, txtLname.Text, 54.89


Reading Data in a Sequential File

Form: Input #fn, item1,…,itemn

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)


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,…)


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


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


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


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


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)


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


Random data files

Output: PutPut #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

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