Control Statements: Part 2

1

6666ControlControl

Statements: Part 2

2009 Pearson Education, Inc. All rights reserved.

2

Who can control his fate?Who can control his fate?– William Shakespeare, Othello

The used key is always bright.– Benjamin Franklinj

Not everything that can be counted counts,and not every thing that counts can be counted.

– Albert Einstein

Every advantage in the past is judgedin the light of the final issue


in the light of the final issue.– Demosthenes

3

OBJECTIVESOBJECTIVESIn this chapter you will learn: The essentials of counter controlled repetition The essentials of counter-controlled repetition. To use the For…Next, Do…Loop While andDo Loop Until repetition statements toDo…Loop Until repetition statements to execute statements in a program repeatedly. To perform multiple selection using thep p gSelect…Case selection statement.


4

OBJECTIVESOBJECTIVES To use the Exit statement to break out of a

repetition statementrepetition statement. To use the Continue statement to break out

of the current iteration of a repetition statement.p To use logical operators to form more complex

conditions.


5

6.1 Introduction 6.2 Essentials of Counter-Controlled Repetition6.3 For…Next Repetition Statement 6 4 Examples Using the For Next Statement6.4 Examples Using the For…Next Statement 6.5 GradeBook Case Study: Select…Case

Multiple-Selection Statement 6 6 R i i S6.6 Do…Loop While Repetition Statement 6.7 Do…Loop Until Repetition Statement 6.8 Using Exit in Repetition Statements6.8 Using Exit in Repetition Statements 6.9 Using Continue in Repetition Statements 6.10 Logical Operators 6.11 (Optional) Software Engineering Case Study:

Identifying Objects’ States and Activities in the ATM System


6

6.2 Essentials of Counter-Controlled RepetitionRepetition

• Counter-controlled repetition requiresh f h l i bl– the name of the control variable

– the initial value– the increment (or decrement) valuethe increment (or decrement) value– the condition that tests for the final value


7Outline

• The example in Fig. 6.1 uses counter-controlled

1 ' Fig. 6.1: WhileCounter.vb

WhileCounter.vb

The example in Fig. 6.1 uses counter controlled repetition to display the even integers in the range 2–10.

1 Fig. 6.1: WhileCounter.vb

2 ' Using the While statement to demonstrate counter-controlled repetition.

3 Module WhileCounter

4 Sub Main()

5 Dim counter As Integer = 2 ' name and initialize loop counter

6

Initializing counterbefore the loop

6 7 While counter <= 10 ' test final value of loop counter

8 Console.Write("{0} ", counter)

9 counter += 2 ' increment counter

10 End While 1111 12 Console.WriteLine() 13 End Sub ' Main 14 End Module ' WhileCounter

2 4 6 8 10 2 4 6 8 10

Fig. 6.1 | Counter-controlled repetition with the While…End While statement.


8Outline

• In this For...Next statement, counter (Fig. 6.2) is

1 ' Fig. 6.2: ForCounter.vb

2 ' Using the For...Next statement for counter-controlled repetition.

3 Module ForCounter

ForCounter.vb

( 1 of 2 )

used to print even numbers from 2 to 10.

3 Module ForCounter

4 Sub Main()

5 ' initialization, repetition condition and

6 ' incrementing are all included in For...Next statement

7 For counter As Integer = 2 To 10 Step 2

8 Console.Write("{0} ", counter) 8 Console.Write( {0} , counter)

9 Next

10 11 Console.WriteLine() 12 End Sub ' Main 13 End Module ' ForCounter 13 End Module ForCounter

2 4 6 8 10

Fig. 6.2 | Counter-controlled repetition with the For…Next statement.

Good Programming Practice 6.1Place a blank line before and after each control

g | p


Place a blank line before and after each control statement to make it stand out in the program.

9Outline

Good Programming Practice 6.2ForCounter.vb

( 2 of 2 )

Vertical spacing above and below control statements, as well as indentation of the bodies of control statements gives programs aof control statements, gives programs atwo-dimensional appearance that enhances readability.

Error-Prevention Tip 6.1 Use a For Next loop for counter-controlledUse a For...Next loop for counter-controlled repetition. Off-by-one errors (which occur when a loop is executed for one more or one less i i h i ) d diiteration than is necessary) tend to disappear, because the terminating value is clear.


10

6.3 For…Next Repetition Statement (Cont.)(Cont.)

• The first line of the For…Next statement sometimes is called the For Next header (Fig 6 3)is called the For…Next header (Fig. 6.3).

Fig. 6.3 | For...Next header components.


11

6.3 For…Next Repetition Statement (Cont.)

• The general form of the For…Next statement is i i i li i fi lV l S i

(Cont.)

For initialization To finalValue Step incrementstatement

NextNext

• A For…Next statement can be represented byan equivalent While statement:q

initializationWhile variable <= finalValuef

statementincrement

End While


End While

12

6.3 For…Next Repetition Statement (Cont.)

• The counter variable may be declared before theFor Next statement:

(Cont.)

For…Next statement:Dim counter As Integer

For counter = 2 To 10 Step 2For counter = 2 To 10 Step 2Console.Write("{0} ", counter)

Next

• Values of a For…Next statement header maycontain arithmetic expressions (evaluated at start).

• If the loop-continuation condition is initially false,the For…Next’s body is not performed.


13


Common Programming Error 6.1Counter-controlled loops should not be controlled

i f i i i i iwith floating-point variables. Floating-point values are represented only approximately in the computer’s memory; this can lead to imprecise counter values andmemory; this can lead to imprecise counter values and inaccurate tests for termination.


14


Error-Prevention Tip 6.2Although the value of the control variable can bechanged in the body of a For…Next loop, avoiddoing so, because this practice can lead to subtle errors.doing so, because this practice can lead to subtle errors.

Common Programming Error 6 2Common Programming Error 6.2In nested For…Next loops, the use of the samecontrol-variable name in more than one loop is acontrol variable name in more than one loop is acompilation error.


15

6.3 For…Next Repetition Statement (Cont.)For counter As Integer = 1 To 10 Step 2

• The activity diagram for a For Next statement

(Cont.)

• The activity diagram for a For…Next statement (Fig. 6.4) is similar to a While statement.


Fig. 6.4 | For...Next repetition statement activity diagram.

16

6.3 For… Next Repetition Statement (Cont.)

• Local type inference infers a local variable’s typebased on the context of initialization

(Cont.)

based on the context of initialization.

Di 7Dim x = 7

• The compiler infers type Integer.

Dim y = -123.45

bl• The compiler infers type Double.


17

6.3 For… Next Repetition Statement (Cont.)For counter As Integer = 1 To 10

Th di For Next h d b itt

(Cont.)

• The preceding For Next header can now be written as:For counter = 1 To 10

• In this case, counter is of type Integer becauseit i i iti li d ith h l b (1)it is initialized with a whole number (1).

• Local type inference can be used on any variablethat is initialized in its declarationthat is initialized in its declaration.


18Outline• The program in Fig. 6.5 uses a For...Next statement

to sum the even integers from 2 to 100.R b t dd f t

1 ' Fig. 6.5: Sum.vb

2 ' Using For...Next statement to demonstrate summation.

• Remember to add a reference to System.Windows.Forms.dll. Sum.vb

( 1 of 2 )g

3 Imports System.Windows.Forms

4 5 Module Sum

6 Sub Main()

7 Dim sum As Integer 0 7 Dim sum As Integer = 0

8 9 ' add even numbers from 2 to 100

10 For number = 2 To 100 Step 2 11 sum += number 12 Next 13 14 MessageBox.Show("The sum is " & sum, _ 15 "Sum even integers from 2 to 100", _ 16 MessageBoxButtons.OK, MessageBoxIcon.Information)

Method MessageBox.Showcan take four arguments.

g , g )

17 End Sub ' Main 18 End Module ' Sum

Fig. 6.5 | For…Next statement used for summation. (Part 1 of 2.)


19Outline

Sum.vb

( 2 of 2 )

Title bar text

Message textMessageBoxIcon.Information

MessageBoxButtons.OK

Fig 6 5 | For Next statement used for summation (Part 2 of 2 )Fig. 6.5 | For…Next statement used for summation. (Part 2 of 2.)


20

6.4 Examples Using the For Next Statement (Cont.)Statement (Cont.)

• Method MessageBox.Show can take four arguments.h fi h d i l b– The first two arguments are the text and title bar.

– The third argument indicates which button(s) to display.Th f th t i di t hi h i– The fourth argument indicates which icon appears.


21


MessageBoxIcon constants Icon Description

i MessageBoxIcon.Warning or MessageBoxIcon.Exclamation

Used to caution the user against potential problems.

MessageBoxIcon.Information Used to display information about the state of the applicationapplication.

MessageBoxIcon.None No icon is displayed.

MessageBoxIcon.Error Used to alert the user to errors or critical situations.

Fig. 6.6 | Message dialog icon constants.


22


MessageBoxButton constants Description

MessageBoxButtons.OK OK button Allows the user to acknowledge aessage o utto s O OK button. Allows the user to acknowledge a message. Included by default.

MessageBoxButtons.OKCancel OK and Cancel buttons. Allow the user to either continue or cancel an operation.

Fig. 6.7 | Message dialog button constants. (Part 1 of 2.)

MessageBoxButtons.YesNo Yes and No buttons. Allow the user to respond to a question.


23


MessageBoxButton constants Description

MessageBoxButtons.YesNoCancel Yes No and Cancel buttons Allow theg Yes, No and Cancel buttons. Allow the user to respond to a question or cancel an operation.

MessageBoxButtons.RetryCancel Retry and Cancel buttons. Allow the user to retry or cancel an operation that has failed.to retry or cancel an operation that has failed.

MessageBoxButtons.AbortRetryIgnore Abort, Retry and Ignore buttons. When one of a series of operations has failed, these buttons allow the user to abort the entire sequence, retry the failed operation or ignore q , y p gthe failed operation and continue.

Fig. 6.7 | Message dialog button constants. (Part 2 of 2.)


24

6.4 Examples Using the For Next Statement (Cont.)

• Consider the following problem statement:

Statement (Cont.)

A person invests $1000.00 in a savings account that yields 5% interest. Calculate the amount of money in the account at the end of each year over a period ofthe account at the end of each year over a period of 10 years. Use the following formula:

a = p (1 + r) na = p (1 + r) n


25

1 ' Fig. 6.8: Interest.vb

2 ' Calculating compound interest.

3 Imports System.Windows.Forms

Outline

4 5 Module Interest

6 Sub Main()

7 Dim amount As Decimal ' dollar amounts on deposit

8 Dim principal As Decimal = 1000.00 ' amount invested

Interest.vb

( 1 of 3 )

9 Dim rate As Double = 0.05 ' interest rate

10 11 ' amount after each year 12 Dim output As String = _ 13 "Year" & vbTab & "Amount on deposit" & vbNewLine

amount and principalare declared as type Decimal3 ea & b ab & ou t o depos t & b e e

14 15 ' calculate amount after each year 16 For yearValue = 1 To 10 17 amount = principal * (1 + rate) ^ yearValue 18 output &= yearValue & vbTab & String Format("{0:C}" amount)

Calculating interest foreach year

18 output &= yearValue & vbTab & String.Format( {0:C} , amount) _

19 & vbNewLine 20 Next 21 22 ' display output 23 MessageBox Show(output "Compound Interest"

String.Formatformats text as directed

23 MessageBox.Show(output, Compound Interest , _

24 MessageBoxButtons.OK, MessageBoxIcon.Information) 25 End Sub ' Main 26 End Module ' Interest


Fig. 6.8 | For…Next statement used to calculatecompound interest. (Part 1 of 2.)

26Outline

Interest.vb

( 2 of 3 )

Fig. 6.8 | For…Next statement used to calculated i t t (P t 2 f 2 )compound interest. (Part 2 of 2.)


27Outline

Error-Prevention Tip 6.3Interest.vb

( 3 of 3 )

o e e t o p 6 3Do not use variables of type Single or Double to perform precise monetary calculations. The imprecision

f fl ti i t b th t lt iof floating-point numbers can cause errors that result in incorrect monetary values. Use the type Decimal for monetary calculations.

Performance Tip 6.1Avoid placing inside a loop the calculation of an expression whose value does not change each time through the loop. Such an expression should be evaluated only once andSuch an expression should be evaluated only once and prior to the loop.


28Outline

• The GradeBook class now uses the Select...Casem ltiple selection statement (Fi 6 9)

1 ' Fig. 6.9: GradeBook.vb

2 ' GradeBook class uses Select...Case statement to count letter grades.

3 Public Class GradeBook

GradeBook.vb

( 1 of 6 )

multiple-selection statement (Fig. 6.9).

4 Private courseNameValue As String ' name of course

5 Private total As Integer ' sum of grades

6 Private gradeCounter As Integer ' number of grades entered

7 Private aCount As Integer ' count of A grades

8 Private bCount As Integer ' count of B grades 8 Private bCount As Integer count of B grades

9 Private cCount As Integer ' count of C grades

10 Private dCount As Integer ' count of D grades 11 Private fCount As Integer ' count of F grades 12 Private perfectScoreCount As Integer ' count of perfect scores 1313 14 ' constructor initializes course name; 15 ' Integer instance variables are initialized to 0 by default 16 Public Sub New(ByVal name As String) 17 CourseName = name ' initializes CourseName 18 End Sub ' New 19

Fig. 6.9 | GradeBook class uses Select…Case statement to countA B C D d F d (P t 1 f 6 )


A, B, C, D and F grades. (Part 1 of 6.)

29Outline

20 ' property that gets and sets the course name; the Set accessor 21 ' ensures that the course name has at most 25 characters 22 Public Property CourseName() As String 23 Get ' retrieve courseNameValue 24 Return courseNameValue

GradeBook.vb

( 2 of 6 )24 Return courseNameValue 25 End Get 26 27 Set(ByVal value As String) ' set courseNameValue 28 If value.Length <= 25 Then ' if value has 25 or fewer characters 29 courseNameValue = value ' store the course name in the object 30 Else ' if name has more than 25 characters 31 ' set courseNameValue to first 25 characters of parameter name 32 ' start at 0, length of 25 33 courseNameValue = value.Substring(0, 25) 33 courseNameValue value.Substring(0, 25)

34 35 Console.WriteLine( _ 36 "Course name (" & value & ") exceeds maximum length (25).") 37 Console.WriteLine( _ 38 "Limiting course name to first 25 characters " & vbNewLine) 38 "Limiting course name to first 25 characters." & vbNewLine) 39 End If 40 End Set 41 End Property ' CourseName


Fig. 6.9 | GradeBook class uses Select…Case statement to countA, B, C, D and F grades. (Part 2 of 6.)

30Outline

42 43 ' display a welcome message to the GradeBook user 44 Public Sub DisplayMessage()

GradeBook.vb

( 3 of 6 )

45 Console.WriteLine("Welcome to the grade book for " _ 46 & vbNewLine & CourseName & "!" & vbNewLine) 47 End Sub ' DisplayMessage 48 49 ' input arbitrary number of grades from user 49 input arbitrary number of grades from user

50 Public Sub InputGrades() 51 Console.Write( _ 52 "Enter the grades in the range 0-100, negative value to quit: ") 53 Dim grade As Integer = Console.ReadLine() ' input first grade 5454 55 ' loop until user enters a sentinel value

56 While grade >= 0 57 total += grade ' add grade to total 58 gradeCounter += 1 ' increment number of grades 59



31Outline

60 ' call method to increment appropriate counter 61 IncrementLetterGradeCounter(grade) 62

GradeBook.vb

( 4 of 6 )

63 ' input next grade 64 Console.Write("Enter the grades in the range 0-100, " & _ 65 "negative value to quit: ") 66 grade = Console.ReadLine() 67 End While 68 End Sub ' InputGrades 69 70 ' add 1 to appropriate counter for specified grade 71 Private Sub IncrementLetterGradeCounter(ByVal grade As Integer) 72 Select Case grade ' determine which grade was entered

Using a Select...Casestatement to determine g g

73 Case 100 ' perfect score 74 perfectScoreCount += 1 ' increment perfectScoreCount 75 aCount += 1 ' increment aCount 76 Case 90 To 99 ' grade was between 90 and 99 77 aCount += 1 ' increment aCount

which counter to increment.

77 aCount += 1 increment aCount



32Outline

78 Case 80 To 89 ' grade was between 80 and 89 79 bCount += 1 ' increment bCount 80 Case 70 To 79 ' grade was between 70 and 79

GradeBook.vb

( 5 of 6 )

81 cCount += 1 ' increment cCount 82 Case 60 To 69 ' grade was between 60 and 69 83 dCount += 1 ' increment dCount 84 Case Else ' grade was less than 60 85 fCount += 1 ' increment fCount 86 End Select 87 End Sub ' IncrementLetterGradeCounter 88 89 ' display a report based on the grades entered by user 90 Public Sub DisplayGradeReport() p y p ()

91 Console.WriteLine(vbNewLine & "Grade Report:") 92 93 ' if user entered at least one grade 94 If (gradeCounter > 0) Then 95 ' calculate average of all grades entered 95 calculate average of all grades entered

96 Dim average As Double = total / gradeCounter



, , , g ( )

33Outline

97 98 ' output summary of results 99 Console.WriteLine("Total of the {0} grades entered is {1}", _

GradeBook.vb

( 6 of 6 )

100 gradeCounter, total) 101 Console.WriteLine("Class average is {0:F2}", average) 102 Console.WriteLine("Number of students who received each grade:") 103 Console.WriteLine("A: " & aCount) ' display number of A grades 104 Console WriteLine("B: " & bCount) ' display number of B grades 104 Console.WriteLine( B: & bCount) display number of B grades

105 Console.WriteLine("C: " & cCount) ' display number of C grades

106 Console.WriteLine("D: " & dCount) ' display number of D grades 107 Console.WriteLine("F: " & fCount) ' display number of F grades 108 Console.WriteLine(vbNewLine & "Number of students who " & _ 109 " i d f " & f ) 109 "received perfect scores: " & perfectScoreCount) 110 Else ' no grades were entered, so output appropriate message 111 Console.WriteLine("No grades were entered") 112 End If 113 End Sub ' DisplayGradeReport 114 End Class ' GradeBook



34

6.5 GradeBook Case Study: Select… Case Multiple-Selection Statement (Cont.)Case Multiple Selection Statement (Cont.)Select Case grade

l• The expression following the keywords SelectCase is called the controlling expression.

• If a matching Case is found for the controlling i h d i h C hexpression, the code in that Case executes, then

program control proceeds to the first statement after the Select Case statementthe Select…Case statement.


35

6.5 GradeBook Case Study: Select… Case Multiple-Selection Statement (Cont.)Case Multiple Selection Statement (Cont.)Common Programming Error 6.3D li t C t t t l i At tiDuplicate Case statements are logic errors. At run time,the first matching Case is executed.

Common Programming Error 6.4Common Programming Error 6.4If the value on the left side of the To keyword in a Case statement is larger than the value on the right side, the Case is ignored.


36

6.5 GradeBook Case Study: Select… Case Multiple-Selection Statement (Cont.)• If no match occurs between the controlling

expression’s value and a Case label the optional

Case Multiple Selection Statement (Cont.)

expression s value and a Case label, the optional Case Else executes.

• Case Else must be the last Case• Case Else must be the last Case.

• Case statements can use relational operators• Case statements can use relational operators.Case Is < 0

• Multiple values can be tested in a Case statement:• Multiple values can be tested in a Case statement:Case 0, 5 To 9

• The controlling expression also may be a String or


The controlling expression also may be a String or Object.

37

6.5 GradeBook Case Study: Select… Case Multiple-Selection Statement (Cont.)Case Multiple Selection Statement (Cont.)Error-Prevention Tip 6.4 P id C El i S l t C t t t CProvide a Case Else in Select...Case statements. Cases not handled in a Select...Case statement are ignored unless a Case Else is provided. The inclusion of a CaseElse statement can facilitate the processing of exceptional conditions.


38Outline

• Module GradeBookTest (Fig. 6.10) outputs a report based on the grades entered

1 ' Fig. 6.10: GradeBookTest.vb

GradeBookTest.vb

( 1 of 2 )

based on the grades entered.

1 Fig. 6.10: GradeBookTest.vb

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

3 Module GradeBookTest

4 Sub Main()

5 ' create GradeBook object gradeBook1 and

6 ' t t t 6 ' pass course name to constructor

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

8 9 gradeBook1.DisplayMessage() ' display welcome message

10 gradeBook1.InputGrades() ' read grades from user 11 gradeBook1.DisplayGradeReport() ' display report based on grades 12 End Sub ' Main 13 End Module ' GradeBookTest

Fig 6 10 | GradeBookTest creates a GradeBook object and invokesFig. 6.10 | GradeBookTest creates a GradeBook object and invokesits methods. (Part 1 of 2.)


39

Welcome to the grade book for

CS101 Introduction to VB!

Outline

CS101 Introduction to VB!

Enter a grade in the range 0-100, negative value to quit: 99




d i h 0 100 i l i 63

GradeBookTest.vb

( 2 of 2 )Enter a grade in the range 0-100, negative value to quit: 63





Enter a grade in the range 0-100, negative value to quit: 100 g g , g q

Enter a grade in the range 0-100, negative value to quit: -1

Grade Report:

Total of the 10 grades entered is 778

Class average is 77.80

Number of students who received each grade: Number of students who received each grade:

A: 4

B: 1

C: 2

D: 1

F: 2

Number of students who received a perfect score: 1

Fig. 6.10 | GradeBookTest creates a GradeBook object and invokes


g | jits methods. (Part 2 of 2.)

40

6.5 GradeBook Case Study: Select… Case Multiple-Selection Statement (Cont.)Case Multiple Selection Statement (Cont.)• Methods declared Private can be called only by

other members of the class in hich the Privateother members of the class in which the Privatemethods are declared.

• Such Private methods are commonly referred to as• Such Private methods are commonly referred to as utility methods or helper methods.


41

6.5 GradeBook Case Study: Select… Case Multiple-Selection Statement (Cont.)Case Multiple Selection Statement (Cont.)• The With statement allows you to make multiple

references to the same objectreferences to the same object.With gradeBook1

.DisplayMessage() ' display welcome messageI tG d () ' d d f .InputGrades() ' read grades from user

.DisplayGradeReport() ' display report based on grades

End With

• These lines of code are collectively known as a With t t t bl kstatement block.


42

6.5 GradeBook Case Study: Select… Case Multiple-Selection Statement (Cont.)Case Multiple Selection Statement (Cont.)

• Figure 6.11 shows the UML activity diagram for the general Select...Case statement.


Fig. 6.11 | Select…Case multiple-selection statement UML activity diagram.

43Outline

• The Do...Loop While repetition statement is similar to the While statement

DoLoopWhile.vb

( 1 of 2 )

to the While statement.• The program in Fig. 6.12 uses a Do...Loop While

statement to output the values 1–5. 1 ' Fig. 6.12: DoLoopWhile.vb

2 ' Demonstrating the Do...Loop While repetition statement.

3 Module DoLoopWhile

4 Sub Main()

5 Dim counter As Integer 1 5 Dim counter As Integer = 1

6 7 ' print values 1 to 5

8 Do


10 counter += 1 11 Loop While counter <= 5 12 13 Console.WriteLine() 14 End Sub ' Main

Condition tested after loop body executes

15 End Module ' DoLoopWhile

1 2 3 4 5

hil


Fig. 6.12 | Do…Loop While repetition statement.

44Outline

DoLoopWhile.vb

( 2 of 2 )

Error-Prevention Tip 6.5 .Infinite loops occur when the loop-continuation p pcondition in a While, Do While...Loop orDo...Loop While statement never becomes false.


45

6.6 Do…Loop While Repetition Statement (Cont.)(Cont.)

• The Do…Loop While UML activity diagram (Fig. 6.13) illustrates th t th l ti ti diti i t l t d til ft ththat the loop-continuation condition is not evaluated until after the statement body.


Fig. 6.13 | Do…Loop While repetition statement activity diagram.

46Outline

• Figure 6.14 uses a Do...Loop Until statement to print the numbers from 1 to 5

1 ' Fig. 6.14: DoLoopUntil.vb

DoLoopUntil.vb

print the numbers from 1 to 5.

2 ' Using Do...Loop Until repetition statement.

3 Module DoLoopUntil

4 Sub Main()

5 Dim counter As Integer = 1

6 7 ' print values 1 to 5

8 Do


10 counter += 1 11 Loop Until counter > 5

Condition tested after loop body executes

12 13 Console.WriteLine() 14 End Sub ' Main 15 End Module ' DoLoopUntil

loop body executes

1 2 3 4 5

Fig. 6.14 | Do…Loop Until repetition statement.


47

6.7 Do...Loop Until Repetition Statement (Cont.)(Cont.) Common Programming Error 6.5I l di i t l ti l t i tIncluding an incorrect relational operator or an incorrectfinal value for a loop counter in the condition of any repetition statement can cause off-by-one errors.y

Error-Prevention Tip 6.6.Infinite loops occur when the loop-termination condition in a Do Until...Loop or Do...Loop Until statement never b tbecomes true.


48

6.7 Do...Loop Until Repetition Statement (Cont.)(Cont.) Error-Prevention Tip 6.7 I t t ll d l th t th t l i bl iIn a counter-controlled loop, ensure that the control variable is incremented (or decremented) appropriately in the body of the loop to avoid an infinite loop.p p


49

6.8 Using Exit in Repetition Statements

• An Exit statement causes the program to exit immediately from a repetition statementimmediately from a repetition statement.

– The Exit Do statement can be executed in any Do statement.– Exit For and Exit While cause immediate exit fromExit For and Exit While cause immediate exit from For…Next and While…End While loops.

• These statements are used to alter a program’s flow of control.


50Outline

• Figure 6.15 demonstrates Exit statements.

1 ' Fig. 6.15: ExitTest.vb

2 ' Using the Exit statement in repetition statements.

3 Module ExitTest

ExitTest.vb

( 1 of 3 )

4 Sub Main()

5 Dim counter As Integer ' loop counter

6 7 ' exit For...Next statement

8 For counter = 1 To 10 8 For counter = 1 To 10

9 ' skip remaining code in loop only if counter = 5

10 If counter = 5 Then 11 Exit For ' break out of loop 12 End If 1313 14 Console.Write("{0} ", counter) ' output counter 15 Next 16

Fig. 6.15 | Exit statement in repetition statements. (Part 1 of 3.)


51Outline

17 Console.WriteLine(vbNewLine & _ 18 "Broke out of For...Next at counter = " & counter & vbNewLine) 19 counter = 1 ' reset counter 20 21 ' i il

ExitTest.vb

( 2 of 3 )21 ' exit Do Until...Loop statement 22 Do Until counter > 10 23 ' skip remaining code in loop only if counter = 5 24 If counter = 5 Then 25 Exit Do ' break out of loop 26 End If 27 28 Console.Write("{0} ", counter) ' output counter 29 counter += 1 ' increment counter 30 Loop 30 Loop 31 32 Console.WriteLine(vbNewLine & "Broke out of Do Until...Loop " & _ 33 " at counter = " & counter & vbNewLine) 34 counter = 1 ' reset counter 3535 36 ' exit While statement



52Outline

37 While counter <= 10 38 ' skip remaining code in loop only if counter = 5 39 If counter = 5 Then 40 Exit While ' break out of loop 41 End If

ExitTest.vb

( 3 of 3 )

42 43 Console.Write("{0} ", counter) ' output counter 44 counter += 1 ' increment counter 45 End While 4646 47 Console.WriteLine( _ 48 vbNewLine & "Broke out of While at counter = " & counter) 49 End Sub ' Main 50 End Module ' ExitTest

1 2 3 4

Broke out of For...Next at counter = 5

1 2 3 4

Broke out of Do Until Loop at counter = 5 Broke out of Do Until...Loop at counter = 5

1 2 3 4

Broke out of While at counter = 5



536.9 Using Continue in Repetition Statements

• The Continue statement skips to the next iteration of the loopof the loop.

– The Continue Do statement can be executed in any Dostatement.

– Continue For and Continue While are used inFor... Next and While loops.

Continue For i t th t l i bl b• Continue For increments the control variable by the Step value.


54Outline

• Figure 6.16 demonstrates the Continue statements.

1 ' Fig. 6.16: ContinueTest.vb

2 ' Using the Continue statement in repetition statements.

3 Module ContinueTest

ContinueTest.vb

( 1 of 3 )

4 Sub Main()

5 Dim counter As Integer ' loop counter

6 7 ' skipping an iteration of a For...Next statement

8 For counter = 1 To 10 8 For counter = 1 To 10

9 If counter = 5 Then

10 Continue For ' skip to next iteration of loop if counter = 5 11 End If 12

l i ("{ } " ) '13 Console.Write("{0} ", counter) ' output counter 14 Next 15 16 Console.WriteLine(vbNewLine & _ 17 "Skipped printing in For...Next at counter = 5" & vbNewLine) 18 counter = 0 ' reset counter 19

Fig. 6.16 | Continue statement in repetition statements. (Part 1 of 3.)


55Outline

20 ' skipping an iteration of a Do Until...Loop statement 21 Do Until counter >= 10 22 counter += 1 ' increment counter 23 24 f 5 h

ContinueTest.vb

( 2 of 3 )24 If counter = 5 Then 25 Continue Do ' skip to next iteration of loop if counter = 5 26 End If 27 28 Console.Write("{0} ", counter) ' output counter 29 Loop 30 31 Console.WriteLine(vbNewLine & _ 32 "Skipped printing in Do Until...Loop at counter = 5" & vbNewLine) 33 counter = 0 ' reset counter 33 counter = 0 reset counter

34 35 ' skipping an iteration of a While statement 36 While counter < 10 37 counter += 1 ' increment counter 3838 39 If counter = 5 Then 40 Continue While ' skip to next iteration of loop if counter = 5 41 End If



56Outline

42 43 Console.Write("{0} ", counter) ' output counter 44 End While

ContinueTest.vb

( 3 of 3 )

45 46 Console.WriteLine( _ 47 vbNewLine & "Skipped printing in While at counter = 5") 48 End Sub ' Main 49 End Module ' ContinueTest 49 End Module ContinueTest

1 2 3 4 6 7 8 9 10

Skipped printing in For...Next at counter = 5

1 2 3 4 6 7 8 9 10

Skipped printing in Do Until...Loop at counter = 5

1 2 3 4 6 7 8 9 10

Skipped printing in While at counter = 5



57

6.10 Logical Operators • The logical And operator can be used as follows:If gender = "F" And age >= 65 Then

seniorFemales += 1

End If

• The If Then statement considers the combined condition:• The If Then statement considers the combined condition:gender = "F" And age >= 65

• Figure 6.17 is a truth table for the And operator. expression1 expression2 expression1 And expression2

False False False

g p

False True False

True False False

True True True


Fig. 6.17 | Truth table for the logical And operator.

58

6.10 Logical Operators (Cont.)

Logical Or Operatorh O i d i h f ll i• The Or operator is used in the following segment:

If (semesterAverage >= 90 Or finalExam >= 90) Then Console.WriteLine("Student grade is A")

End If

• Figure 6.18 provides a truth table for the Or operator.

expression1 expression2 expression1 Or expression2

False False False

False True True False True True

True False True

True True True


Fig. 6.18 | Truth table for the logical Or operator.

59


Logical AndAlso and OrElse Operators• AndAlso and OrElse are similar to the And and Or• AndAlso and OrElse are similar to the And and Or

operators.(gender = "F" AndAlso age >= 65)(gender F AndAlso age > 65)

• The preceding expression stops evaluation immediately if gender is not equal to "F"; this is called short-circuit

l tievaluation.Performance Tip 6.2In expressions using operator AndAlso if the separate conditions areIn expressions using operator AndAlso, if the separate conditions are independent of one an-other, place the condition most likely to be false as the leftmost condition. In expres-sions using operator OrElse, make the condition most likely to be true the leftmost condition. Each of these


condition most likely to be true the leftmost condition. Each of these suggestions can reduce a program’s execution time.

60


• AndAlso and OrElse operators can be used in place of Andand Orand Or.

• An exception to this rule occurs when the right operand of a condition produces a side effect:

Console.WriteLine("How old are you?")

If (gender = "F" And Console.ReadLine() >= 65) Then(g () )Console.WriteLine("You are a female senior citizen.")

End If

Error-Prevention Tip 6.8Avoid expressions with side effects in conditions,


because side effects often cause subtle errors.

61


Logical Xor OperatorA di i i i h l i l l i OR ( )• A condition containing the logical exclusive OR (Xor)operator is true if and only if one of its operands results in a true value and the other results in a false value.

• Figure 6.19 presents a truth table for the Xor operator.

expression1 expression2 expression1 Xor expression2

False False False

False True True False True True

True False True

True True False


Fig. 6.19 | Truth table for the logical exclusive OR (Xor) operator.

62


Logical Not OperatorTh N t bl “ ” h• The Not operator enables you to “reverse” the meaning of a condition.Th l i l ti t i t• The logical negation operator is a unary operator, requiring only one operand.

If Not (grade = sentinelValue) ThenIf Not (grade = sentinelValue) ThenConsole.WriteLine("The next grade is " & grade)

End If

• The parentheses are necessary because Not has a higher precedence than the equality operator.


63


• You can avoid using Not by expressing the condition differently:differently:

If grade <> sentinelValue Then

Console.WriteLine("The next grade is " & grade)

End If

• Figure 6.20 provides a truth table for Not.

expression Not expression

False True

Fig. 6.20 | Truth table for operator Not (logical negation).

True False


64Outline• Figure 6.21 demonstrates the logical operators by

displaying their truth tables.

1 ' Fig. 6.21: LogicalOperators.vb

2 ' Using logical operators.

3 Module LogicalOperators

LogicalOperators.vb

( 1 of 4 ) 4 Sub Main()

5 ' display truth table for And

6 Console.WriteLine("And" & vbNewLine & _

7 "False And False: " & (False And False) & vbNewLine & _

8 "False And True: " & (False And True) & vbNewLine &

( 1 of 4 )

8 False And True: & (False And True) & vbNewLine & _

9 "True And False: " & (True And False) & vbNewLine & _

10 "True And True: " & (True And True) & vbNewLine) 11 12 ' display truth table for Or 13 l i i (" " & b i & 13 Console.WriteLine("Or" & vbNewLine & _ 14 "False Or False: " & (False Or False) & vbNewLine & _ 15 "False Or True: " & (False Or True) & vbNewLine & _ 16 "True Or False: " & (True Or False) & vbNewLine & _ 17 "True Or True: " & (True Or True) & vbNewLine) 18

Fig. 6.21 | Logical operator truth tables. (Part 1 of 4.)


65Outline

19 ' display truth table for AndAlso 20 Console.WriteLine("AndAlso" & vbNewLine & _ 21 "False AndAlso False: " & (False AndAlso False) & vbNewLine & _ 22 "False AndAlso True: " & (False AndAlso True) & vbNewLine &

LogicalOperators.vb

( 2 of 4 )22 False AndAlso True: & (False AndAlso True) & vbNewLine & _

23 "True AndAlso False: " & (True AndAlso False) & vbNewLine & _ 24 "True AndAlso True: " & (True AndAlso True) & vbNewLine) 25 26 ' display truth table for OrElse

( 2 of 4 )

27 Console.WriteLine("OrElse" & vbNewLine & _ 28 "False OrElse False: " & (False OrElse False) & vbNewLine & _ 29 "False OrElse True: " & (False OrElse True) & vbNewLine & _ 30 "True OrElse False: " & (True OrElse False) & vbNewLine & _ 31 "True OrElse True: " & (True OrElse True) & vbNewLine) ( ) )

32 33 ' display truth table for Xor 34 Console.WriteLine("Xor" & vbNewLine & _ 35 "False Xor False: " & (False Xor False) & vbNewLine & _ 36 "False Xor True: " & (False Xor True) & vbNewLine & 36 False Xor True: & (False Xor True) & vbNewLine & _

37 "True Xor False: " & (True Xor False) & vbNewLine & _ 38 "True Xor True: " & (True Xor True) & vbNewLine)




66Outline

39 40 ' display truth table for Not 41 Console.WriteLine("Not" & vbNewLine & "Not False: " & _ 42 (Not False) & vbNewLine & "Not True: " & (Not True) & vbNewLine) 43 End Sub ' Main

LogicalOperators.vb

( 3 of 4 )44 End Module ' LogicalOperators

And

False And False: False

False And True: False

( 3 of 4 )

True And False: False

True And True: True

Or

False Or False: False

False Or True: True False Or True: True

True Or False: True

True Or True: True

AndAlso

False AndAlso False: False False AndAlso False: False

False AndAlso True: False

True AndAlso False: False

True AndAlso True: True

(continued on next page...)



67Outline

(continued from previous page…) And

False And False: False

False And True: False

T A d F l F l

LogicalOperators.vb

( 4 of 4 )True And False: False

True And True: True

Or

False Or False: False

False Or True: True

( 4 of 4 )

False Or True: True

True Or False: True

True Or True: True

AndAlso

False AndAlso False: False

False AndAlso True: False

True AndAlso False: False

True AndAlso True: True



68


• Figure 6.22 displays the operators in decreasing orderof precedenceof precedence.

Operators Type

^ exponentiation

+ - unary plus and minus

* / multiplicative operatorsp p

\ integer division

Mod modulus

+ - additive operators

Fig. 6.22 | Precedence of the operators discussed so far. (Part 1 of 2.)

additive operators


69


Operators Type

& concatenation

< <= > >= = <> relational and equality < <= > >= = <> relational and equality

Not logical NOT

And AndAlso logical AND

Or OrElse logical inclusive OR Or OrElse logical inclusive OR

Xor logical exclusive OR

= += -= *= /= \= ^= &= assignment

Fig. 6.22 | Precedence of the operators discussed so far. (Part 2 of 2.)


70

6.11 Software Engineering Case Study: Identifying Objects’ States and ActivitiesIdentifying Objects States and Activities in the ATM System

E h bj t i t th h i f di t• Each object in a system goes through a series of discrete states.

• State machine diagrams model key states of an objectState machine diagrams model key states of an object and show under what circumstances the object changes state.

• Figure 6.23 models some of the states of an ATM object.


Fig. 6.23 | State machine diagram for some of the states of the ATM object.

71

6.11 Software Engineering Case Study: Identifying Objects’ States and ActivitiesIdentifying Objects States and Activities in the ATM System (Cont.)

• An activity diagram models an object’s workflow• An activity diagram models an object’s workflow.• The activity diagram in Fig. 6.24 models the actions involved

in executing a BalanceInquiry transaction.g q y


Fig. 6.24 | Activity diagram for a BalanceInquiry transaction.

72

6.11 Software Engineering Case Study: Identifying Objects’ States and Activities in the ATM System (Cont.)

• Figure 6 25 shows a moreFigure 6.25 shows a more complex activity diagram for a Withdrawal.


Fig. 6.25 | Activity diagram for a Withdrawal transaction.

Documents

Control Statements: Part 2