2009 Pearson Education, Inc. All rights reserved. 1 7 7 Methods: A Deeper Look

2009 Pearson Education, Inc. All rights reserved.

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77

Methods: ADeeper Look


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The greatest invention of the nineteenth century was the invention of the method of invention.

– Alfred North Whitehead

Form ever follows function.– Louis Henri Sullivan

Call me Ishmael.– Herman Melville, Moby Dick

When you call me that, smile!– Owen Wister


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O! call back yesterday, bid time return.– William Shakespeare

Answer me in one word.– William Shakespeare

There is a point at whichmethods devour themselves.

– Frantz Fanon


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OBJECTIVES

In this chapter you will learn: To construct programs modularly from

methods. That Shared methods are associated with a

class rather than a specific instance of the class.

To use common Math methods from the Framework Class Library.

To create new methods.


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OBJECTIVES

The mechanisms used to pass information between methods.

Simulation techniques that employ random number generation.

How the visibility of identifiers is limited to specific regions of programs.

To write and use recursive methods(methods that call themselves).


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7.1 Introduction

7.2 Modules, Classes and Methods

7.3 Subroutines: Methods That Do Not Return a Value

7.4 Functions: Methods That Return a Value

7.5 Shared Methods and Class Math

7.6 GradeBook Case Study: Declaring Methods with Multiple Parameters

7.7 Notes on Declaring and Using Methods

7.8 Method Call Stack and Activation Records

7.9 Implicit Argument Conversions

7.10 Option Strict and Data-Type Conversions


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7.11 Value Types and Reference Types

7.12 Framework Class Library Namespaces

7.13 Passing Arguments: Pass-by-Value vs. Pass-by-Reference

7.14 Scope of Declarations

7.15 Case Study: Random Number Generation

7.16 Case Study: A Game of Chance

7.17 Method Overloading

7.18 Optional Parameters

7.19 Recursion

7.20 (Optional) Software Engineering Case Study: Identifying Class Operations in the ATM System


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• Programs consist of many pieces, including modules and classes.

• Modules and classes are composed of methods, fields and properties.

• You combine new modules and classes with those available in class libraries.

• Related classes are grouped into namespaces.

7.2 Modules, Classes and Methods


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Software Engineering Observation 7.1When possible, use .NET Framework classes and methods instead of writing new classes and methods. This reduces program development time and can prevent the introduction of errors.

Performance Tip 7.1.NET Framework Class Library methods are written to perform efficiently.

7.2 Modules, Classes and Methods (Cont.)


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• A method is invoked by a method call.– The method call specifies the method name and provides

information.

– When the method completes its task, it returns control to the caller.

• In some cases, the method also returns a result to the caller.


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Fig. 7.1 | Hierarchical boss-method/worker-method relationship.

• A boss (the caller) asks a worker (the callee) to perform a task and return the results when the task is done.

• The worker might call other workers—the boss would be unaware of this (Fig. 7.1).


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Software Engineering Observation 7.2To promote reusability, the capabilities of each method should be limited to the performance of a single, well-defined task, and the name of the method should express that task effectively.

Software Engineering Observation 7.3If you cannot choose a concise method name that expresses the task performed by a method, the method could be attempting to perform too many diverse tasks. Consider dividing such a method into several smaller methods.


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1 ' Fig. 7.2: Payment.vb

2 ' Subroutine that prints payment information.

3 Module Payment

4 Sub Main()

5 ' call subroutine PrintPay 4 times

6 PrintPay(40, 10.5)

7 PrintPay(38, 21.75)

8 PrintPay(20, 13)

9 PrintPay(30, 14)

10 End Sub ' Main

Outline

Payment.vb

(1 of 2 )

• Subroutines (such as Console.WriteLine) do notreturn a value.

• The console application in Fig. 7.2 uses a subroutineto print a worker’s payment information.

Main makes four calls to the subroutine PrintPay.

Fig. 7.2 | Subroutine for printing payment information. (Part 1 of 2.)


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11

12 ' print dollar amount earned in console window

13 Sub PrintPay(ByVal hours As Double, ByVal wage As Decimal)

14 ' pay = hours * wage

15 Console.WriteLine("The payment is {0:C}", hours * wage)

16 End Sub ' PrintPay

17 End Module ' Payment The payment is $420.00 The payment is $826.50 The payment is $260.00 The payment is $420.00

Outline

Payment.vb

(2 of 2 )

Method PrintPayis a subroutine.

Fig. 7.2 | Subroutine for printing payment information. (Part 2 of 2.)


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7.3 Subroutines: Methods That Do Not Return a Value (Cont.)

• The format of a subroutine declaration is:Sub method-name(parameter-list)

declarations and statements

End Sub

• The parameter-list is a comma-separated list of each parameter’s type and name.

– The type of each argument must be consistent with its corresponding parameter’s type.

– If a method does not receive any values, the method name is followed by an empty set of parentheses.


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Common Programming Error 7.1Declaring a variable in the method’s body with the same name as a parameter variable in the method header is a compilation error.

Error-Prevention Tip 7.1Although it is allowable, an argument passed to a method should not have the same name as the corresponding parameter name in the method declaration. This prevents ambiguity that could lead to logic errors.



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Software Engineering Observation 7.4Method names tend to be verbs because methods typically perform actions. By convention, method names begin withan uppercase first letter. For example, a method that sends an e-mail message might be named SendMail.


• The method body (also referred to as a block) contains declarations and statements.

• The body of a method declared with Sub must be terminated with End Sub.


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Error-Prevention Tip 7.2Small methods are easier to test, debug and understandthan large methods.

Good Programming Practice 7.1When a parameter is declared without a type, its type is assumed to be Object. Explicitly declaring a parameter’s type improves program clarity.



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1 ' Fig. 7.3: SquareInteger.vb

2 ' Function that squares a number.

3 Module SquareInteger

4 Sub Main()

5 Console.WriteLine("Number" & vbTab & "Square")

6

7 ' square integers from 1 to 10

8 For counter = 1 To 10

9 Console.WriteLine(counter & vbTab & Square(counter))

10 Next

11 End Sub ' Main

Outline

SquareInteger.vb

(1 of 2 )

• Functions are methods that return a valueto the caller.

• The console application in Fig. 7.3 uses the functionSquare to calculate the squares of the integersfrom 1–10.

The For…Next statement displays the results of squaring the integers.

Fig. 7.3 | Function for squaring an integer. (Part 1 of 2.)


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13 ' function Square is executed when it is explicitly called

14 Function Square(ByVal y As Integer) As Integer

15 Return y ^ 2 ' return square of parameter value

16 End Function ' Square

17 End Module ' SquareInteger Number Square 1 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81 10 100

Outline

SquareInteger.vb

(2 of 2 )

Square receives a copy of counter’s value.

The Return statement terminates the method and returns the result.

Fig. 7.3 | Function for squaring an integer. (Part 2 of 2.)


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7.4 Functions: Methods That Return a Value (Cont.)

• The Return statement terminates the method and returns the result.

• The return statement can occur anywhere in a function body.

Return expression

• The return-type indicates the type of the result returned from the function.


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7.4 Functions: Methods That Return a Value (Cont.)

Common Programming Error 7.2If the expression in a Return statement cannot be converted to the function’s return-type, a run time error is generated.

Common Programming Error 7.3Failure to return a value from a function (e.g., by forgetting to provide a Return statement) causes the function to return the default value for the return-type, possibly producing incorrect output.


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7.5 Shared Methods and Class Math

• A method which performs a task that does not depend on an object is known as a Shared method.

– Place the Shared modifier before the keyword Sub or Function.

• To call a Shared method:ClassName.MethodName(arguments)

• Class Math provides a collection of Shared methods:Math.Sqrt(900.0)


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7.5 Shared Methods and Class Math (Cont.)

Software Engineering Observation 7.5It is not necessary to add an assembly reference to use the Math class methods in a program, because class Math is located in the assembly mscorlib.dll, which is referenced by every .NET application. Also it is not necessary to import class Math’s namespace (System), because it is implicitly imported in all .NET applications.


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Method Description Example

Abs(x) returns the absolute value of x Abs(23.7) is 23.7 Abs(0) is 0 Abs(-23.7) is 23.7

Ceiling(x) rounds x to the smallest integer not less than x

Ceiling(9.2) is 10.0

Ceiling(-9.8) is -9.0

Cos(x) returns the trigonometric cosine of x (x in radians)

Cos(0.0) is 1.0

Exp(x) returns the exponential ex Exp(1.0) is approximately 2.71828182845905 Exp(2.0) is approximately 7.38905609893065

• Figure 7.4 summarizes several Math class methods.In the figure, x and y are of type Double.

Fig. 7.4 | Math class methods. (Part 1 of 3.)



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Floor(x) rounds x to the closets integer less than or equal to x

Floor(9.2) is 9.0

Floor(-9.8) is -10.0

Log(x) returns the natural logarithm of x (base e)

Log(2.7182818284590451) is approximately 1.0 Log(7.3890560989306504) is approximately 2.0

Max(x, y) returns the larger value of x and y Max(2.3, 12.7) is 12.7 Max(-2.3, -12.7) is -2.3

Min(x, y) returns the smaller value of x and y Min(2.3, 12.7) is 2.3 Min(-2.3, -12.7) is -12.7


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Pow(x, y) calculates x raised to the power y (xy) Pow(2.0, 7.0) is 128.0 Pow(9.0, .5) is 3.0

Sin(x) returns the trigonometric sine of x (x in radians)

Sin(0.0) is 0.0

Sqrt(x) returns the square root of x Sqrt(9.0) is 3.0 Sqrt(2.0) is 1.4142135623731

Tan(x) returns the trigonometric tangent of x (x in radians)

Tan(0.0) is 0.0




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• Math.PI and Math.E are declared in class Math with the modifiers Public and Const.

– Const declares a constant—a value that cannot be changed.

– Constants are implicitly Shared.


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1 ' Fig. 7.5: GradeBook.vb

2 ' Definition of class GradeBook that finds the maximum of three grades.

3 Public Class GradeBook

4 Private courseNameValue As String ' name of course

5 Private maximumGrade As Integer ' maximum of three grades

6

7 ' constructor initializes course name

8 Public Sub New(ByVal name As String)

9 CourseName = name ' initializes CourseName

10 maximumGrade = 0 ' this value will be replaced by maximum grade

11 End Sub ' New

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Outline

GradeBook.vb

(1 of 5 )

• The application in Figs. 7.5–7.6 uses auser-declared method called Maximum todetermine the largest of three Integer values.

Fig. 7.5 | User-declared method Maximum that has threeInteger parameters. (Part 1 of 5.)


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13 ' property that gets and sets the course name; the Set accessor

14 ' ensures that the course name has at most 25 characters

15 Public Property CourseName() As String

16 Get ' retrieve courseNameValue

17 Return courseNameValue

18 End Get

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20 Set(ByVal value As String) ' set courseNameValue

21 If value.Length <= 25 Then ' if value has 25 or fewer characters

22 courseNameValue = value ' store the course name in the object

23 Else ' if name has more than 25 characters

24 ' set courseNameValue to first 25 characters of parameter name

25 ' start at 0, length of 25

26 courseNameValue = value.Substring(0, 25)

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28 Console.WriteLine( _

29 "Course name (" & value & ") exceeds maximum length (25).")

30 Console.WriteLine( _

31 "Limiting course name to first 25 characters." & vbNewLine)

32 End If

33 End Set

34 End Property ' CourseName

Outline

GradeBook.vb

(2 of 5 )



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36 ' display a welcome message to the GradeBook user

37 Public Sub DisplayMessage()

38 Console.WriteLine("Welcome to the grade book for " _

39 & vbNewLine & CourseName & "!" & vbNewLine)

40 End Sub ' DisplayMessage

41

42 ' input three grades from user

43 Public Sub InputGrades()

44 Dim grade1 As Integer ' first grade entered by user

45 Dim grade2 As Integer ' second grade entered by user

46 Dim grade3 As Integer ' third grade entered by user

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48 Console.Write("Enter the first grade: ")

49 grade1 = Console.ReadLine()

50 Console.Write("Enter the second grade: ")


52 Console.Write("Enter the third grade: ")


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55 ' store the maximum in maximumGrade

56 maximumGrade = Maximum(grade1, grade2, grade3)

57 End Sub ' InputGrades

Outline

GradeBook.vb

(3 of 5 )



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58

59 ' returns the maximum of its three integer parameters

60 Function Maximum(ByVal x As Integer, ByVal y As Integer, _

61 ByVal z As Integer) As Integer

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63 Dim maximumValue As Integer = x ' assume x is the largest to start

64

65 ' determine whether y is greater than maximumValue

66 If (y > maximumValue) Then

67 maximumValue = y ' make y the new maximumValue

68 End If

69

Outline

GradeBook.vb

(4 of 5 )



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70 ' determine whether z is greater than maximumValue

71 If (z > maximumValue) Then

72 maximumValue = z ' make z the new maximumValue

73 End If

74

75 Return maximumValue

76 End Function ' Maximum

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78 ' display a report based on the grades entered by user

79 Public Sub DisplayGradeReport()

80 ' output maximum of grades entered

81 Console.WriteLine("Maximum of grades entered: " & maximumGrade)

82 End Sub ' DisplayGradeReport

83 End Class ' GradeBook

Outline

GradeBook.vb

(5 of 5 )



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1 ' Fig. 7.6: GradeBookTest.vb

2 ' Create a GradeBook object, input grades and display grade report.

3 Module GradeBookTest

4 Sub Main()

5 ' create GradeBook object

6 Dim gradeBook1 As New GradeBook("CS101 Introduction to VB")

7

8 gradeBook1.DisplayMessage() ' display welcome message

9 gradeBook1.InputGrades() ' read grades from user

10 gradeBook1.DisplayGradeReport() ' display report based on grades

11 End Sub ' Main

12 End Module ' GradeBookTest Welcome to the grade book for CS101 Introduction to VB! Enter the first grade: 65 Enter the second grade: 87 Enter the third grade: 45 Maximum of grades entered: 87 (continued on next page...)

Outline

GradeBookTest.vb

(1 of 2 )

Fig. 7.6 | Application to test class GradeBook’s Maximum method. (Part 1 of 2.)


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(continued from previous page…) Welcome to the grade book for CS101 Introduction to VB! Enter the first grade: 45 Enter the second grade: 65 Enter the third grade: 87 Maximum of grades entered: 87 Welcome to the grade book for CS101 Introduction to VB! Enter the first grade: 87 Enter the second grade: 45 Enter the third grade: 65 Maximum of grades entered: 87

Outline

GradeBookTest.vb

(2 of 2 )

Fig. 7.6 | Application to test class GradeBook’s Maximum method. (Part 2 of 2.)


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• There are three ways to call a method:– Using a method name by itself within the same class or module.

– Using a reference to an object, followed by a dot (.) and the method name to call a method of the object.

– Using the class or module name and a dot (.) to call a Shared method.

• A Shared method can call only other Shared methods.

7.7 Notes on Declaring and Using Methods


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Common Programming Error 7.4Declaring a method outside the body of a class or module declaration or inside the body of another method is a syntax error.

Common Programming Error 7.5Omitting the return-value-type in a method declaration, if that method is a function, is a syntax error.

Common Programming Error 7.6Redeclaring a method parameter as a local variable in the method’s body is a compilation error.

Common Programming Error 7.7Returning a value from a method that is a subroutine is a compilation error.

7.7 Notes on Declaring and Using Methods (Cont.)


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7.8 Method Call Stack and Activation Records

• A data structure known as a stack is analogous to a pile of dishes.

– When a dish is placed on the pile, it is placed at the top(pushing onto the stack).

– When a dish is removed from the pile, it is removed from the top (popping off the stack).

– Stacks are known as last-in, first-out (LIFO) data structures.


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7.8 Method Call Stack and Activation Records (Cont.)

• When a program calls a method, the return address of the caller is pushed onto the method call stack.

• Successive return addresses are pushed onto the stack.

• The memory for the local variables in each method is in the activation record or stack frame.

• When the method returns to its caller, the activation record is popped off the stack.

• If too many method calls occur then stack overflow occurs.


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7.9 Implicit Argument Conversions

• An important feature of argument passing is implicit argument conversion.

– A widening conversion occurs when an argument is converted to another type that can hold more data.

– A narrowing conversion occurs when there is potential for data loss during the conversion.


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7.9 Implicit Argument Conversions (Cont.)

• Figure 7.7 lists the widening conversions supported by Visual Basic.

Fig. 7.7 | Widening conversions between primitive types. (Part 1 of 2.)

Type Conversion types

Boolean no possible widening conversions to other primitive types

Byte UShort, Short, UInteger, Integer, ULong, Long, Decimal, Single or Double

Char String

Date no possible widening conversions to other primitive types

Decimal Single or Double

Double no possible widening conversions to other primitive types

Integer Long, Decimal, Single or Double

Long Decimal, Single or Double


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Type Conversion types

SByte Short, Integer, Long, Decimal, Single or Double

Short Integer, Long, Decimal, Single or Double

Single Double

String no possible widening conversions to other primitive types

UInteger ULong, Long, Decimal, Single or Double

ULong Decimal, Single or Double

UShort UInteger, Integer, ULong, Long, Decimal, Single or Double

Fig. 7.7 | Widening conversions between primitive types. (Part 2 of 2.)


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Common Programming Error 7.8Converting a primitive-type value to a value of another primitive type may change the value if the conversion is not a widening conversion. For example, converting a floating-point value to an integral value truncates any fractional part of the floating-point value (e.g., 4.7 becomes 4).


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• Conversions occur in expressions containing two or more types.

• Temporary copies of the values are converted to the “widest” type in the expression.

• The value of integerNumber is converted to type Single for this operation:

singleNumber + integerNumber


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7.10 Option Strict and Data-Type Conversions• Option Explicit forces an explicit declaration of variables before

they are used.

• To set Option Explicit to Off, double click My Project in Solution Explorer (Fig. 7.8). Click the Compile tab, then select the value Off from the Option Explicit drop-down list.

Fig. 7.8 | Modifying Option Strict and Option Explicit.

Compile tab

Drop-down list to modify

Option Explicit

Drop-down list to modify Option Strict

Drop-down list to modify Option

Infer


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7.10 Option Strict and Data-Type Conversions (Cont.)

• Option Strict requires explicit conversion for all narrowing conversions.

• An explicit conversion uses a cast operator or a method.

• The methods of class Convert explicitly convert data from one type to another:number = Convert.ToDouble(Console.ReadLine())


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7.10 Option Strict and Data-Type Conversions (Cont.)

• Option Infer enables the compiler to infer a variable’s type based on its initializer value.

• You can set the default values of Option Explicit, Option Strict and Option Infer.

– Select Tools > Options...

– Under Projects and Solutions, select VB Defaults

– Choose the appropriate value in each option’s ComboBox.

Error-Prevention Tip 7.3From this point forward, all code examples have Option Strict set to On.


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7.11 Value Types and Reference Types

• All Visual Basic types can be categorized as either value types or reference types.

– A variable of a value type contains data of that type.

– A variable of a reference type contains the address of the location in memory where the data is stored.


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7.11 Value Types and Reference Types (Cont.)

Fig. 7.9 | Primitive types. (Part 1 of 3.)

Type

Size in bits Values Standard

Name of .NET class or structure

Boolean True or False Boolean

Byte 8 0 to 255 Byte

SByte 8 -128 to 127 SByte

Char 16 One Unicode character (Unicode character set)

Char

Date 64 1 January 0001 to 31 December 9999 12:00:00 AM to 11:59:59 PM

DateTime

Decimal 128 1.0E-28 to 7.9E+28 Decimal


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Type



Double 64 ±5.0E–324 to ±1.7E+308 (IEEE 754 floating point)

Double

Integer 32 –2,147,483,648 to 2,147,483,647

Int32

Long 64 –9,223,372,036,854,775,808 to 9,223,372,036,854,775,807

Int64

Short 16 –32,768 to 32,767 Int16

Single 32 ±1.5E-45 to ±3.4E+38 (IEEE 754 floating point)

Single



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Type



String 0 to ~2000000000 Unicode characters

(Unicode character set)

String

UInteger 32 0 to 4,294,967,295 UInt32

ULong 64 0 to 18,446,744,073,709,551,615 UInt64

UShort 16 0 to 65,535 UInt16



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7.12 Framework Class Library Namespaces

• The .NET framework contains many classes grouped into over 100 namespaces.

• A program includes the an Imports declaration to use classes from that namespace:

Imports System.Windows.Forms


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7.12 Framework Class Library Namespaces (Cont.)

• Some key namespaces are described in Fig. 7.10.

Fig. 7.10 | .NET Framework Class Library namespaces (a subset). (Part 1 of 2.)

Namespace Description

System.Windows.Forms Contains the classes required to create and manipulate GUIs.

System.IO Contains classes that enable programs to input and output data.

System.Data Contains classes that enable programs to access and manipulate databases (i.e., an organized collection of data).

System.Web Contains classes used for creating and maintaining Web applications, which are accessible over the Internet.

System.Xml Contains classes for creating and manipulating XML data. Data can be read from or written to XML files.


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Namespace Description

System.Collections Contains classes that define data structures for maintaining collections of data.

System.Net Contains classes that enable programs to communicate via computer networks like the Internet.

System.Text Contains classes and interfaces that enable programs to manipulate characters and strings.

System.Threading Contains classes that enable programs to perform several tasks concurrently, or at the same time.

System.Drawing Contains classes that enable programs to display basic graphics, such as displaying shapes and arcs.

Fig. 7.10 | .NET Framework Class Library namespaces (a subset). (Part 2 of 2.)


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Good Programming Practice 7.2The Visual Studio .NET documentation is easy to search and provides many details about each class. As you learn a class in this book, you should read about the class in the online documentation.


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7.13 Passing Arguments: Pass-by-Value vs. Pass-by-Reference

• Arguments are passed pass-by-value orpass-by-reference.

– When an argument is passed by value, the program makes a copy of the argument’s value and passes the copy. Changes to the copy do not affect the original variable.

– When an argument is passed by reference, the original data can be accessed and modified directly.


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1 ' Fig. 7.11: ByRefTest.vb

2 ' Demonstrates passing by value and by reference.

3 Module ByRefTest

4 Sub Main()

5 Dim number1 As Integer = 2

6

7 Console.WriteLine("Passing a value-type argument by value:")

8 Console.WriteLine("Before calling SquareByValue, " & _

9 "number1 is {0}", number1)

10 SquareByValue(number1) ' passes number1 by value

11 Console.WriteLine("After returning from SquareByValue, " & _

12 "number1 is {0}" & vbNewLine, number1)

13

Outline

ByRefTest.vb

(1 of 4 )

• Figure 7.11 demonstrates passing argumentsby value and by reference.

Fig. 7.11 | ByVal and ByRef used to pass value-type arguments. (Part 1 of 4.)


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15

16 Console.WriteLine("Passing a value-type argument" & _

17 " by reference:")

18 Console.WriteLine("Before calling SquareByReference, " & _

19 "number2 is {0}", number2)

20 SquareByReference(number2) ' passes number2 by reference

21 Console.WriteLine("After returning from " & _

22 "SquareByReference, number2 is {0}" & vbNewLine, number2)

23


25

26 Console.WriteLine("Passing a value-type argument" & _

27 " by reference, but in parentheses:")

28 Console.WriteLine("Before calling SquareByReference " & _

29 "using parentheses, number3 is {0}", number3)

30 SquareByReference((number3)) ' passes number3 by value

31 Console.WriteLine("After returning from " & _

32 "SquareByReference, number3 is {0}", number3)

33 End Sub ' Main

Outline

Squaring number2 by reference.

ByRefTest.vb

(2 of 4 )

The inner set of parentheses evaluates number3 to its value and passes this value.

Squaring number1 by value.



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34

35 ' squares number by value (note ByVal keyword)

36 Sub SquareByValue(ByVal number As Integer)

37 Console.WriteLine("After entering SquareByValue, " & _

38 "number is {0}", number)

39 number *= number

40 Console.WriteLine("Before exiting SquareByValue, " & _

41 "number is {0}", number)

42 End Sub ' SquareByValue

43

44 ' squares number by reference (note ByRef keyword)

45 Sub SquareByReference(ByRef number As Integer)

46 Console.WriteLine("After entering SquareByReference" & _

47 ", number is {0}", number)

48 number *= number

49 Console.WriteLine("Before exiting SquareByReference" & _

50 ", number is {0}", number)

51 End Sub ' SquareByReference

52 End Module ' ByRefTest

Outline

ByVal indicates that its argument is passed by value.

Squaring the number parameter value.

ByRef receives its parameter by reference.

Squaring the number parameter’s reference.

ByRefTest.vb

(3 of 4 )



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Passing a value-type argument by value: Before calling SquareByValue, number1 is 2 After entering SquareByValue, number is 2 Before exiting SquareByValue, number is 4 After returning from SquareByValue, number1 is 2 Passing a value-type argument by reference: Before calling SquareByReference, number2 is 2 After entering SquareByReference, number is 2 Before exiting SquareByReference, number is 4 After returning from SquareByReference, number2 is 4 Passing a value-type argument by reference, but in parentheses: Before calling SquareByReference using parentheses, number3 is 2 After entering SquareByReference, number is 2 Before exiting SquareByReference, number is 4 After returning from SquareByReference, number3 is 2

Outline

ByRefTest.vb

(4 of 4)



61

Outline

ByRefTest.vb

(5 of 5 )

Error-Prevention Tip 7.4When passing arguments by value, changes to the called method’s copy do not affect the original variable’s value. This prevents possible side effects that could hinder the development of correct and reliable software systems. Always pass value-type arguments by value unless you explicitly intend for the called method to modify the caller’s data.


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7.14 Scope of Declarations

• A declaration’s scope is the portion of the program that can refer to the declared entity by its name without qualification.

• The basic scopes are:– Block scope—from the point of the declaration to the end of the

block

– Method scope—from the point of the declaration to the end of that method

– Module scope—the entire body of the class or module.

– Namespace scope—accessible to all other elements in the same namespace


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7.14 Scope of Declarations (cont.)

• If a local variable has the same name as a field of the class, the field is hidden—this is called shadowing.

• A variable’s lifetime is the period during which the variable exists in memory.

– Variables normally exist as long as their container exists.

Error-Prevention Tip 7.5Use different names for fields and local variables to help prevent subtle logic errors that occur when a method is called and a local variable of the method shadows a field of the same name in the class.


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1 ' Fig. 7.12: Scope.vb

2 ' Scope class demonstrates instance and local variable scopes.

3 Public Class Scope

4 ' instance variable that is accessible to all methods of this class

5 Private x As Integer = 1

6

7 ' method Begin creates and initializes local variable x and

8 ' calls methods UseLocalVariable and UseInstanceVariable

9 Public Sub Begin()

10 ' method's local variable x shadows instance variable x

11 Dim x As Integer = 5

12

13 Console.WriteLine("local x in method Begin is " & x)

14

15 UseLocalVariable() ' UseLocalVariable has local x

16 UseInstanceVariable() ' uses class Scope's instance variable x

17 UseLocalVariable() ' UseLocalVariable reinitializes local x

18 UseInstanceVariable() ' Scope's instance variable x retains value

19

20 Console.WriteLine(vbNewLine & "local x in method Begin is " & x)

21 End Sub ' Begin

Outline

Scope.vb

(1 of 2 )

This instance variable is shadowed by any local variable named x.

Method Begin declares a local variable x

Fig. 7.12 | Scoping rules in a class. (Part 1 of 2.)


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22

23 ' create and initialize local variable x during each call

24 Sub UseLocalVariable()

25 Dim x As Integer = 25 ' initialized in each call

26

27 Console.WriteLine(vbNewLine & _

28 "local x on entering method UseLocalVariable is " & x)

29 x += 1 ' modifies this method's local variable x

30 Console.WriteLine("local variable x before exiting method " & _

31 "UseLocalVariable is " & x)

32 End Sub ' UseLocalVariable

33

34 ' modify class Scope's instance variable x during each call

35 Sub UseInstanceVariable()

36 Console.WriteLine(vbNewLine & "instance variable" & _

37 " x on entering method UseInstanceVariable is " & x)

38 x *= 10 ' modifies class Scope's instance variable x

39 Console.WriteLine("instance variable " & _

40 "x before exiting method UseInstanceVariable is " & x)

41 End Sub ' UseInstanceVariable

42 End Class ' Scope

Outline

Subroutine UseLocalVariable declares a local variable x

Modifying the local variable x

UseInstanceVariable does not declare local variables, so x refers to the instance variable.

Scope.vb

(2 of 2 )

Fig. 7.12 | Scoping rules in a class. (Part 2 of 2.)


66

1 ' Fig. 7.13: ScopeTest.vb

2 ' Testing class Scope.

3 Module ScopeTest

4 Sub Main()

5 Dim testScope As New Scope()

6 testScope.Begin()

7 End Sub ' Main

8 End Module ' ScopeTest local x in method Begin is 5 local x on entering method UseLocalVariable is 25 local variable x before exiting method UseLocalVariable is 26 instance variable x on entering method UseInstanceVariable is 1 instance variable x before exiting method UseInstanceVariable is 10 local x on entering method UseLocalVariable is 25 local variable x before exiting method UseLocalVariable is 26 instance variable x on entering method UseInstanceVariable is 10 instance variable x before exiting method UseInstanceVariable is 100 local x in method Begin is 5

Outline

ScopeTest.vb

Fig. 7.13 | Module to test class Scope.


67

7.15 Case Study: Random NumberGeneration

• The element of chance can be introduced through class Random.

Dim randomObject As New Random()

Dim randomNumber As Integer = randomObject.Next()

• Next generates a positive Integer between zero and Int32.MaxValue.

• The values returned by Next are actuallypseudo-random numbers.

• The default seed value is based on the current time.


68

7.15 Case Study: Random NumberGeneration (Cont.)

• We can produce integers in the range 1–6 by passing an argument to method Next:

value = 1 + randomObject.Next(6)

• The number 6 is the scaling factor.

• We shift the range of numbers produced by adding 1.

• Simplify the process by passing two arguments to Next:

value = randomObject.Next(1, 7)


69

1 ' Fig. 7.14: RandomInteger.vb

2 ' Random integers from 1 to 6 created by calling Random method Next

3 Module RandomInteger

4 Sub Main()

5 Dim randomObject As New Random() ' create Random object

6 Dim randomNumber As Integer

7

8 ' generate 20 random numbers between 1 and 6

9 For i = 1 To 20

10 randomNumber = randomObject.Next(1, 7)

11 Console.Write(randomNumber & " ")

12

13 If i Mod 5 = 0 Then ' is i a multiple of 5?

14 Console.WriteLine()

15 End If

16 Next

17 End Sub ' Main

18 End Module ' RandomInteger 1 1 3 1 1 2 1 1 3 6 6 3 4 6 6 2 2 3 6 5

Outline

RandomInteger.vb

• Figure 7.14 demonstrates the use of class Random andmethod Next by simulating 20 rolls of a six-sided die.

Random numbers are generated by an object of class Random.

The values that are produced are in the range from 1 to 6, inclusive.

Fig. 7.14 | Random integers from 1 to 6 created bycalling Random method Next.


70

1 ' Fig. 7.15: RollDice.vb

2 ' Rolling four dice.

3 Public Class RollDice

4 Dim randomObject As New Random() ' create Random object

5

6 ' display results of four rolls

7 Private Sub rollButton_Click(ByVal sender As System.Object, _

8 ByVal e As System.EventArgs) Handles rollButton.Click

9 ' method randomly assigns a face to each die

10 DisplayDie(die1PictureBox)




14 End Sub ' rollButton_Click

15

Outline

RollDice.vb

(1 of 2 )

Initializing a Random object.

Fig. 7.15 | Demonstrating four die rolls with graphical output. (Part 1 of 2.)


71

16 ' get a random die image

17 Sub DisplayDie(ByVal diePictureBox As PictureBox)

18 ' generate random integer in range 1 to 6

19 Dim face As Integer = randomObject.Next(1, 7)

20

21 ' retrieve specific die image from resources

22 Dim pictureResource = My.Resources.ResourceManager.GetObject( _

23 String.Format("die{0}", face))

24

25 ' convert pictureResource to type Image and display in PictureBox

26 diePictureBox.Image = CType(pictureResource, Image)

27 End Sub ' DisplayDie

28 End Class ' RollDice

Outline

The values that are produced are in the range from 1 to 6, inclusive.

CType performs conversion from Object to Image.

Retrieving the picture from My.Resources.

RollDice.vb

(2 of 2 )

a) b)

Fig. 7.15 | Demonstrating four die rolls with graphical output. (Part 2 of 2.)


72

7.15 Case Study: Random Number Generation (cont.)

• The Button control can be simply dragged and dropped onto the Form from the Toolbox.

– Name the control rollButton and set its Text to Roll.

• Double click the Button to create the Click event handler.

– Note that the event handler is a Sub procedure.


73


• The event handler calls method DisplayDie once for each PictureBox.

• Calling DisplayDie causes dice to appear as if they are being rolled.

• The images are embedded with the project as resources.


74


– Double click My Project in the Solution Explorer.

– Click the Resources tab, then click the down arrow next to the Add Resource button (Fig. 7.16) to select Add Existing File...

– Locate the resources you wish to add and click the Open button.

– Save your project.


75


Fig. 7.16 | Images added as resources.


76


• To access the project’s resources, use the method My.Resources.ResourceManager.GetObject.

• You must convert this Object to type Image to assign it to the PictureBox’s Image property;

• Visual Basic’s CType function converts its first argument to the type specified in its second argument.


77

1 ' Fig. 7.17: RollTwelveDice.vb

2 ' Rolling 12 dice with frequency chart.

3 Public Class RollTwelveDice

4 Dim randomObject As New Random() ' generate random number

5 Dim ones As Integer ' count of die face 1

6 Dim twos As Integer ' count of die face 2

7 Dim threes As Integer ' count of die face 3

8 Dim fours As Integer ' count of die face 4

9 Dim fives As Integer ' count of die face 5

10 Dim sixes As Integer ' count of die face 6

11

12 ' display result of twelve rolls



15

16 ' assign random faces to 12 dice using DisplayDie





Outline

RollTwelveDice.vb

(1 of 5 )

Initializing a Random object.

Fig. 7.17 | Random class used to simulate rolling12 six-sided dice. (Part 1 of 5.)


78









29

30 Dim total As Integer = ones + twos + threes + fours + fives + sixes

31 Dim output As String

32

33 ' display frequencies of faces

34 output = ("Face" & vbTab & "Frequency" & vbTab & "Percent")

35 output &= (vbNewLine & "1" & vbTab & ones & _

36 vbTab & vbTab & String.Format("{0:P2}", ones / total))

37 output &= (vbNewLine & "2" & vbTab & twos & vbTab & _

38 vbTab & String.Format("{0:P2}", twos / total))

39 output &= (vbNewLine & "3" & vbTab & threes & vbTab & _

40 vbTab & String.Format("{0:P2}", threes / total))

Outline

Using String.Format to print percentages to two decimal places.

RollTwelveDice.vb

(2 of 5 )



79

41 output &= (vbNewLine & "4" & vbTab & fours & vbTab & _

42 vbTab & String.Format("{0:P2}", fours / total))

43 output &= (vbNewLine & "5" & vbTab & fives & vbTab & _

44 vbTab & String.Format("{0:P2}", fives / total))

45 output &= (vbNewLine & "6" & vbTab & sixes & vbTab & _

46 vbTab & String.Format("{0:P2}", sixes / total) & vbNewLine)

47 displayTextBox.Text = output


49

50 ' display a single die image

51 Sub DisplayDie(ByVal diePictureBox As PictureBox)

52 Dim face As Integer = randomObject.Next(1, 7)

53




57

58 ' convert pictureResource to image type and load into PictureBox


60

Outline

CType performs conversion from Object to Image.

Retrieving the picture from My.Resources.

Using String.Format to print percentages to two decimal places.

RollTwelveDice.vb

(3 of 5 )



80

61 ' maintain count of die faces

62 Select Case face

63 Case 1 ' die face 1

64 ones += 1


66 twos += 1


68 threes += 1


70 fours += 1


72 fives += 1


74 sixes += 1

75 End Select


77 End Class ' RollTwelveDice

Outline

RollTwelveDice.vb

(4 of 5 )



81

Outline

RollTwelveDice.vb

(5 of 5 )

a) b)

displayTextBox



82


• The TextBox control can be used both for displaying and inputting data.

– The data in the TextBox can be accessed via the Text property.

– This TextBox’s name is displayTextBox and the font size has been set to 9.

– The Multiline property has been set to True.


83

7.16 Case Study: A Game of Chance

• One of the most popular games of chance is a dice game known as “craps":

A player rolls two dice. After the dice have come to rest, the sum of the spots on the two upward faces is calculated. If the sum is 7 or 11 on the first throw, the player wins. If the sum is 2, 3 or 12 on the first throw (called “craps”), the player loses. If the sum is 4, 5, 6, 8, 9 or 10 on the first throw, that sum becomes the player’s “point.” To win, players must continue rolling the dice until they “make their point” (i.e., roll their point value). The player loses by rolling a 7 before making the point.


84

1 ' Fig 7.18: CrapsGame.vb

2 ' Craps game using class Random.

3 Public Class CrapsGame

4 ' die-roll constants

5 Enum DiceNames

6 SNAKE_EYES = 2

7 TREY = 3

8 LUCKY_SEVEN = 7

9 CRAPS = 7

10 YO_LEVEN = 11

11 BOX_CARS = 12

12 End Enum

13

14 Dim myPoint As Integer ' total point

15 Dim myDie1 As Integer ' die1 face

16 Dim myDie2 As Integer ' die2 face

17 Dim randomObject As New Random() ' generate random number

18

Outline

CrapsGame.vb

(1 of 7 )

• The application in Fig. 7.18 simulates the game of craps.

Declaring constants through an enumeration.

Fig. 7.18 | Craps game using class Random. (Part 1 of 7.)


85

19 ' begins new game and determines point

20 Private Sub playButton_Click(ByVal sender As System.Object, _

21 ByVal e As System.EventArgs) Handles playButton.Click

22

23 ' initialize variables for new game

24 myPoint = 0

25 pointGroupBox.Text = "Point"

26 statusLabel.Text = String.Empty

27

28 ' remove point-die images

29 pointDie1PictureBox.Image = Nothing

30 pointDie2PictureBox.Image = Nothing

31

32 Dim sum As Integer = RollDice() ' roll dice and calculate sum

33

34 ' check die roll

35 Select Case sum

36 Case DiceNames.LUCKY_SEVEN, DiceNames.YO_LEVEN

37 rollButton.Enabled = False ' disable Roll button

38 statusLabel.Text = "You Win!!!"

Outline

CrapsGame.vb

(2 of 7 )

Setting the Image to Nothing causes a PictureBox to appear blank

String.Empty represents a string with no characters ("").

The Select Case statement analyzes the roll.



86

39 Case DiceNames.SNAKE_EYES, DiceNames.TREY, DiceNames.BOX_CARS


41 statusLabel.Text = "Sorry. You Lose."

42 Case Else

43 myPoint = sum

44 pointGroupBox.Text = "Point is " & sum

45 statusLabel.Text = "Roll Again!"

46 DisplayDie(pointDie1PictureBox, myDie1)

47 DisplayDie(pointDie2PictureBox, myDie2)

48 playButton.Enabled = False ' disable Play button

49 rollButton.Enabled = True ' enable Roll button

50 End Select

51 End Sub ' playButton_Click

52

Outline

CrapsGame.vb

(3 of 7 )



87

53 ' determines outcome of next roll



56

57 Dim sum As Integer = RollDice() ' roll dice and calculate sum

58

59 ' check outcome of roll

60 If sum = myPoint Then ' win

61 statusLabel.Text = "You Win!!!"


63 playButton.Enabled = True ' enable Play button

64 ElseIf sum = DiceNames.CRAPS Then ' lose

65 statusLabel.Text = "Sorry. You Lose."


67 playButton.Enabled = True ' enable Play button

68 End If


70

Outline

CrapsGame.vb

(4 of 7 )

Determining if a losing roll is made.



88

71 ' display die image

72 Sub DisplayDie(ByVal diePictureBox As PictureBox, _

73 ByVal face As Integer)

74




78

79 ' convert pictureResource to image type and load into PictureBox



82

Outline

CrapsGame.vb

(5 of 7 )



89

83 ' generate random die rolls

84 Function RollDice() As Integer

85 ' determine random integer

86 myDie1 = randomObject.Next(1, 7)

87 myDie2 = randomObject.Next(1, 7)

88

89 ' display rolls

90 DisplayDie(die1PictureBox, myDie1)

91 DisplayDie(die2PictureBox, myDie2)

92

93 Return myDie1 + myDie2 ' return sum

94 End Function ' RollDice

95 End Class ' CrapsGame

Outline

CrapsGame.vb

(6 of 7 )

a) b)

rollButtonplayButtonpointGroupBox



90

Outline

CrapsGame.vb

(7 of 7 ) c) d)



91

• String.Empty represents a string with nocharacters ("").

• Setting the Image to Nothing causes a PictureBox to appear blank.

7.16 Case Study: A Game of Chance (Cont.)


92

• The GroupBox control is a container used to group related controls.

– Within the GroupBox pointDiceGroup, we add two PictureBoxes.

– To add controls to a GroupBox, drag and drop the controls onto it.

– We set the GroupBox’s Text property to Point.



93

• Enumerations enhance program readability by providing descriptive identifiers for constant numbers or Strings.

• In this application, we created an Enumeration for the various dice combinations.

• Buttons are enabled and disabled with the Enabled property.



94

1 ' Fig. 7.19: Overload.vb

2 ' Using overloaded methods.

3 Module Overload

4 Sub Main() ' call Square methods with Integer then with Double

5 Console.WriteLine("The square of Integer 7 is " & Square(7) & _

6 vbNewLine & "The square of Double 7.5 is " & Square(7.5))

7 End Sub ' Main

8

9 ' method Square takes an Integer and returns an Integer

10 Function Square(ByVal value As Integer) As Integer

11 Return Convert.ToInt32(value ^ 2)


13

Outline

Overload.vb

(1 of 2 )

• Method overloading allows you tocreate methods with the same name but differentparameters.

• Figure 7.19 uses overloaded method Square tocalculate the square of both an Integer and a Double.

Fig. 7.19 | Using overloaded methods. (Part 1 of 2.)


95

14 ' method Square takes a Double and returns a Double

15 Function Square(ByVal value As Double) As Double

16 Return value ^ 2


18 End Module ' Overload The square of Integer 7 is 49 The square of Double 7.5 is 56.25

Outline

Overload.vb

(2 of 2 )

Good Programming Practice 7.3

Overloading methods that perform closely related tasks can make programs clearer.

Fig. 7.19 | Using overloaded methods. (Part 2 of 2.)


96

• Overload resolution determines which method to call.

– This process first finds methods that could be used.

– Visual Basic converts variables as necessary when they are passed as arguments.

– The compiler selects the closest match.

7.17 Method Overloading (Cont.)


97

1 ' Fig. 7.20: Overload2.vb

2 ' Using overloaded methods with identical signatures and

3 ' different return types.

4 Module Overload2

5 Sub Main() ' call Square methods with Integer and Double

6 Console.WriteLine("The square of Integer 7 is " & Square(7) & _

7 vbNewLine & "The square of Double 7.5 is " & Square(7.5))

8 End Sub ' Main

9

10 ' method takes a Double and returns an Integer

11 Function Square(ByVal value As Double) As Integer

12 Return Convert.ToInt32(value ^ 2)


14

Outline

Overload2.vb

(1 of 2 )

The program in Fig. 7.20 illustrates the syntaxerror that is generated when two methods havethe same signature and different return types.

Fig. 7.20 | Syntax error generated from overloaded methods with identical parameter lists and different return types. (Part 1 of 2.)


98

15 ' method takes a Double and returns a Double

16 Function Square(ByVal value As Double) As Double

17 Return value ^ 2


19 End Module ' Overload2

Outline

Overload2.vb

(2 of 2 )

Common Programming Error 7.9

Creating overloaded methods with identical parameter lists and different return types is a compilation error.

Fig. 7.20 | Syntax error generated from overloaded methods with identical parameter lists and different return types. (Part 2 of 2.)


99

• Optional parameters specify a default value that is assigned to the parameter if that argument is not passed.

Sub ExampleMethod(ByVal value1 As Boolean, _ Optional ByVal value2 As Integer = 0)

• Both of these calls to ExampleMethod are valid:ExampleMethod(True)

ExampleMethod(False, 10)

7.18 Optional Parameters


100

Common Programming Error 7.11Not specifying a default value for an Optional parameter is a syntax error.

7.18 Optional Parameters (Cont.)

Common Programming Error 7.10Declaring a non-Optional parameter to the rightof an Optional parameter is a syntax error.


101

1 ' Fig 7.21: Power.vb

2 ' Optional argument demonstration with method Power.

3 Public Class Power

4 ' reads input and displays result

5 Private Sub calculateButton_Click(ByVal sender As System.Object, _

6 ByVal e As System.EventArgs) Handles calculateButton.Click

7

8 Dim value As Integer

9

10 ' call version of Power depending on power input

11 If Not String.IsNullOrEmpty(powerTextBox.Text) Then

12 value = Power(Convert.ToInt32(baseTextBox.Text), _

13 Convert.ToInt32(powerTextBox.Text))

14 Else

15 value = Power(Convert.ToInt32(baseTextBox.Text))

16 powerTextBox.Text = Convert.ToString(2)

17 End If

18

19 outputLabel.Text = Convert.ToString(value)

20 End Sub ' calculateButton_Click

Outline

Power.vb

(1 of 3 )

• The example in Fig. 7.21 demonstrates the useof optional parameters.

Using values from both TextBoxes.

Using the value of one TextBox and omitting the optional parameter.

Fig. 7.21 | Optional argument demonstration withmethod Power. (Part 1 of 3.)


102

21

22 ' use iteration to calculate power

23 Function Power(ByVal base As Integer, _

24 Optional ByVal exponent As Integer = 2) As Integer

25

26 Dim total As Integer = 1 ' initialize total

27

28 For i = 1 To exponent ' calculate power

29 total *= base

30 Next

31

32 Return total ' return result

33 End Function ' Power

34 End Class ' Power

Outline

Power.vb

(2 of 3 )

Using values from both TextBoxes.

a)

baseTextBox powerTextBox



103

Outline

Power.vb

(3 of 3 ) b)

Power not necessary, optional parameter

c)

calculateButton



104

• A recursive method calls itself either directly or indirectly.

• Recursive problem-solving approaches have a number of elements in common:

– The method solves only the simplest case—the base case.

– When presented with a more complicated problem, the method invokes a fresh copy of itself to work on a smaller problem.

• A sequence of returns travels back up the line until the original method call returns the final result.

7.19 Recursion


105

Determining a Factorial Value Recursively

• The factorial of a nonnegative integer n, written n! (and read “n factorial”), is the product

n . ( n – 1 ) . ( n – 2 ) . … . 1

• We arrive at a recursive definition of the factorial method by observing the following relationship:

n! = n . ( n – 1 )!

7.19 Recursion (Cont.)


106

Fig. 7.22 | Recursive evaluation of 5!.

• A recursive evaluation of 5! would proceed as in Fig. 7.22.



107

1 ' Fig. 7.23: FactorialCalculator.vb

2 ' Calculating factorials using recursion.

3 Public Class FactorialCalculator

4 ' calculate factorial

5 Private Sub calculateButton_Click(ByVal sender As System.Object, _

6 ByVal e As System.EventArgs) Handles calculateButton.Click

7

8 ' convert text in TextBox to Integer

9 Dim value As Integer = Convert.ToInt32(inputTextBox.Text)

10 displayTextBox.Clear() ' reset TextBox

11

12 ' call Factorial to perform calculation

13 For i = 0 To value

14 displayTextBox.Text &= i & "! = " & Factorial(i) & vbNewLine

15 Next

16 End Sub ' calculateButton_Click

17

Outline

FactorialCalculator.vb

(1 of 2 )

• Figure 7.23 recursively calculates and prints factorials.• If number is less than or equal to 1, Factorial returns1 and the method returns.

• If number is greater than 1, the answer is expressed asthe product of number and a recursive call to Factorial.

Fig. 7.23 | Recursive factorial program. (Part 1 of 2.)


108

18 ' recursively generates factorial of number

19 Function Factorial(ByVal number As Long) As Long

20 If number <= 1 Then ' base case

21 Return 1

22 Else

23 Return number * Factorial(number - 1)

24 End If

25 End Function ' Factorial

26 End Class ' FactorialCalculator

Outline

FactorialCalculator.vb

(2 of 2 )

Determining if the factorial has been resolved to its simplest form.

Factorial makes recursive calls.

Fig. 7.23 | Recursive factorial program. (Part 2 of 2.)


109

• Set the output TextBox’s ScrollBars property to Vertical to display more outputs.

• Type Long is designed to store integer values in a range much larger than that of type Integer(such as factorials greater than 12!).

• The TextBox’s Clear method clears the text.

Common Programming Error 7.12Forgetting to return a value from a recursive methodcan result in logic errors.



110

Common Programming Error 7.13Omitting the base case or writing the recursion step so that it does not converge on the base case will cause infinite recursion, eventually exhausting memory. This is analogous to the problem of an infinite loop in an iterative (nonrecursive) solution.



111

7.20 (Optional) Software Engineering Case Study: Identifying Class Operations in the ATM System

• An operation is a service that objects of a class provide to clients of the class.

• We can derive many of the operations of the classesin our ATM system by examining the verbs and verb phrases in the requirements document (Fig. 7.24).


112

Fig. 7.24 | Verbs and verb phrases for each class in the ATM system.

7.20 (Optional) Software Engineering Case Study: Identifying Class Operations in the ATM System (Cont.)

Class Verbs and verb phrases

ATM executes financial transactions

BalanceInquiry [none in the requirements document]

Withdrawal [none in the requirements document]

Deposit [none in the requirements document]

BankDatabase authenticates a user, retrieves an account balance, credits an account by a deposit amount, debits an account by a withdrawal amount

Account retrieves an account balance, credits an account by a deposit amount, debits an account by a withdrawal amount

Screen displays a message to the user

Keypad receives numeric input from the user

CashDispenser dispenses cash, indicates whether it contains enough cash to satisfy a withdrawal request

DepositSlot receives a deposit envelope


113


• The UML represents operations (which are implemented as methods) as follows:

operationName( parameter1, parameter2, … , parameterN ) : returnType

• Each parameter consists of a parameter name and type:parameterName : parameterType


114


Fig. 7.25 | Classes in the ATM system with attributes and operations.


115


Fig. 7.26 | Class BankDatabase with operation parameters.

Fig. 7.27 | Class Account with operation parameters.


116


Fig. 7.28 | Class Screen with an operation parameters.

Fig. 7.29 | Class CashDispenser with operation parameters.

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