Chapter 6: The Repetition Structure

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Chapter 6: The Repetition Structure. Programming with Microsoft Visual Basic .NET, Second Edition. Homework/Project #1 : Calculator. Due: 03/02. Homework #1 Calculator. Use Message boxes to inform user of eventual problems. For example: Division by 0 forbidden!. The Basic FOR Loop.

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### Chapter 6: The Repetition Structure

Programming with Microsoft Visual Basic .NET, Second Edition

Homework #1 Calculator
• Use Message boxes to inform user of eventual problems. For example:
• Division by 0 forbidden!
The 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.

Set control variable to initial value

FOR Loop Flowchart

Iscontrol variable > terminating value?

Yes

No

Execute statements within loop

Increment control variable (automatic)

Execute statements following the loop

The For…Next Loop (continued)

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

FOR 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)

NextintCounter

Screen Output

1 2 3 4 5 6 7 8 9 10

FOR Loop use

Dim intStar as integer

For intStar =1 To 15

lblDisplay.Text = lblDisplay.Text & " * "

NextintStar

* * * * * * * * * * * * * * *

Screen Output

Exiting from a loop before it ends

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=9999

end if

Next

Exiting from a loop before it ends

2 ways:

#2 Use Exit for statement (recommended)

For intLoop = 1 to 1000

if intTime > 100 then

Exit for

end if

Next

FOR Loop Exercise
• Write the code to display the first 10 odd numbers.

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

FOR Loop Exercise

Dim intCounter as Integer

For IntCounter = 1 to 19 step 2lblDisplay.text = lblDisplay.Text & convert.toString(IntCounter) & “ - “

Next IntCounter

ORFor IntCounter = 1 to 10

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

Next IntCounter

Structured 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.)
Iteration Control Structure
• There are 3 types of iteration structures:
• For (condition) Loop
• Do While (condition)
• Do Until (condition)
Do While Control Structure

Do WhileCondition Statement(s)Loop

Loopstatements

Do While(Test condition)

Yes

No

Do Until Control Structure

Do Statement(s)Loop Until Condition

Loopstatement(s)

No

Do Until(test condition)

Yes

Difference between Do While / Do Until
• If there are several statements that need to be repeated, you need to decide when to stoprepeatingthem.
• You can decide to stop them:
• at the beginning of the loop, usingDo While
• or at the endof the loop using Do Until
Difference 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.
The Do…Loop Statement (continued)

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

Exit to a Loop statement

Do

Statements

If condition then

Exit Do  Provide an alternate way to exit a loopEnd If Statements

Loop Until condition

Do While condition

Statements

Exit Do

Statements

Loop

Exercise
• 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

Exercise 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

Exercise 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

Exercise #7

Exercise #7

By default hidden – visible after number have been displayed

Lab 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.
Lab 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
Lab 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

### Chapter 6: The Repetition Structure

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

The For…Next Loop (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

The For…Next Loop (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

Programming with Microsoft Visual Basic .NET, Second Edition

The For…Next Loop (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

The For…Next Loop (continued)

For…Next loop examples:

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

The Do…Loop Statement (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

The Do…Loop Statement (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

The Do…Loop Statement (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

The Do…Loop Statement (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

Nested Repetition StructuresLesson B Objectives
• Nest repetition structures

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

Nesting Repetition Structures (continued)

Figure 6-16: Nested loops used by a clock

Programming with Microsoft Visual Basic .NET, Second Edition

• 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

Programming with Microsoft Visual Basic .NET, Second Edition

Programming with Microsoft Visual Basic .NET, Second Edition

• 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

Programming with Microsoft Visual Basic .NET, Second Edition

• 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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

Coding the Shoppers Haven ApplicationLesson C Objectives
• Select the existing text in a text box
• Prevent a form from closing

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

Shoppers Haven (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

Shoppers Haven (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

Shoppers Haven (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

Shoppers Haven (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

Selecting the Existing Text in a Text Box (continued)

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition

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

Programming with Microsoft Visual Basic .NET, Second Edition