Chapter 6: The Repetition Structure Programming with Microsoft Visual Basic.NET, Second Edition.

Chapter 6: The Repetition Structure

Programming with Microsoft Visual Basic .NET, Second Edition

Programming with Microsoft Visual Basic .NET, Second Edition 2

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: The Repetition Structure Programming with Microsoft Visual Basic.NET, Second Edition.

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Transcript of Chapter 6: The Repetition Structure Programming with Microsoft Visual Basic.NET, Second Edition.