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CSM18 Further Constructs in VB

CSM18 Further Constructs in VB. 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

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CSM18 Further Constructs in VB

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  1. CSM18 Further Constructs in VB • 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 Department of Computing UniS

  2. CSM18 Example Statements Declares variables Dim x As Integer, y As Single x = 2 x = x + 4 y = Sqr(x) Print y Assigns a value to the variable Assigns the result of an expression Uses an in-built function Executes a pre-defined operation Department of Computing UniS

  3. CSM18 Forms • Forms are Windows but are also VB Objects which contain other objects - Interaction Objects eg input, output and command controls • Input - InputBox, Text Box, Check Box, Lists etc. • Command - buttons, menus etc • Output - Text Box, Message Box, Picture Box etc • 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 Department of Computing UniS

  4. CSM18 Event Driven Program • A special form of Subroutine (Sub) that Visual Basic associates with an event • Mouse operation • Key-press • Signal from another application • An event handler is called automatically when the event happens • Program can respond to external stimuli Private Sub Botton1_Click() Statements…. End Sub Department of Computing UniS

  5. CSM18 Input & Output • Input use an InputBox or TextBox • Output use a MsgBox or TextBox or Print Private Sub Botton1_Click() Dim Data As Integer Data = InputBox(“Enter a number”) Text1.Text = “The number is “ & Data Print “The number is “ & Data End Sub Department of Computing UniS

  6. CSM18 Event-Driven User Interfaces • Interaction Objects are created on the Form • User interacts with objects to input data, control the program and to receive data/information. • Objects have a set of Properties that describe the state of the object. • Values of Properties can be changed by program statements Department of Computing UniS

  7. CSM18 Event-Driven User Interfaces • To make the button visible set it’s property ‘visible’ to True • Button1.Visible = False Department of Computing UniS

  8. CMS18 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’ Department of Computing UniS

  9. CSM18 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 Department of Computing UniS

  10. CSM18 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 Department of Computing UniS

  11. CSM18 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 Department of Computing UniS

  12. CSM18 Logical Conditions These are not assignments: 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 Department of Computing UniS

  13. CSM18 Conditions to control flow… • Using a condition, can direct the flow of a program… If Time > “12:00” Then Sub Afternoon() Else Sub Morning() End If Department of Computing UniS

  14. CSM18 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 Department of Computing UniS

  15. CSM18 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 Department of Computing UniS

  16. CSM18 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 eg • Print a number of pages • Get input from a user until a valid entry • Calculate a succession of loanpayments Department of Computing UniS

  17. CSM18 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 <> “” Department of Computing UniS

  18. CSM18 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. Department of Computing UniS

  19. CSM18 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, nationalinsurance number, credit cards, etc..) • We use data structures to group information together Department of Computing UniS

  20. CSM18 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 Department of Computing UniS

  21. John Smith Jane Green 1240.00 1450.25 1603.33 Mary Brown 1775.50 1825.23 1733.24 Mike Stone 1602.45 1743.10 1679.95 Ashok Din 1100.70 1250.50 1432.55 CSM18 Arrays Dim Profits(1998 To 2000, 1 To 4) As Single Dim Students(1 To 6) As String Index Students Profits 1 2 3 4 5 6 Quarters 1998 1999 2000 1 2 3 4 1-Dimensional Array 2-Dimensional Array Element Index Department of Computing UniS

  22. CSM18 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 Next Department of Computing UniS

  23. CSM18 Program in a Module • Use a module to provide code accessible form 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 Department of Computing UniS

  24. CSM18 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() ….. Department of Computing UniS

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