1. Random Access Files
Dr. John Abraham

2. Advantages of Random Access
Access one record without disturbing others
Modify that record
Create Index if needed to access it many ways

3. Structure Module modulePersonRec Structure personRecord
<VBFixedString(7)> Public RecordNumber As String
<VBFixedString(9)> Public SS As String
<VBFixedString(40)> Public Name As String
<VBFixedString(10)> Public Tele As String
<VBFixedString(2)> Public LineBreak As String
End Structure

4. Using Structure Public DATAFILE As String = "mydb.mdf"
Public Person As personRecord
Public Position As Long
Public LastRecord As Long
Public FileLength As Long
Public indexSS(100) As String
Public indexKey(100) As Long
End Module

5. Opening a Random file
Private Sub btnOpen_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnOpen.Click
Position = 1
' Calculate the record length.
RecLength = Len(Person)
FileOpen(1, DATAFILE, OpenMode.Random, , , RecLength)
FileLength = LOF(1)
LastRecord = FileLength / RecLength
lblConnected.Text = "Connected... Total " & LastRecord & " Records Found. "
lblRecNo.Text = LastRecord + 1
End Sub

6. Writing to a Random File
Person.SS = txtSSN.Text
Person.Name = txtName.Text
Person.Tele = txtTele.Text
Person.LineBreak = Chr(13) & Chr(10)
LastRecord = LOF(1) / RecLength
Person.RecordNumber = LastRecord + 1
'If LastRecord > 0 Then findLink(link)
FilePut(1, Person, LastRecord + 1)

7. Reading from a Random File
FileGet(1, Person, Val(txtKey.Text))
txtSSN.Text = Person.SS
txtName.Text = Person.Name
txtTele.Text = Person.Tele

8. Reading one after another (scanning)
Private Sub tabScan_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles tabScan.Click
Position = 1
lblPosition.Text = Position
FileGet(1, Person, Position)
lblSS.Text = Person.SS
lblName.Text = Person.Name
lblTele.Text = Person.Tele
End Sub

9. Displaying a record Dim tmps As String With Person
lblPosition.Text = Person.RecordNumber
tmps = Trim(Person.SS)
lblSS.Text = tmps.Substring(0, 3) & "-" & tmps.Substring(3, 2) & "-" & tmps.Substring(5)
lblName.Text = Trim(Person.Name)
tmps = Trim(Person.Tele)
lblTele.Text = tmps.Substring(0, 3) & "-" & tmps.Substring(3, 3) & "-" & tmps.Substring(6)
End With

10. Linear Search For i = 1 To LOF(1) / RecLength FileGet(1, Person, i)
If Person.Name.IndexOf(txtNameSearch.Text) > -1 Then
lblNameS.Text = Person.Name
lblTeleS.Text = Person.Tele
lblSSnS.Text = Person.SS
lblRecS.Text = i
Exit For
Else
lblNameS.Text = "Not Found"
lblTeleS.Text = ""
lblSSnS.Text = ""
lblRecS.Text = 0
End If

11. Creating and Index on SS
LastRecord = LOF(1) / RecLength
For i = 1 To LastRecord
FileGet(1, Person, i)
indexSS(i) = Person.SS
indexKey(i) = i
Next

12. Sorting Index i = LastRecord Do sorted = True For j = 1 To i - 1
If indexSS(j) > indexSS(j + 1) Then
sorted = False
temp1 = indexSS(j) : temp2 = indexKey(j)
indexSS(j) = indexSS(j + 1) : indexKey(j) = indexKey(j + 1)
indexSS(j + 1) = temp1 : indexKey(j + 1) = temp2
End If
Next j
i = i - 1
Loop While Not sorted ' (sorted = False)
MessageBox.Show("Index Sorted.", "Sort")

13. Binary Search Lookfor = Trim(txtSSnS.Text) nfound = False
first = 1 : last = LastRecord
While (Not (nfound) And first <= last)
middle = (first + last) \ 2
MessageBox.Show(Lookfor & " " & indexSS(middle), "Binary search demonstration ")
If Lookfor = Trim(indexSS(middle)) Then
nfound = True
FileGet(1, Person, indexKey(middle))
lblNameS.Text = Person.Name
lblTeleS.Text = Person.Tele
lblSSnS.Text = Person.SS
lblRecS.Text = indexKey(middle)
ElseIf Lookfor < indexSS(middle) Then
last = middle - 1
Else
first = middle + 1
End If
End While
If Not (nfound) Then lblNameS.Text = "Not Found!"