Random I/O and VB Structures

Alternative I/O Methods to streamreader/streamwriter --

• fileopen(filenum, "testfile.xxx", openmode.[input|output|append|random|binary] )
• fileclose(filenum)
• input(filenum, string)          'reads up to a comma, endofline
• inputstring(filenum), #ofcharsToReadIn)
• x = lineinput(filenum)
• print(filenum, stuff)            'no linefeed
• printline(filenum, stuff)       'linefeed
• write(filenum, stuff)            'unformatted
• writeline(filenum, stuff)       'unformatted with linefeed

Binary Files  - 

• these are files that cannot be read by Notepad - they are in pure binary form, and are more efficient to use than sequential files.  
• Why?  Think of an integer variable which will be used to hold the number 15000.  
  • In a sequential file, 5 bytes would be needed (1 5 0 0 0, each is in character form in the file).  
  • In a binary file, only 2 bytes are needed, because integer's only use 2 bytes.

Random Files -

• fileopen( filenum, "myfile.xxx", openmode.random, , recordlength) 

  recordlength = fixed length of each record in chars
   

• fileget(filenum, employee, position)
• employee = structure that will contain the entire record
  position = record number in the file
   
• fileput(filenum, employee, position)    'replace in place
  
  lastrecord += 1    'to add another record at the end
• fileput(filenum, employee, lastrecord)

'  What it was in VB6.0

open filename for random as filenumber len = recordlength

For example, open commondialog1.filename for random as #1 len=56

Visual Basic Structures -

• defined using a structure statement, which 
• is commonly defined in a Visual Basic module
• modules  are added to a project using the Project - Add Module from the menu
• modules are chunks of code, but they are not associated with any specific event
• Record layouts for a random file are defined using Visual Basic structures --

Consider the following record layout for the employees file --

structure person

    public    ID as integer

    public    salary as long

    public    reviewDate as long

    <vbfixedstring(15)> public first as string

    <vbfixedstring(20)> public last as string

end structure

'  Other variables ...

public employee as person

public position as long    'record number in file

public lastrecord as long    'record number of last record

For the VB6 Weasel example - type/end type instead of structure/end structure

Weasel

end type

For record layouts, you must fix the size of string variables

• Sum up record length: 32 + 12 + 2 + 2 + 8 = 56 for Weasel
• 2+ 4 + 4 + 15 + 20 for person

Reading & Writing to Random Access Files

• fileget(fileNumber, employee, recordnumber)
• fileput( fileNumber, employee, recordnumber)

Download the Weasel project components and run the program; then examine the code; convert it to VB.NET.  We will review this in class as well.

weasels for both VB6 AND VB.NET

weasel.vbp    weasel.frm        weasel.dat    weasel.bas 
project file    form/code file  data file    module file

Created & maintained by DrB. Last update:  Wednesday, February 18, 2009