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Microsoft Visual Basic 2010: Reloaded Fourth Edition. Chapter Six Repeating Program Instructions. Repetition Structure Do While...Loop, Do Until...Loop , Do Loop...While, and Do Loop...Until repetition statements String Concatenation Using ListBoxes

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microsoft visual basic 2010 reloaded fourth edition

Microsoft Visual Basic 2010: ReloadedFourth Edition

Chapter Six

Repeating Program Instructions

overview
Repetition Structure

DoWhile...Loop, DoUntil...Loop, DoLoop...While, and DoLoop...Until repetition statements

String Concatenation

Using ListBoxes

Display a dialog box using the InputBox function

Enable and disable a control

Refresh the screen

Delay program execution

Overview
the repetition structure
The Repetition Structure
  • Repetition structure (or loop): a structure that repeatedly processes one or more program instructions until a condition is met
  • Looping condition: the requirement for repeating the instructions
  • Loop exit condition: the requirement for not repeating the instructions
  • Pretest loop: condition is evaluated before the instructions within the loop are processed (0 or more times)
  • Posttest loop:condition is evaluated after the instructions within the loop are processed (at least once)
do while loop repetition statement
DoWhile...Loop Repetition Statement
  • A repetition statement can repeat actions, depending on the value of a condition.
  • If you go to the grocery store with a list of items to purchase, you go through the list until you have each item:

Do while there are more items on my shopping list Put next item in cart Cross it off my list

do while loop repetition statement1
DoWhile...Loop Repetition Statement
  • Using a DoWhile...Loop statement, this codefinds the first power of 3 greater than 50.

Dim product As Integer = 3

Do While product <= 50 product *= 3Loop

  • The condition in the DoWhile...Loopstatement, product<=50, is referred to as theloop-continuationcondition.
  • When the loop-continuation condition becomes false, the DoWhile...Loop statement finishes executing.
do until loop repetition statement
DoUntil...Loop Repetition Statement
  • These statements describe the repetitive actions that occur during a shopping trip:

Do until there are no more items on my shopping list Put next item in cart Cross it off my list

  • Statements in the body of a DoUntil...Loop are executed repeatedly for as long as the loop-termination condition remains False.
    • This is known as aloop-termination condition.
do until loop repetition statement1
DoUntil...Loop Repetition Statement
  • Using aDoUntil...Loop, this code finds the first power of3 larger than 50:

Dim product As Integer = 3

Do Until product > 50 product *= 3Loop

  • Failure to provide the body of a DoUntil...Loop statement with an action that eventually causes the condition in the DoUntil...Loop to become true creates an infinite loop.
do loop while repetition statement
Do...LoopWhile Repetition Statement
  • Do...LoopWhile repetition statement is similar to the Do...WhileLoop statement, except that the loop-termination condition is tested after the loop body is performed.
  • Consider the example of packing a suitcase:
    • You place an item in the suitcase, then determine whether the suitcase is full.
    • As long as the suitcase is not full, you continue to put items in the suitcase.
do loop while repetition statement1
Do...LoopWhile Repetition Statement
  • The following application segment displays the numbers 1 through 3 in a ListBox:

DimcounterAsInteger=1DodisplayListBox.Items.Add(counter)counter+=1LoopWhilecounter<=3

do loop until repetition statement
Do...LoopUntil Repetition Statement
  • The Do...LoopUntil statement is similar to the Do...UntilLoop statement, except that the loop-termination condition is tested after the loop body is performed.
  • Imagine that you place an item in the suitcase, then determine whether the suitcase is full. As long as the condition “the suitcase is full” isFalse, you continue to put items into the suitcase.
do loop until repetition statement1
Do...LoopUntil Repetition Statement
  • This application segment displays the numbers 1 through 3 in a ListBox:

DimcounterAsInteger=1DodisplayListBox.Items.Add(counter)counter+=1LoopUntilcounter>3

car payment calculator application
CarPayment Calculator Application
  • Typically, banks offer car loans for periods ranging from two to five years. Borrowers repay the loans in monthly installments. The amount of each monthly payment is based on the length of the loan, the amount borrowed and the interest rate. Create an application that allows the customer to enter the price of a car, the down-payment amount and the annual interest rate of the loan. The application should display the loan’s duration in months and the monthly payments for two-, three-, four- and five-year loans. The variety of options allows the user to easily compare repayment plans and choose the most appropriate.
car payment calculator application1
CarPayment Calculator Application

Results displayed in tabular format

ListBox control

designing the car payment calculator application
Designing the CarPaymentCalculator Application

When the user clicks the Calculate Button

Initialize loan length to two years

Clear the ListBox of any previous calculation results

Add a header to the ListBox

Get down payment from a TextBox

Get sticker price from a TextBox

Get annual interest rate from a TextBox

Calculate loan amount (sticker price – down payment)

Calculate monthly interest rate (annual interest rate / 12)

designing the car payment calculator application1
Designing the CarPaymentCalculator Application

Do while loan length is less than or equal to five years

Convert the loan length in years to number of months

Calculate monthly payment based on loan amount, monthly interest rate and loan length in months

Insert result into ListBox

Increment loan length in years by one year

clearing and changing a listbox s contents
Clearing and Changing a ListBox’s Contents
  • The ListBox is initially cleared and then displays the number of monthly payments and the amount per payment.
  • To clarify what information is being displayed, a line of text—called aheader—is added to the ListBox by the Method Add
concatenate strings
Concatenate Strings
  • The ampersand symbol (&) is called the string-concatenation operator. This operator combines its two operands into one string value.
  • The constant ControlChars.Tab inserts a tab character into the string.
calculating the monthly payment amounts with a do while loop repetition statement
Calculating the Monthly Payment Amounts witha DoWhile...Loop Repetition Statement
  • This loop is an example ofcounter-controlledrepetition.
    • This uses a counter (years) to control the number of times that a set of statements executes.
    • Counter-controlled repetition also is called definite repetition, because the number of repetitions is known before the repetition statement begins.
accessing items in a list box
Accessing Items in a List Box

Figure 6-27: How to access an item in a list box

the selecteditem and selectedindex properties
The SelectedItem and SelectedIndex Properties
  • SelectedItem property:
    • Contains the value of the selected item in the list
    • If nothing is selected, it contains the empty string
  • SelectedIndex property:
    • Contains the index of the selected item in the list
    • If nothing is selected, it contains the value -1
  • Default list box item: the item that is selected by default when the interface first appears
slide25

The SelectedItem and SelectedIndex Properties

Figure 6-29: Item selected in the animalListBox

slide27

The SelectedItem and SelectedIndex Properties

Figure 6-31: How to select the default list box item

class average application
ClassAverage Application
  • A teacher regularly gives quizzes to a class of 10 students. The grades on these quizzes are integers in the range from 0 to 100 (0 and 100 are both valid grades). The teacher would like you to develop an application that computes the class average forone quiz.
class average application1
ClassAverage Application
  • Enter nine other grades between 0 and 100.
  • Note that the Add GradeButton is disabled once you have entered 10 grades (Fig. 10.3).

Disabled Add GradeButton

Ten quiz grades entered

Figure 10.3|Class Average application after 10 grades have been input.

class average application2
ClassAverage Application
  • Click the AverageButton to calculate theaverage of the 10 quizzes (Fig. 10.4).

Label displaying average

Click to calculate class average

Figure 10.4|Displaying the class average.

designing the class average application
Designing the Class Average Application

When the user clicks the Add Grade Button

If an average has already been calculated for a set of grades

Clear the output Label and the ListBox

Retrieve grade entered by user in the Enter grade:TextBox

Display the grade in the ListBox

Clear the Enter grade: TextBox

Transfer focus to the Enter grade: TextBox

If the user has entered 10 grades

Disable the Add Grade Button

Transfer focus to the Average Button

designing the class average application1
Designing the Class Average Application

When the user clicks the Average Button

Set total to zero

Set grade counter to zero

Do

Read the next grade in the ListBox

Add the grade to the total

Add one to the grade counter

Loop While the grade counter is less than 10

Calculate the class average by dividing the total by 10

Display the class average

Enable the Add Grade Button

Transfer focus to the Enter grade: TextBox

checking if average has been calculated
Checking if Average Has Been Calculated
  • This code tests whether averageResultLabel displays any text by comparing the Text property’s value to the empty string.

Figure 10.10|Clearing the output Label and ListBox after a calculation.

adding entered grades to listbox
Adding Entered Grades to ListBox
  • Line 13 (Fig. 10.11) Adds the grade entered in gradeTextBox to gradesListBox’s Items property. The grade is displayed in the ListBox.
  • Method Clear deletes the grade from the TextBox so that the next grade can be entered.

Figure 10.11|Adding the grade input to the ListBox andclearing the Entergrade:TextBox.

transferring the focus to a control
Transferring the Focus to a Control
  • Calling gradeTextBox’s Focus method places the cursor in the TextBox for the next grade input (Fig. 10.12).
  • This process is called transferring the focus.

Figure 10.12|Transferring the focus to the TextBox control.

disabling a button
Disabling a Button
  • Your application should accept exactly 10 grades.
    • Items’s Count property returns the number of items displayed in the Grade list:ListBox.
    • If 10 grades have been entered, addButton’s Enabled property is set to False (Fig. 10.13).

Figure 10.13|Application accepts only 10 grades.

calculating the class average
Calculating the Class Average
  • Use the Integertotal (Figure 10.14 ) to calculate the sum of the 10 grades.
  • The result of the averaging calculation can be a floating-point value; therefore, you declare a Double variable to store the class average.

Figure 10.14|Initialization phase of class-average calculation.

calculating the class average cont
Calculating the Class Average (Cont.)
  • The Do...LoopUntil statement should iterate until the value of gradeCounter is greater than or equal to 10.
  • The items in aListBoxare accessed by their position number, starting from position number0.

Figure 10.15|Do...LoopUntil summing grades.

calculating the class average cont1
Calculating the Class Average (Cont.)
  • After the average is displayed, the application resets, and another list of grades can be entered.

Figure 10.16|Displaying the result of the average calculation.

adding items to a list box cont d
Adding Items to a List Box (cont'd.)

Figure 6-19: How to use the Items collection’s Add method

the inputbox function
The InputBox Function
  • InputBox function: displays a predefined dialog box that allows the user to enter data
    • Contains a text message, an OK button, a Cancel button, and an input area
  • InputBox function returns:
    • The user’s entry if the user clicks the OK button
    • An empty string if the user clicks the Cancel button or the Close button on the title bar
the inputbox function1
The InputBox Function
  • InputBox function arguments:
    • prompt: the message to display inside the dialog box
    • title: the text to display in the dialog box’s title bar
    • defaultResponse: a prefilled value for the user input
the inputbox function2
The InputBox Function

Figure 6-16: How to use the InputBox function (cont'd.)

the color viewer application
The Color Viewer Application
  • Enabled property: used to enable or disable a control
    • When False, the control appears dimmed (grayed out), indicating it is not available for use
  • Refresh method: ensures that the computer processes any previous lines of code that affect the interface’s appearance
  • Sleep method: delays program execution
    • Argument is specified in milliseconds
the color viewer application1
The Color Viewer Application

Figure 6-38: MainForm in the Color Viewer application

the color viewer application2
The Color Viewer Application

Figure 6-39: View Colors button’s Click event procedure