MS Visual Basic Applications

Walter Milner

Event-driven programming

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

Dim x as Integer Dim x,y as Integer NO! Is case sensitive (kind of) Variable naming conventions

• Microsoft Simonyi Hungarian Reddick

• House rules

Assignment statement – x = 4

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

Data types

Integer Long Single Double Currency Byte unsigned 0 - 255 String Boolean Date Object Variant NO!

Data type conversion

Conversionfunction

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

DIM x as integer

x = Cint("10")

Controls

Private Sub CommandButton1_Click()Dim x As IntegerDim y As IntegerDim z As Integerx = TextBox1.Valuey = TextBox2.Valuez = x + yLabel1.Caption = zEnd Sub

Copy this

Correct it

Add other buttons

VB Core II

Conditional statements Exception handling Loops Arrays Debugging

if statements

If c > 5 Then x = 1 y = 3End If

If c > 5 Then x = 1: y = 3

If c > 5 Then x = 1 y = 3Else z = 7End If

If c > 5 Then x = 1 y = 3ElseIf c = 4 Then z = 7Else x = 9End If

select

Dim Number Number = 8 ' Initialize variable. Select Case Number ' Evaluate Number.

Case 1 To 5 ' Number between 1 and 5, inclusive. x=4Case 6, 7, 8 ' Number between 6 and 8. x=5Case 9 To 10 ' Number is 9 or 10. x=6Case Else ' Other values. x=7

End Select

Error and exception handling

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 numbersPrivate Sub Command1_Click()Dim num1 As IntegerDim num2 As IntegerDim result As Integer

On Error GoTo myErrorHandlernum1 = Textbox1.Textnum2 = Textbox2.Textresult = num1 + num2Label1.Caption = resultExit Sub

myErrorHandler:If Err.Number = 13 Then MsgBox ("Please enter a valid number")Else MsgBox (Err.Description)End IfResume Next

End Sub

Exercise

Try this out in the calculator program

Then deal with divide by zero (11)

For next loops

Dim x as Integer, total As Integertotal = 0For x = 1 To 5 total = total + xNext

Dim x as Integer, total As Integertotal = 0For x = 1 To 5 Step 2 total = total + xNext

Debugging

Exercises – Use the debugger to watch..

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 Integerc = 1Do While c < 5 c = c + 1Loop

Dim c As Integerc = 1Do Until c >4 c = c + 1Loop

Dim c As Integerc = 1Do c = c + 1Loop While c < 5

Dim c As Integer, x as integerc = 1x=2Do c = c + 1Loop Until c>4 And x<>3

Arrays (fixed size)

Dim x(100) As IntegerDim i As IntegerFor i = 0 To 100 x(i) = 99Next

Exercise – use the debugger – check each stage

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 IntegerDim i as integer, j As IntegerFor i = 1 To 3 For j = 1 To 3 x(i, j) = 99 NextNext

Dynamic arrays

Dim x() As IntegerReDim x(5)

Dim i, j As IntegerFor i = 1 To 5 x(i) = 99Next

ReDim Preserve x(10)For i = 6 To 10 x(i) = 100Next

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