Lec2 P 1 CP2030 Visual Basic For C++ Programmers Copyright © University of Wolverhampton CP2030...

Lec2 P 1CP2030 Visual Basic For C++ Programmers Copyright © University of Wolverhampton

CP2030 VBFCLecture 2

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Basic Data Types Arithmetic Operators Inputting Strings Handling Numbers Scope Of Variables


Variable Identifiers Declaring variables:

Dim VariableName As type

Examples:Students to add example

The rules for naming identifiers are:

– Must begin with a letter

– Can only contain letters, numbers and the underscore character ( _ )

– Must not exceed 40 characters

– Cannot be a Visual Basic reserved word


Visual Basic Standard Data Types

Data type Storage size Example Name Integer 2 bytes iCount Long (integer) 4 bytes lDebt Single(floating-point) 4 bytes fArea Double(floating point) 8 bytes dWidth Currency(scaled integer) 8 bytes cPrice String 1 byte/char sName Variant Varies vData User-defined(Type) size bytes uPaymentDetails

(VB4 provides Byte, Boolean, Date, etc)


Note

String - used for textual information,length is determined when assigned

Variant - for compatibility with earlier BASICs,the type is determined when it is assigned to.

DO NOT USE

Boolean - No boolean type available, (up to VB4)but they can be expressed as an integer - declare

as Integer. 0 = FALSE

anything else = TRUE


Naming Conventions

Prefix each name with a lower case letter showing the data type– i for Integer

– l for Long

– d for Double

– s for String

– c for Currency

– f for Single Float

– u for User Defined

Capitalise the first/significant letter of each word in the name– e.g. iNumberOfSeats

Choose meaningful names– if you cannot pronounce it then it is not meaningful!


Variable Declaration 1

Variables can be declared in a control event procedure

In this case they are only available within the procedure in which they are declared.

Local to the procedure (Sub routine)


Constant Identifiers and Naming

Use meaningful names

By capitilising the whole identifier it is easier to see where constants are used in code

Examples:Const SEATCOST = 2.5 ‘price of a seat

'total number of seats availableConst SEATSAVAILABLE = 100

Note the use of Comments


Constant Declaration

Constants can be declared in a control event procedure

In this case they are only available within the procedure in which they are declared

Local to the procedure (Sub routine)


Implicit / Explicit Declaration Implicit Declaration:

You can just use any name,Variable type determined by the data assigned

Explicit Declaration:You must pre-declare any variables used:

Dim sName As StringDim iCount As Integer

Explicit declaration is a much better approach to adopt,implicit declaration is for backwards compatibility

Can force Explicit Declaration via Options/Environment option


Arithmetic Operators

Arithmetic operation Symbol Example

Exponentiation ^ 2^3 = 8 (2*2*2 or 23)

Multiplication & division *, / 2/4 = 0.5Addition & subtraction +, - 2+2=4

Integer division \ 7\2 = 3 (goes 3 times)

Modulo arithmetic Mod 7 Mod 2 = 1 (remainder 1)

String addition + "abc" + "def "= “abcdef"String concatenation & "abc" & "def "= "abcdef"


Operator Order of Precedence

In expressions containing a mixture of operators the order of evaluation is arithmetic, comparison and logical operators

Within individual categories, operators are evaluated in the order of precedence shown below:

Arithmetic Comparison LogicalExponentiation (^) Equality (=) NotNegation ( - ) Inequality (<>) AndMultiplication, division (*,/ ) Less than (<) OrInteger division (\) Greater than (>) XorModulo arithmetic (Mod) Less than or Equal to (<=) EqvAddition, subtraction (+,-) Greater than or Equal to (>=) ImpString concatenation (&) Like

Is


Assignment In Basic the = character is both the assignment character and

the logical equivalence character:Students to add an example

Some type conversions are automatic:Students to add example

Use the string concatenator & to allow automatic conversion:Students to add example

Some type conversions must be coded


Val and Str$

Converting a String to Number

Students to add an example

Converting a Number to a String Students to add an example


Question Assume that the following variable have been declared :

inum1, inum2, snum3, snum4 and

iresult1, sresult2.

Show the statements for the following :

1. Assign inum1 + inum2 to iresult1

2. Assign inum1 + inum2 to sresult2

3. Assign snum3 + snum4 to iresult1

4. Assign snum3 + snum4 to sresult2

5. Assign inum1 + snum2 to iresult1

6. Assign inum2 + snum4 to sresult2


Inputting Strings

eg.

To input a persons first and second name and assign to string variables.

sFirstName = Text1.Text

sSecondName = Text2.Text


Concatenating Strings Concatenating the input text (strings )

Students to add code


Handling Numbers VB will automatically convert numbers input from and output to

text and label boxes.

Students to add notes


Question

Write an application to input two numbers via text boxes. Use four separate commands to calculate either the addition, subtraction, multiplication or division of the numbers. The result is to be displayed in a Label box.

For this question use FULL CONVERSIONS via Str$() and Val()


Referencing properties

Assigning a labels’ caption equal to the text in a text box we would write:

Label1.Caption = Text1.text Label1.Caption = Text1.text

We could write :Form1.Label1.Caption = Form1.Text1.TexForm1.Label1.Caption = Form1.Text1.Text

The Form associated with the statement is taken as the default.

To reference properties on other forms :

Students to add statementStudents to add statement


SetFocus Methods We may want to use code to set the Focus (point of input) to a

particular control, for example in our earlier program after adding the numbers we may want to set the Focus back to the first text box

Students to add statementStudents to add statement

Our code in our earlier program would be modified to

This sets the Focus back to text box Text1Text1


TabIndex

If we want to be able to move between controls using the Tab key we can set the TabIndex property - at design or run time.

For our earlier program we may want to Tab from control Text1 to Text2 Then Command1

To do this we would set in the properties window– Students to add statements


Form Load

Code can be attached to a form and executed whenever the form is loaded.

Used for initialisationInitialise variables, arrays, controls

E.g.Text1.Text = ‘xxxxxxx’

Text2.Text = ‘xxxxxxx’


Scope Of VariablesVariable Declaration 2

Variables can also be declared in the general declarations section of a form (Double click on the form)

In this case they are available anywhere within the form in which they are declared - ie. in any control code on the form BUT not on other forms


Scope of Variables

Shows declarationsat form level, knownas: General Declarations

Shows variabledeclarations withinan event handler

Form1General Declarations

Sub Command1_Click ()

Sub Command2_Click ()

Dim sName1 As StringDim iNum1 As Integer



Available variables:sName1, sName2, iNum1, iNum2

Available variables:sName1, sName3, iNum1, iNum3


Static Variables A static variable will hold its value when it goes out of scope:

Sub Command1_Click() 'declare variables

Dim iDimCount As Integer Static iStaticCount As Integer

'increment variables iDimCount = iDimCount + 1 iStaticCount = iStaticCount + 1 'display variables Label3.Caption = Str$(iDimCount) Label4.Caption = Str$(iStaticCount)End Sub

A static variable can only be declared inside a procedure and only available inside the procedure


Static Variables: Effects

The effect of using a static variable can be seen below:

Students to add diagrams

This has the same effect as if the variable had been declared at the form’s general declaration levelExcept the scope is local to the procedure


Scope Of VariablesCode Modules

A code module can be thought of as being a form without a screen attached

In a code module you can declare:

Students to add examples

We can also write code in a module,we will not be looking at this …....yet

Variables declared as Global have scope throughout the application, that is they can be seen across all forms


Adding a Module to an Application

Use either the tool bar Module icon or the New Module menu option to add a code module to the application

The tool bar Module icon

The menu New Module option


Code Module Windows Available

A code module consists only ofdeclarations and code

It doesn’t have a form associatedwith it

You can only getaccess to a codeentry window


Global Variables

In a code module we can declare a variable using the keyword Global

General form for declaring global variables:

Global VariableName As type

Example:

Global giTotalSeatsBooked As Integer

Prefix each global variable identifier with a lower case letter g to show that it is a global variable and the lower case letter indicating the data type. e.g. i for integer


Visual Basic Declaration Locations

Form 1

Control 1

Code Module 1Dim iNum1 As IntegerGlobal giNum1 As Integer

Dim iNum2 As Integer

Dim iNum3 As IntegerStatic iNum4 As Integer

Control 2Dim iNum5 As IntegerStatic iNum7 As Integer

Form 2

Control 1

Dim iNum8 As Integer

Dim iNum9 As IntegerStatic iNumA As Integer

Control 2Dim iNumB As Integer


Using Global Variables

Global variables are ideal for storing information which has to be accessed from more than one form

They have scope throughout all forms and modules within an application

Students to add code Students to add code

Global gsName As String

Module1.Bas


Global Constants

A global constant is basicaly a global variable that you cannot change once the program is running

You must initialise a global constant with the required value when it is declared, just as with an ordinary constant

The syntax for a Global Const statement is:Global Const identifier = expression

You should use meaningful identifies names Example:

'total number of seats available in hallGlobal Const gTOTALSEATSAVAILABLE% = 100


Question The application is to calculate the average age of a class of

students. Form1 is used to enter and accumulate the students ages and form2 used to calculate the average and display the results.

Show the code for all command buttons and clearly show where the variables are declared.

Lec2 P 1 CP2030 Visual Basic For C++ Programmers Copyright © University of Wolverhampton CP2030...

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