Chapter 4

Chapter 4 - Visual Basic Schneider 1

Chapter 4

General Procedures


Outline & Objective

Creating Visual Basic Sub Procedures Creating User-defined Function Procedures Parameter Passing Mechanism Modularizing in Programming Languages


What is Modularization

Breaking the program into subtasks A Sub procedure or Function performs a

well-defined task Easier to test, debug and correct


Why use Sub procedures and Functions?

Provide abstract operations Make programs easier to write, debug and

maintain.


Modularizing Programs in Visual Basic

In Visual Basic, there are two types of procedures: – Sub– Function

Note: To distinguish procedures from event procedures, Sub and Functions are referred to as general procedures.


Passing Arguments to Subs:

When you define a Sub procedure; sometimes you need to transfer variables that are used in different Subs. This is called passing in programming languages.


Sub Procedures Properties:

may be called may be passed data called arguments may return values to the calling program may change the data stored in a received

variable


Components of Sub Procedure:

name: used to identify the Sub procedure parameters: a Sub procedure accepts values

from the caller through its parameters; it may also send values back to the caller through it’s parameters.


Sub Procedure's Name

The rules for naming Sub Procedures are the same as naming variables.

In this text, Sub procedure names begin with uppercase letters in order to distinguish them from variable names.


Syntax of a Sub Procedure

Private Sub ProcedureName ( )

statement(s)

End Sub


Creating Visual Basic Sub Procedure:

Activate a code window Select Add Procedure from the Tools menu Type in the name of the Sub procedure Click on Private in Scope frame Press the Enter key or click the OK button Type the statements of the Sub procedure

into this window


Example of Call to a Sub Procedure:

Private Sub cmdCompute_Click()

Dim num As Single

num = Val(InputBox("Enter a number:"))

Call Triple(num)

End Sub


Sub Procedure Triple:

Private Sub Triple(num As Single)

' Multiply the value of the number by 3

picResult.Print "The number is"; 3 * num

End Sub


Passing Arguments to Sub Procedures

Arguments : Variables or expressions placed in parentheses in a Call statement.

Not only is the value of the argument passed to the parameter, but the value of the parameter is passed back to the argument.


Example of Arguments

Call Triple(num)


Parameters

Variables placed in parentheses after a Sub Procedure's name.

When the procedure is called, the values of the corresponding arguments are placed in the parameters.


Example of Parameters



Passing arguments to parameters

Call Triple(num )

Private Sub Triple (num As Single)

Argument

Parameter


Passing Arguments to Parameters

Call Add (x, y )

Private Sub Add ( num1 As Single, num2 As Single)

Parameters

Arguments


Passing Arguments

The Sub Procedure receives the location of the arguments, the Sub Procedure may use and modify the value of the arguments.

Two way into and out of the Sub Procedure.


Passing Arguments

Private Sub cmdDisplay_Click()

Dim amt As Single

amt = 2

picResults.Print amt;

Call Triple(amt)

picResults.Print amt

End Sub


Sub Triple


' Triple a number

picResults.Print num;

num = 3 * num


End Sub


Passing Arguments

Call Triple(amt)

Private Sub Triple (num As Single)

amt

num


Passing Data - by Reference

2 2 6 6

amt amt amt

num num


Important Rules for Passing Arguments to a Sub

The number of arguments and parameters must match.

The data type of each argument must match its corresponding parameter.


Local Variables:

A variable that is used only in a specific procedure (Sub or Function).

The scope of the local variable is the portion of a Sub or Function in which that variable has been defined.


Local Variables:

Declared within a procedure definition Private to a procedure definition Variables in different procedures are totally independent different procedures can have variables with the same

names; however, each variable will have its own memory location


Advantages of Local Variables

Extremely useful for team programming To protect side effect (which is an

accidental change of the value of the variable)


Example of Local Variables

Private Sub cmdButton_Click()

Dim var1 As Integer, var2 As Integer,num As Integer

var1 = 2

var2 = 4

Call Add(num)

picBox.Print num

End Sub


Sub Add

Private Sub Add(num As Integer)

Dim var1 As Integer, var2 As Integer

num = var1 + var2

End Sub


Form-Level Variables

Form-level variables are visible to every procedure (Global variable).

Form-level variables appear at the top of the code window.


How to create Form-Level Variables?

Activate the code window Click on the down arrow to the right of the object

list box Click on General Click on Declaration in the procedure list box Type in Dim statements for form-level variables


Example

' In Declaration section of General

Dim num1 As Single, num2 As Single


Review

Visual Basic has two types of procedures:– Sub– Function

Each Sub procedure performs a distinct task.

The Call statement causes a Sub procedure to be executed.


Review

A Sub procedure can receive many values

A Sub procedure can return many values


Review

Values can be passed between the calling program and Sub by passing arguments.

The number and type of arguments in the calling program and Sub must match.


Review

Variables that are used in a particular Sub are local variables.

Values that are assigned to them are not returned to the calling module.


Review

Structure charts are useful in determining how to divide a program into Sub procedures.

Modularizing programs offers significant advantages.– Easier to maintain

– Extremely useful for team programming

– Side effect can be prevented


Common Errors

Passing incorrect data types. Not returning the result of the computation

back to the calling program.


Review

A sub is like a black box; you know what goes in and what comes out; but you do not have to know what happens inside.

To specify the result of the computation inside the box ; the result can only be returned through its arguments.


Another Example


' Demonstrate that variables in a Sub procedure do

' not retain their values in subsequent calls

Call Three

Call Three

End Sub


Sub Three

Private Sub Three()

Dim num As Single

' Display the value of num and assign it the value 3


num = 3

End Sub


What is a function?

A function designed to perform a specific task also.

A function designed to return a single value to the calling program.


Types of Functions

Standard functions (built-in) User-defined functions


User-Defined Function

A function designed to return a single value.

The value is returned in the function itself. The arguments of a function should not be

changed in the function body.


The Function Syntax

Private Function FunctionName (parameter-list) As datatype

Statement(s)……

…..

FunctionName = ……..

End Function


Example of a Function(using a function to change from Fahrenheit to Celsius)

Private Sub cmdConvert_Click()

picTempC.Cls

picTempC.Print FtoC(Val(txtTempF.Text))

End Sub

Private Function FtoC(t As Single) As Single

‘ Convert Fahrenheit temperature to Celsius

FtoC = (5 / 9) * (t - 32)

End Function


Rule for Defining and Calling a Function

User-defined function must include a statement that assigns the function name a value.

User-defined functions are called in the same way that built-in functions are called.

A user-defined function may be called in an expression.


Returning Value

A function can receive many values

Only one value can be directly returned


Example of a Function

Private Sub cmdDetermine_Click()

Dim nom As String

' Determine a person's first name

nom = txtFullName.Text

picFirstName.Cls

picFirstName.Print "The first name is "; FirstName(nom)

End Sub


Function FirstName

Private Function FirstName(nom As String) As String

Dim firstSpace As Integer

' Extract the first name from a full name

firstSpace = InStr(nom, " ")

FirstName = Left(nom, firstSpace - 1)

End Function


Common Errors

Passing incorrect data types Not specifying the data type of the returned

value Forgetting the data type of a function's

parameter


Common Errors

Not assigning a value to the function name inside the function definition

Misspelling of the Function name Wrong invoking of the function in an

expression


Further Examples

Private Sub cmdDisplay_Click ()

' Compute Volume of a Cylinder

Dim r As Single, h As Single

r =1

h = 2

Call DisplayVolume (r, h)

r =3

h = 4

Call DisplayVolume (r, h)

End Sub


Function Area

Private Function Area (r As Single) As Single

' Compute area of a circle of radius r

Area = 3.14159 * r ^ 2

End Function


Sub DisplayVolume

Private Sub DisplayVolume ( r As Single, h As Single)

PicOutput.Print "Volume of cylinder having base area"; Area( r)

PicOutput.Print "and height"; h; "is"; h * Area (r )

End Sub


Another Example


Dim a As String

' Demonstrates local variables

a = “Choo “

picOutput.Print = TypeOfTrain()

End Sub


Function TypeOfTrain

Private Function TypeOf Train( ) As String

Dim a As String

a = a & a

TypeOfTrain = a & “train”

End Function


Last Example

Private Sub cmdDisplay_Click ( )

Dim num As Single

' Triple a number

num = 5

picOutput.Print Triple (num)

picOutput.Print num

End Sub


Function Triple

Private Function Triple(x As Single) As Single

Dim num As Single

num = 3

Triple = num * x

End Function

Documents

Chapter 4