1. CS285 Visual Basic 2 Department of Computing UniS 1 Statements in Visual Basic A statement is the fundamental syntactical element of a program smallest piece of code that the language will accept in its own right A statement can be used to set aside space for data storage (variables) assign a value to a variable perform a calculation and assign its result to a variable execute a previously-defined operation control other statements CS285 Programming in VB

2. CS285 Visual Basic 2 Department of Computing UniS 2 Visual Basic - Example Statements Dim x As Integer, y As Single x = 2 x = x + 4 y = Sqr(x) Print y Declares variables Assigns a value to the variable Assigns the result of an expression Uses an in-built function Executes a pre- defined operation Prints values on the current form from the top down

3. CS285 Visual Basic 2 Department of Computing UniS 3 Visual Basic - Input & Output Forms are Windows but are also VB Objects which contain other objects - Interaction Objects e.g. input, output and command controls Input - InputBox, Text Box, Check Box, Lists etc. Output - Text Box, Message Box, Print, Picture Box Command - Buttons, Menus etc Private Sub Button1_Click() Dim Data As Integer Data = InputBox(“Enter a number”) Text1.Text = “The number is “ & Data MsgBox (“The number is “ & Data) Print “The number is “ & Data End Sub

4. CS285 Visual Basic 2 Department of Computing UniS 4 Visual Basic - Design Issues A Form should allow user-input in any order Processing triggered when a command is issued Users should be kept informed of appropriate actions required and results of actions Make sure inappropriate actions are disabled Private Sub Command1_Click() Command2.Enabled = False Command3.Visible = True End Sub _Click()

5. CS285 Visual Basic 2 Department of Computing UniS 5 VB - Structures within Software Structure is apparent at a number of levels A project can involve a number of modules, each occupying a separate file Each module will contain a number of procedures (Subs and Functions) and variable declarations Within a Sub or Function, certain standard code constructs are used to define whether actions happen or not, or how often they are executed Data variables in a module can be grouped in several ways to form data structures The last two categories are central to the idea of ‘Structured Programming’

6. CS285 Visual Basic 2 Department of Computing UniS 6 VB - Structured Programming Born out of chaotic nature of software designs before late 1960s Idea is to limit the power of the programmer to direct a program with no restrictions Code statements are organised in a small number of standard forms Structure of data should mirror real-life information structure wherever possible

7. CS285 Visual Basic 2 Department of Computing UniS 7 VB - Program Control Constructs Three main structuring principles Code statements should be grouped into functional units Subs, Functions Possible to select whether one or more statements will execute or not, based on a simple logical criterion If...Then, If...Then...Else, Select Case Possible to repeat a statement or group of statements either a given number of times or until a specific condition is true For...Next, Do...Loop, While...WEnd

8. CS285 Visual Basic 2 Department of Computing UniS 8 Visual Basic - Conditions Central to the principles of programming The flow of execution depends on prevailing conditions within a program A condition is an expression that evaluates to True or False. Can be… A single Boolean variable A combination of Boolean variables The result of a comparison for equality or relative size A combination of such comparisons

9. CS285 Visual Basic 2 Department of Computing UniS 9 Visual Basic - Logical Conditions X=0‘ True if X is zero, False otherwise Y=X‘ True if Y equals X X < Y‘ True is X is bigger than Y X = 0 And Y >2 ‘ True if both are true X = 0 Or Y > 2 ‘ True if either is true These are not assignments:

10. CS285 Visual Basic 2 Department of Computing UniS 10 VB - Conditions to control flow Using a condition, can direct the flow of a program… If Time > “12:00” Then Call Afternoon() Else Call Morning() End If

11. CS285 Visual Basic 2 Department of Computing UniS 11 VB - If...Then flexible control If Time < “12:00” Then MsgBox “Good morning” ElseIf Time > “12:00” And Time < “18:00” Then MsgBox “Good afternoon” Else MsgBox “Good evening” End If

12. CS285 Visual Basic 2 Department of Computing UniS 12 Visual Basic - Case Structure When dealing with many possible conditions use a Case Construct to Select one of a number of possible Cases Select Case Variable Select Case MonthNo Case 4, 6, 9, 11 MsgBox “This month has 30 days” Case 2 MsgBox “This month has 28 days” Case Else MsgBox “This month has 31 days” End Select

13. CS285 Visual Basic 2 Department of Computing UniS 13 Visual Basic - Iteration Iteration is repetition of code Can execute one or more statements A given number of times Until a condition becomes True While a condition remains True This allows the same code to be reused For a number of similar variables For the same variable(s) containing a succession of values e.g. Print a number of pages Get input from a user until a valid entry Calculate a succession of loan payments

14. CS285 Visual Basic 2 Department of Computing UniS 14 Visual Basic - Iteration For index = 1 To 12 ‘ Print a table of squares Print index, index * index Next Do ‘Repeat until user enters something Name = InputBox (“Enter your name”) Loop Until Name <> “”

15. CS285 Visual Basic 2 Department of Computing UniS 15 Visual Basic - Iteration While Time < “18:00” DoWork () ‘Sub DoWork() defined elsewhere Wend Note: This code will not be executed unless the initial condition is met. Compare with Do…Loop which executes the code at least once.

16. CS285 Visual Basic 2 Department of Computing UniS 16 Visual Basic - Structured Data May need to work with sets of data Classes of students Lists of library books Normally, information is complex and has structure Items are related by.. Similarity (e.g. class of students) Hierarchy (e.g. a book has several chapters) Grouping (e.g. a person has a name, address, phone number, national insurance number, credit cards, etc..) We use data structures to group information together

17. CS285 Visual Basic 2 Department of Computing UniS 17 Visual Basic - Arrays Simplest form of data structure A number of variables of the same type, related in some way A list of names A Table of profit figures All elements share the same name Each element has an index indicating its position in the array

18. CS285 Visual Basic 2 Department of Computing UniS 18 Visual Basic Arrays Students John Smith Jane Green Mary Brown Mike Stone Ashok Din Profits 1240.00 1775.50 1602.45 1100.70 1450.25 1825.23 1743.10 1250.50 1603.33 1733.24 1679.95 1432.55 1998 1999 2000 12341234 123456123456 Quarters 1-Dimensional Array 2-Dimensional Array Element Index Dim Students(1 To 6) As String Dim Profits(1998 To 2000, 1 To 4) As Single

19. CS285 Visual Basic 2 Department of Computing UniS 19 VB - Programming with Arrays Use For..Next construct to loop through an array Dim ID As Integer, Year As Integer, Quarter As Integer …… For ID = 1 To 6 Print Students(ID) Next For Year = 1998 To 2000 ‘Note nested For loops For Quarter = 1 To 4 TotalProfit = TotalProfit + Profits(Year, Quarter) Next

20. CS285 Visual Basic 2 Department of Computing UniS 20 VB - Programming with Arrays Use of a Control Array An array of Control Objects: Create the first Control Object and name it Create the second Control Object and give it the same name. VB asks if this is an array. Reference by Name(0), Name(1), Name(2) etc.

21. CS285 Visual Basic 2 Department of Computing UniS 21 VB - Use of Control Arrays For i = 0 To 3 TextBox(i).Text = "Text Box " & i Next

22. CS285 Visual Basic 2 Department of Computing UniS 22 Visual Basic - Use of ListBox Sub Form_Load() List1.AddItem "Red" List1.AddItem "Blue" List1.AddItem "Green" End Sub Sub List1_Click() Select Case List1.ListIndex Case 0 TextBox(0).BackColor = &HFF Case 1 TextBox(1).BackColor = &HFF00 Case 2 TextBox(2).BackColor = &HFF0000 End Select End Sub

23. CS285 Visual Basic 2 Department of Computing UniS 23 Visual Basic - Program in a Module Use a module to provide code accessible from any where in the project Add Module, make Option Explicit - forces declaration of variables - good practice Data Storage with Private Procedures Access with Public Procedures

24. CS285 Visual Basic 2 Department of Computing UniS 24 Visual Basic - Program in a Module Option Explicit ‘Forces declaration of variables Private Const Max = 10 Private Names (1 To Max) As String Public Sub AddEntry() ….. Public Sub DisplayList() …..