Chapter 6

Chapter 6: The Repetition Structure

Programming with Microsoft Visual Basic .NET, Second Edition

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Homework/Project #1 : Calculator

Due: 03/02

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Homework #1 Calculator

• Use Message boxes to inform user of eventual problems. For example:– Division by 0 forbidden!

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The Basic FOR LoopThe Basic FOR Loop

• A counter loop is also known as a For loop, or a For/Next loop.

• You use a counter loop when you want the computer to perform a task a specific number of times.

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FOR LoopFOR Loop Flowchart Flowchart

Set control variable to initial value

Iscontrol variable >

terminating value?

Execute statements within loop

Increment control variable (automatic)

Execute statements following the loop

Yes

No

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The For…Next Loop (continued)The For…Next Loop (continued)

Figure 6-4: Pseudocode and flowchart for the first example shown in Figure 6-2

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FOR Loop useFOR Loop use

For ControlVariable= InitVal To TerminatingVal [Step StepSize]

Statement(s) to be Repeated

Next ControlVariable

Dim intCounter as integer

For intCounter =1 To 10

txtDisplay.Text = txtDisplay.Text & convert.toString(IntCounter)

Next intCounter

1 2 3 4 5 6 7 8 9 10Screen Output

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FOR Loop useFOR Loop use

Dim intStar as integer

For intStar =1 To 15

lblDisplay.Text = lblDisplay.Text & " * "

Next intStar

* * * * * * * * * * * * * * *Screen Output

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Exiting from a loop before it Exiting from a loop before it endsends

2 ways:

#1 Assign a value to the control variable that will make the loop condition false (> terminating value)

For intLoop = 1 to 1000…if intTime > 100 then

intLoop=9999end if…

Next

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Exiting from a loop before it Exiting from a loop before it endsends

2 ways:

#2 Use Exit for statement (recommended)

For intLoop = 1 to 1000…if intTime > 100 then

Exit forend if…

Next

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FOR Loop ExerciseFOR Loop Exercise

• Write the code to display the first 10 odd numbers.

1 – 3 – 5 – 7 – 9 – 11 – 13 – 15 - 17 - 19

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FOR Loop ExerciseFOR Loop Exercise

Dim intCounter as Integer

For IntCounter = 1 to 19 step 2

lblDisplay.text = lblDisplay.Text & convert.toString(IntCounter) & “ - “

Next IntCounterOR

For IntCounter = 1 to 10

lblDisplay.Text = lblDisplay.Text & convert.toString (IntCounter*2) & “ - “

Next IntCounter

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Structured program designStructured program design

• Control structures:

– Sequence control structure (one program statement follows another in logical order).

– Selection control structure (represents choices)

– Iteration, loop structure (when a process may be repeated as long as a certain condition remains true.)

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• There are 3 types of iteration structures:

• For (condition) Loop

• Do While (condition)

• Do Until (condition)

Iteration Control StructureIteration Control Structure

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Do While Control StructureDo While Control Structure

strPassword=InputBox(“Password?”)

Do While strPassword <> “007”

strPassword=InputBox(“Password?”)

Loop

Loopstatements

Yes

No

Do While Condition Statement(s)Loop

Do While(Test condition)

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Do Until Control StructureDo Until Control Structure

Loopstatement(s)

Do Until(test condition)

Yes

No

Do Statement(s)Loop Until Condition

Do

strPassword = InputBox(“Password?”)

Loop Until strPassword = “007”

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Difference between Do While / Do UntilDifference between Do While / Do Until

• If there are several statements that need to be repeated, you need to decide when to stop repeating them.

• You can decide to stop them:

– at the beginning of the loop, using Do While

– or at the end of the loop using Do Until

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Difference between Do While / Do UntilDifference between Do While / Do Until

• The Do Until iterations means that the loop

statement will be executed at least once.

This is because the iteration statements are

executed before you asked whether to stop.

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The Do…Loop Statement The Do…Loop Statement (continued)(continued)

Figure 6-7: Syntax and examples of the Do...Loop statement

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Exit to a Loop statementExit to a Loop statementDo

StatementsIf condition then Exit Do Provide an alternate way to exit a loop

End If StatementsLoop Until condition

Do While conditionStatementsExit DoStatements

Loop

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ExerciseExercise

• Write a program to display all the numbers between 1 and 1000 that are perfect squares. (A perfect square is an integer that is the square of another integer; 1,4,9,16, …)

• Use: #1 Do While

#2 Do Until

#3 For Next

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Exercise SolutionExercise Solution

Dim intX, intY as integer

intX=1

IntY = IntX * IntX

lblDisp.text= "The perfect squares between 1 and 1000 are"

Do While IntY <= 1000 Do

lblDisp.text= lblDisp.text & lblDisp.text =lblDisp.text &

conver.toString(IntY) & “,” conver.toString(IntY) & “,”

IntX = IntX + 1 IntX = IntX + 1

IntY = IntX * IntX IntY = IntX * IntX

Loop Loop Until IntY >= 1000

1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289,

324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900, 961

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Exercise SolutionExercise Solution

lblDisp.text= "The perfect squares between 1 and 1000 are"

For intX=1 to 100 (Could have used another end value)

intY = intX * intX

If intY > 1000 Then

Exit For

Else

lblDisp.text=convert.toString(IntY)

End If

Next IntX

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Exercise #7Exercise #7Exercise #7

By default hidden – visible after number have been displayed

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Lab ExercisesLab Exercises

• #8 Display a row of 50 stars (asterisks).

• #9 Request a number from 1 to 20 (using an input box) and display a row of that many stars. If number >20 or <1 display a warning message (message box) and request the number again.

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Lab ExerciseLab Exercise

• #10 Find the sum 1 + ½ + 1/3 + ¼ + … + 1/100 Solution: 5.187378

• #11 You are offered two salary options for ten days of work.

Option 1: $100 per day

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Lab ExerciseLab Exercise

Option2: $1 the first day, $2 the second day, $4 the third day, and so on, with the amount of doubling each day.

Write a program to determine which option pays better.

Solution: Option1: $1000 Option 2: $1023

That’s all Folks!That’s all Folks!

Chapter 6: The Repetition Structure

Programming with Microsoft Visual Basic .NET, Second Edition

Programming with Microsoft Visual Basic .NET, Second Edition 30

The Repetition Structure (Looping)Lesson A Objectives

• Code the repetition structure using the For...Next and Do...Loop statements

• Write pseudocode for the repetition structure

• Create a flowchart for the repetition structure

• Initialize and update counters and accumulators


The Repetition Structure

• Use the repetition structure to repeatedly process one or more program instructions until some condition is met, at which time the repetition ends

• The repetition structure is referred to as a loop


The Repetition Structure (continued)

• Pretest loop: evaluation occurs before the instructions within the loop are processed

• Posttest loop: evaluation occurs after the instructions within the loop are processed


The For…Next Loop

• Use the For…Next statement to code a loop that repeats for a specific number of times

Figure 6-2: Syntax and examples of the For...Next statement


The For…Next Loop (continued)

Figure 6-2: Syntax and examples of the For...Next statement (continued)



• counter is a numeric variable that keeps track of how many times the loop instructions are repeated

• startvalue, endvalue, and stepvalue

– Must be numeric

– Can be positive or negative, integer or non-integer

– Default stepvalue is 1



Figure 6-4: Pseudocode and flowchart for the first example shown in Figure 6-2



Dim count As Integer

For count = 0 to 3 Step 1

Debug.WriteLine(count)

Next count


For count = 3 to 0 Step -1


Next count


For count = 0 to 10 Step 2


Next count

Dim loc As Single

For loc = 0.5 To 15 Step 0.5

Debug.WriteLine(loc)

Next loc

For…Next loop examples:


The Do…Loop Statement

• Unlike the For…Next statement, the Do…Loop statement can be used to code both a pretest loop and a posttest loop

• The Do…Loop statement begins with the Do clause and ends with the Loop clause


The Do…Loop Statement (continued)

Figure 6-7: Syntax and examples of the Do...Loop statement



Figure 6-7: Syntax and examples of the Do...Loop statement (continued)



Figure 6-9: Flowcharts for the examples shown in Figure 6-7



Figure 6-9: Flowcharts for the examples shown in Figure 6-7 (continued)


Using Counters and Accumulators

• Counters and accumulators are used within a repetition structure to calculate subtotals, totals, and averages

• A counter is a numeric variable used for counting something and is typically updated by 1

• An accumulator is a numeric variable used for accumulating and is updated by an amount that varies


Using Counters and Accumulators (continued)

• Initializing: assigning a beginning value to the counter or accumulator

• Updating (incrementing): adding a number to the value stored in the counter or accumulator


Nested Repetition StructuresLesson B Objectives

• Nest repetition structures


Nesting Repetition Structures

• In a nested repetition structure, one loop, referred to as the inner loop, is placed entirely within another loop, called the outer loop

• A clock uses nested loops to keep track of the time


Nesting Repetition Structures (continued)

Figure 6-16: Nested loops used by a clock


The Grade Calculator Application

• Professor Arkins needs an application that allows him to assign a grade to any number of students

• Each student’s grade is based on three test scores, with each test worth 100 points

• The application should total the test scores and then assign the appropriate grade, using the table shown on the next slide


The Grade Calculator Application (continued)

Total points earned Grade

270–300 A

240–269 B

210–239 C

180–209 D

below 180 F



• uiAssignGradeButton’s Click event procedure

– Allows Professor Arkins to enter each student’s test scores, and then assign the appropriate grade

– Contains two loops, one nested within the other

– A For...Next statement controls the inner loop



• uiAssignGradeButton’s Click event procedure (continued)

– A Do...Loop statement controls the outer loop

– The inner loop is a pretest loop

– The outer loop is a posttest loop



Figure 6-20: Sample run of the application that contains the procedure


Coding the Shoppers Haven Application

Lesson C Objectives• Select the existing text in a text box

• Prevent a form from closing


Shoppers Haven

• The manager of Shoppers Haven wants an application that the store clerks can use to calculate the discounted price of an item, using discount rates from 10% through 30% in increments of 5%

• The clerks will enter the item’s original price

• The application should display the discount rates and the discounted prices in the interface


Shoppers Haven (continued)

Figure 6-21: User interface for the Shoppers Haven application



Figure 6-22: TOE chart for the Shoppers Haven application



Figure 6-23: Pseudocode for the Calculate button’s Click event procedure



Figure 6-27: Discounted prices shown in the Shoppers Haven application


Selecting the Existing Text in a Text Box

• Use the SelectAll method to select all of the text contained in a text box

• Syntax: textbox.SelectAll()

• textbox is the name of the text box whose text you want to select


Selecting the Existing Text in a Text Box (continued)

• Enter the SelectAll method in a text box control’s Enter event

• A text box control’s Enter event occurs when the user tabs to the control, and when the Focus method is used in code to send the focus to the control

• The uiOriginalTextBox control’s Enter event is responsible for highlighting the existing text in the control


Selecting the Existing Text in a Text Box (continued)

Figure 6-29: Text selected in the Shoppers Haven application


Coding the TextChanged Event Procedure

• A control’s TextChanged event occurs when the contents of a control’s Text property change

• Use the uiOriginalTextBox’s TextChanged event to clear the contents of the uiDiscPricesLabel when the user changes the original price


Coding the ShoppersForm’s Closing Event Procedure

• A form’s Closing event occurs when a form is about to be closed

• In the Shoppers Haven application, the Closing event procedure is responsible for:

– Verifying that the user wants to exit the application

– Taking an action based on the user’s response


Coding the ShoppersForm’s Closing Event Procedure (continued)

Figure 6-31: Pseudocode for the ShoppersForm’s Closing event procedure


Coding the ShoppersForm’s Closing Event Procedure (continued)

Figure 6-33: Message box displayed by the form’s Closing event


Summary

• Repetition structure (loop): the computer repeats a set of instructions until some condition is met

• Code a repetition structure in Visual Basic .NET using one of the following statements: For...Next, Do...Loop, and For Each…Next

• The For...Next statement is pretest loops only

• The Do...Loop statement can code pretest and posttest loops


Summary (continued)

• To use a counter or accumulator:

– Initialize, if necessary

– Update using an assignment statement in a repetition structure

• To nest a repetition structure, place the entire inner loop within the outer loop


Summary (continued)

• To process code when the user tabs to a control, or when the Focus method is used in code to send the focus to the control, enter the code in the control’s Enter event procedure

• To process code when a form is about to be closed, enter the code in the form’s Closing event procedure

Chapter 6

Documents

ends2 ways

perfect squares

inttime gt

statements

counter loop

test condition

text lbldisplay

text amp