CSM18 Visual Basic Section1 Department of Computing UniS 1 CSM18 Interactive Computing Introduce...

CSM18 Visual Basic Section1 Department of Computing UniS1

CSM18 Interactive Computing

• Introduce Visual Basic language - provides excellent facilities for building a user interface

• Need to build User-Friendly interfaces• Introduction to Data Base system Access • Control of Access via Visual Basic• Plan and Implement a small project


CSM18 Brief historic overview

Dataoperations

on data

Stored program - keyboard & printer

Keyboard & screen - text only

Keyboard & screen - screen input WYSIWYG

Apple, IBM PC, Microsoft MSDos, Windows


CSM18 Procedural Programming

• List of instructions - code

• Execution proceeds from one instruction to next

• Need for sequence control - logical condition• Hence branching and repeating structures• The concept of Algorithm:

a finite number of steps to carryout a specific task eg receipt to bake a cake

• Hence branching and repeating structures


CSM18 Algorithms

Algebra

A = B + C

B = A - C

Assignments

A B + C assigns B + C to A

A A + 1 increments A

X A

A B interchanges two numbers

B X


CSM18 Declarative Programming

• Declare statements or facts that are true• Declare relations between facts• Interrogate the system - is this true? Response

Yes or No• Good for solving logical problems - don’t have

to tell the computer how to do it as you do in the case of procedural programming via the algorithm


CSM18 Spreadsheet computations

• Declare relation between one cell and another

• Limited capability eg can’t put data into a cell based upon the result of a calculation

= B3*1.1

10 11

Cell B3


CSM18 Real Time Programming

• Procedural language that can respond to time• At 1200 do . . . . • Used for control of real systems such as power

stations • System can collect data directly from the real

world eg read the temperature of furnace


CSM18 Event Driven Programming

• Procedural language to support a user interface• Processes are initiated by an event on screen

eg user clicks a button • Large code split into smaller blocks -

subroutines that are associated with events• A major feature of Visual Basic • All the features of the windows environment are

readily available in Visual Basic


CSM18 Visual Basic

• Complex but easy to use development environment

• Integrated Development Environment - IDE provides for all the facilities to build, test and execute a program

• Large code split into smaller blocks - subroutines that are associated with events


CSM18 Visual Basic

• A major feature of Visual Basic • All the features of the windows environment

are readily available in Visual Basic

• Exploration of the IDE is the first lab assignment


CSM18 Visual Basic IDE

• User Form • Menu Bar• Tool Bar• Tool Box• Project window• Properties window• Immediate window• Locals window• Watch window• Form layout window


CSM18 Programming Constructs

• data stored in variables, whose value can change over time

• they can be altered or manipulated by operations, known as program statements

2000 197525

Age = CurrentYear – YearOfBirth


CSM18 Programming Constructs

• Assign a value to a variable with the = sign yet conceptually it can be represented as

• Operators work on values & variables to perform• simple arithmetic (+, -, *, /...) • string (text) manipulation• date manipulation

a = 24.5

a 24.5


• Values, variables and operators can be put together (using a form of grammar) to form expressions

• An expression

• is a combination of values, variables and operators

• has a value (found by calculating the expression)• is a type of data (e.g. number, string, date…)• can be a direct replacement for a value of the

appropriate type

CSM18 Expressions


CSM18 Expressions

Examples:

x+2

“Joe” & “ Bloggs”

Date + 7 (1 week from today)

z * (x + 2) (value 9 if z=3, x=1)

Examples:

x+2

“Joe” & “ Bloggs”

Date + 7 (1 week from today)

z * (x + 2) (value 9 if z=3, x=1)


• Variables need to be declared • with the name and the type of data• Integer types (Byte, Integer, Long) for whole

numbers• Floating point types (Single, Double) for numbers

with fractional parts• Dates and times (one type, Date, for both)• Fixed point types (Decimal, Currency) for high

precision (many digits)• String type for text

CSM18 Declarations


CSM18 Declarations

Dim index As integer Can be placed anywhere

Private name As String Declarations in General section Only accessible from same module

Public today As Date Declarations in General sectionAccessible anywhere

Dim index As integer Can be placed anywhere

Private name As String Declarations in General section Only accessible from same module

Public today As Date Declarations in General sectionAccessible anywhere

Module - part of the structure of a program or application:


CSM18 ModulesThree types of code file• Form module

Defines look and behaviour of Windows

• Class ModuleDefines content and behaviour of objects

• Code (or Standard) ModuleDefines general purpose Subroutines and Functions

Form Module

CodeModule

Class ModuleObject

ObjectObject

Object

FormForm

FormForm

Code

Design Time Run Time


• User-interface development• Form modules

Screen layout plus data content plus behaviour

• Description of type of object• Class Module

Can have more than one object of the type

• Definition of general purpose code• Standard Module

Can only be one copy of each module in a program

CSM18 Types of Module


• Every module has• General Declarations section• Subroutines (Subs) and Functions

• Form modules also have• User interface elements (which appear on-screen)• Event handlers

• Class modules also have• Event receptors (objects that can respond to

events)

CSM18 Module Structure


• Where a variable can be accessed depends on how it is declared• Local scope (Dim) – only within Procedure (Sub or

Function) in which it is declared• Module Scope (Private) – available in every

Procedure within a module• Global Scope (Public) – available throughout

application

• Controlling scope makes it possible to work within contexts, so that variable names can be reused with no conflicts

CSM18 Variable Scope


• Public variable is visible throughout all modules

• No two Public variables with the same name

• Private in one module is invisible in another

• Local variable overrides Private or Public with same name

CSM18 Scope Rules


• Local variables (within Sub or Function)• Declared with Dim are

reset to zero every time Procedure is called

• Declared with Static, retain their value

Sub Forgettable( )Dim Number As Integer

Number = Number + 1End Sub Always 1 at End Sub

Sub Persistent()Static CallCount As Integer

CallCount = CallCount + 1End Sub Counts up

Sub Forgettable( )Dim Number As Integer

Number = Number + 1End Sub Always 1 at End Sub

Sub Persistent()Static CallCount As Integer

CallCount = CallCount + 1End Sub Counts up

CSM18 Local & Static Variables


• A Variant is an all purpose variable• Type is always Variant• Sub-Type matches whatever value is assigned• Sub-Type changes to suit

Private Sub SquareRoot_Click()Dim V As Variant V = 2 Assign an integer V = Sqr(V) Assign a double V = “Root 2 = “ & V Assign a stringEnd Sub

Private Sub SquareRoot_Click()Dim V As Variant V = 2 Assign an integer V = Sqr(V) Assign a double V = “Root 2 = “ & V Assign a stringEnd Sub

CSM18 Variants


• A UDT is composed of simple types (Integers, strings etc.)

• Each instance contains one of each component variable

• Must be placed in Standard Modules• Definition is Global (available

throughout program)• Useful for grouping related

information

CSM18 User Defined Types


CSM18 An example UDF

Type PhoneEntry Name As String Telephone As StringEnd Type

Dim PE As PhoneEntry PE.Name = “Fred Bloggs” PE.Telephone = “2468” Print PE.Name, PE.Telephone


All executable statements must be part of a Sub or Function in VB

• Sub – an operation, delineated by Sub and End Sub, containing a sequence of statements

• a Sub ‘call’ is a statement that invokes the Sub (executes the operation)

• Function – similar to a Sub, but returning a value as a result

• a function call is an expression – can be used as a value in another expression or statement

• Both Subs and Functions can have parameters or arguments – variables used to get information in and/or out of the Sub/Function

CSM18 Subs & Functions


CSM18 Subs & Functions

Sub Greeting (Name As String)

MsgBox “Hello “ & Name

End Sub

Greeting (“John”)

Greeting “John”

Call Greeting (“John”)


• A special form of 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

CSM18 Event Handlers

Private Sub Botton1_Click()

MsgBox “Hello World”

End Sub

End of Section 1

CSM18 Visual Basic Section1 Department of Computing UniS 1 CSM18 Interactive Computing Introduce...

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