Transcript MS Visual Basic 6

MS Visual Basic
Applications
Walter Milner
Event-driven programming
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Standard approach for GUIs
Contrast with old character interfaces – program
determines what happens
In GUI, the user triggers what application does
(mostly)
Event examples are key press, mouse move, timer
timeouts
Correspond to native Windows Messages (next
slide)
Event handler = a subroutine which will execute
when that event happens
Variables
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Dim x as Integer
Dim x,y as Integer NO!
Is case sensitive (kind of)
Variable naming conventions
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Microsoft Simonyi Hungarian Reddick
House rules
Assignment statement –
x=4
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Option Explicit YES!
Constants –
Private Const MyInt As Integer = 5
Comments, line continuation and hex
Comments start with an apostrophe
 'and run to the end of the line
 A very long statement can use a_
to continue onto the next line
 Hex constants written like &HFF0012
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Data types
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Integer Long
Single Double
Currency
Byte unsigned 0 - 255
String
Boolean
Date
Object
Variant NO!
Data type conversion
DIM x as integer
x = Cint("10")
Conversion
function
Converts an expression to
Cbool
Boolean
Cbyte
Byte
Ccur
Currency
Cdate
Date
CDbl
Double
Cint
Integer
CLng
Long
CSng
Single
CStr
String
Cvar
Variant
CVErr
Error
Controls
Private Sub CommandButton1_Click()
Dim x As Integer
Dim y As Integer
Dim z As Integer
x = TextBox1.Value
y = TextBox2.Value
z=x+y
Label1.Caption = z
End Sub
Copy this
Correct it
Add other buttons
VB Core II
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Conditional statements
Exception handling
Loops
Arrays
Debugging
if statements
If c > 5 Then x = 1: y = 3
If c > 5 Then
x=1
y=3
End If
If c > 5 Then
x=1
y=3
Else
z=7
End If
If c > 5 Then
x=1
y=3
ElseIf c = 4 Then
z=7
Else
x=9
End If
select
Dim Number
Number = 8 ' Initialize variable.
Select Case Number ' Evaluate Number.
Case 1 To 5 ' Number between 1 and 5, inclusive.
x=4
Case 6, 7, 8 ' Number between 6 and 8.
x=5
Case 9 To 10 ' Number is 9 or 10.
x=6
Case Else ' Other values.
x=7
End Select
Error and exception handling
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exception = problem event at run-time
usually related to I/O
eg file not found, server connection lost,
invalid user input
not a programming bug
VB calls exception errors
(Unlike Java) VB does not force
exception handling – but should do
Error handlers – example - invalid numbers
Private Sub Command1_Click()
Dim num1 As Integer
Dim num2 As Integer
Dim result As Integer
On Error GoTo myErrorHandler
num1 = Textbox1.Text
num2 = Textbox2.Text
result = num1 + num2
Label1.Caption = result
Exit Sub
Exercise
Try this out in the calculator
program
Then deal with divide by zero (11)
myErrorHandler:
If Err.Number = 13 Then
MsgBox ("Please enter a valid number")
Else
MsgBox (Err.Description)
End If
Resume Next
End Sub
For next loops
Dim x as Integer, total As Integer
total = 0
For x = 1 To 5
total = total + x
Next
Dim x as Integer, total As Integer
total = 0
For x = 1 To 5 Step 2
total = total + x
Next
Debugging
Exercises – Use the debugger to watch..
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a for next loop looping from 1 to 5
outputting the multiples of 3 from 99 to 3
adding up the odd numbers between 1
and 9
Other loops
Dim c As Integer
c=1
Do While c < 5
c=c+1
Loop
Dim c As Integer
c=1
Do
c=c+1
Loop While c < 5
Dim c As Integer
c=1
Do Until c >4
c=c+1
Loop
Dim c As Integer, x as integer
c=1
x=2
Do
c=c+1
Loop Until c>4 And x<>3
Arrays (fixed size)
Dim x(100) As Integer
Dim i As Integer
For i = 0 To 100
x(i) = 99
Next
Exercise – use the debugger – check each stage
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Fill an array with 25 random integers in
the range 1 to 100 (Rnd() gives a
random single between 0 and 1 )
Find the total
Find the largest
Search the array for a value given in
cell(1,1)
Arrays (2 d)
Dim x(1 To 3, 1 To 3) As Integer
Dim i as integer, j As Integer
For i = 1 To 3
For j = 1 To 3
x(i, j) = 99
Next
Next
Dynamic arrays
Dim x() As Integer
ReDim x(5)
Dim i, j As Integer
For i = 1 To 5
x(i) = 99
Next
ReDim Preserve x(10)
For i = 6 To 10
x(i) = 100
Next
Exercise
Use the =RAND() function to fill a column of a sheet
with 20 random numbers
Then set up a button and write VB code which will –
•read those 20 numbers into an array
•sort the array into increasing order (bubblesort)
•display the numbers in the next column of the
spreadsheet