csm18 visual basic section 2 department of computing unis 1 csm18 further constructs in vb a...
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CSM18 Visual Basic Section 2 Department of Computing UniS1
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
CSM18 Visual Basic Section 2 Department of Computing UniS2
CSM18 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
CSM18 Visual Basic Section 2 Department of Computing UniS3
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
CSM18 Visual Basic Section 2 Department of Computing UniS4
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
CSM18 Visual Basic Section 2 Department of Computing UniS5
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
CSM18 Visual Basic Section 2 Department of Computing UniS6
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
CSM18 Visual Basic Section 2 Department of Computing UniS7
CSM18 Event-Driven User Interfaces
• To make the button visible set it’s property ‘visible’ to True
•Button1.Visible = False
CSM18 Visual Basic Section 2 Department of Computing UniS8
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’
CSM18 Visual Basic Section 2 Department of Computing UniS9
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
CSM18 Visual Basic Section 2 Department of Computing UniS10
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
CSM18 Visual Basic Section 2 Department of Computing UniS11
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
CSM18 Visual Basic Section 2 Department of Computing UniS12
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
CSM18 Visual Basic Section 2 Department of Computing UniS13
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
CSM18 Visual Basic Section 2 Department of Computing UniS14
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
CSM18 Visual Basic Section 2 Department of Computing UniS15
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
CSM18 Visual Basic Section 2 Department of Computing UniS16
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 loan payments
CSM18 Visual Basic Section 2 Department of Computing UniS17
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 <> “”
CSM18 Visual Basic Section 2 Department of Computing UniS18
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.
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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, national insurance number, credit cards, etc..)
• We use data structures to group information together
CSM18 Visual Basic Section 2 Department of Computing UniS20
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
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CSM18 Arrays
StudentsJohn 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
1
2
3
4
123456
Quarters
1-Dimensional Array 2-Dimensional
ArrayEleme
ntIndex
IndexDim Students(1 To 6) As String
Dim Students(1 To 6) As String
Dim Profits(1998 To 2000, 1 To 4) As Single
Dim Profits(1998 To 2000, 1 To 4) As Single
CSM18 Visual Basic Section 2 Department of Computing UniS22
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) NextNext
CSM18 Visual Basic Section 2 Department of Computing UniS23
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
CSM18 Visual Basic Section 2 Department of Computing UniS24
CSM18 Program in a Module
Option Explicit ‘Forces declaration of variables
Private Const Max = 10Private Names (1 To Max) As String
Public Sub AddEntry()…..
Public Sub DisplayList()…..