Chapter 9: Writing Procedures Visual Basic.NET Programming: From Problem Analysis to Program Design.

Chapter 9: Writing Procedures

Visual Basic .NET Programming:

From Problem Analysis to Program Design

Visual Basic .NET Programming: From Problem Analysis to Program Design 2

Objectives

• Explore the client-server model

• Review the syntax of procedures

• Write user-defined procedures

• Design and write overloaded procedures


Exploring the Client-Server Model

• Methods

– Procedures within class definition

– Provide services to other objects in system

• Objects send messages to invoke procedures

– Client object sends message

– Server object receives message


Exploring the Client-Server Model (continued)

• Client sends message invoking server procedure

– Optionally sending values in form of arguments

• Server procedure performs requested task

– Optionally returns value to client



• Calling method example:

– Console.WriteLine(“Hello World Wide Web”)

• Class: Console

• Method: WriteLine

• Argument: “Hello World Wide Web”



• Calling method example:

– doubleResult = Math.Sqrt(firstInt)

• Class: Math

• Method: Sqrt

• Argument: firstInt

• Return value: doubleResult



• Unified Modeling Language (UML) sequence diagram– Maps interactions between objects

– Shows objects as rectangles

– Lifelines• Vertical lines below objects

• Represent sequence of time

• Shown as either dashed line or narrow box



• Unified Modeling Language (UML) sequence diagram (continued)

– Active object

• Object executing or controlling part of interaction

– Messages

• Represented using arrows


Example 9-1: Invoking Math.Pow

1. Option Explicit On

2. Option Strict On

3. Module Chapter9Example1

4. Sub Main()

5. Dim answer As Double

6. answer = Math.Pow(4, 2)

7. Console.WriteLine(answer)

8. End Sub

9. End Module


Reviewing the Syntax of Procedures

• Using procedures

– Improves code clarity

– Enhances error detection

– Simplifies maintenance

• Types of procedure in VB .NET:

– Sub

– Function


Using Sub Procedures

• Sub procedure

– Begins with procedure header

– Followed by one or more statements

– Ends with keywords End Sub


Using Sub Procedures (continued)

• Procedure header parts:

– Accessibility

– Sub

– Procedure name

– Parameter list



• Parameters

– Values passed to procedure

– Declared using keywords

• ByVal

• ByRef



• By value – Procedure does not have direct access to argument

variable • Cannot change its contents

– Procedure is given copy of variable

• By reference– Procedure code given direct access to argument

variable • May change its value


Using Function Procedures

• Function procedure definition

– Begins with procedure header

– Followed by one or more statements

– Ends with keywords End Function


Using Function Procedures (continued)

• Procedure header:

– Accessibility

– Function

– Procedure name

– Parameter list

– As dataType


Example 9-3: Writing and invoking a Function procedure

1. Option Explicit On

2. Option Strict On

3. Module Client

4. Sub Main()

5. Dim total As Integer

6. total = Server.AddEmUp(2, 3)

7. Console.WriteLine(total)

8. End Sub

9. End Module



(continued)3. Module Server

4. Public Function AddEmUp(ByVal a As Integer, ByVal b As Integer) As Integer

5. Dim sum As Integer

6. sum = a + b

7. Return sum

8. End Sub

9. End Module



(continued)

• Returning a variable

– Use keyword Return

– Followed by variable

– Returns control of execution to invoking client


Writing User-Defined Procedures

• User-defined procedures

– Procedures that you design and write

– Contrast to methods contained in FCL classes

– Use to give code structure and organization

– Can be Private or Public


Writing User-Defined Procedures (continued)

• Private procedures

– Provide services only to module in which they are defined

– Cannot be invoked by code in other objects

• Public procedures

– Designed specifically to be invoked by other objects


Writing Private Procedures

• From Example 9-4:13. Private Sub InputFahrenheit()

14. Console.WriteLine("Please enter a Fahrenheit temperature:")

15. fahrenheit =

Convert.ToDouble(Console.ReadLine())

16. End Sub


Writing Private Procedures (continued)

• Scope:

– Determined by where variable is declared

– Controls where variable can be accessed

• Module scope

– Variable declared within module

– Can be accessed by code in module



• Procedure scope

– Variable declared within procedure

– Only code within procedure can access variable

• Block scope

– Declared within If, Do, or For statement

– Can only be accessed within block



• Use variables with smallest or narrowest scope possible– Minimizes impact that changes in one procedure

may have in others

• Hide internal workings of procedure from outside code

• Public procedures can be invoked by any object• Design public procedures to provide services that

are required by multiple objects or applications


Using Optional Parameters

• May sometimes want to make some arguments optional

• Use keyword Optional

– In parameter declaration

– Provide default value

– Must be placed at end of parameter list


Example 9-7: Using an Optional Parameter

• Excerpt:Public Function ComputeShippingCharge(Optional

ByVal distance As Integer = 100) As Double


Designing and Writing Overloaded Procedures

• Procedure’s signature consists of

– Name

– Parameter list

• VB .NET identifies procedures by signature

– Can have two or more procedures with same name

– As long as parameter lists are different


Designing and Writing Overloaded Procedures

(continued)• Overloading a procedure

– Write multiple procedures with same name but with different signatures

– Convenient technique to avoid using unique names for procedures

– Use Overloads keyword

• Return data type is not part of procedure’s signature


Example 9-8: Overloading a procedure (Excepts)

Public Overloads Function ComputeSalesTax(ByVal amount As Single) As Double

Public Overloads Function ComputeSalesTax(ByVal amount As Double) As Double


Programming Example: Electricity Billing

• Input

– Previous and current meter reading

• Output

– Electric bill containing:

1. Kilowatt hours (kwh) consumed

2. Electric charge formatted as currency

3. Sales tax formatted as currency

4. Bill total formatted as currency


Programming Example: Electricity Billing (continued)

• Problem Analysis and Algorithm Design

– Purpose of program:

• Input previous and current electric meter readings

• Compute and display total bill

– Using sentinel-controlled loop


Programming Example: Electricity Billing (continued)

• Procedures used:

– Main

– GetPreviousReading

– GetCurrentReading

– ComputeElectricityCharge:

– DisplayBill


Summary

• Two types of procedures in Visual Basic .NET:

– Sub procedures

– Function procedures

• Arguments are passed into parameters

• Two categories of user-defined procedures:

– Public

– Private


Summary (continued)

• Scope of variable

– Represents visibility or accessibility

– Determined by where you declare it

• Create optional argument by adding keyword Optional

• Procedure’s signature consists

– Name

– Parameter list


Summary (continued)

• Overloading procedure:

– Write multiple procedures with same name but different signatures

Chapter 9: Writing Procedures Visual Basic.NET Programming: From Problem Analysis to Program Design.

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