CS 4 Intro to Programming using Visual Basic

Do Loops

Patchrawat UthaisombutUniversity of Pittsburgh

1based on lecture notes by D. Schneider

Chapter 6 - VB 2005 by Schneider 2

Chapter 6 – Repetition

• 6.1 Do Loops

• 6.2 Processing Lists of Data with Do

Loops

• 6.3 For...Next Loops

• 6.4 A Case Study: Analyze a Loan

How to write the following program?

• The user inputs a number x.

• The program displays the multiplication table for x.

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User Interface Design

• Use a Text Box to accept a number

• Use a Button to accept the “print table” command from the user

• Use a List Box to show the multiplication table.

• Put a Label near the Text Box to give a brief instruction to the user

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User Interface Design

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Expected Results

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Technical issues

• When does the program respond to the user?

• How do we convert text input from the Text Box into an integer?

• How do we add a line into a List Box?

• How do we construct the right string on each line of the List Box? (How do we construct the string “5 * 1 = 5”, etc. ?)

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Technical Issues

• How do we add exactly 12 lines into the List Box?

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One solution

• Put 12 lstTable.Items.Add(…) in the code

• But there is a better solution …

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6.1 Do Loops

• A loop is one of the most important structures in programming.

• Used to repeat a sequence of statements a number of times.

• The Do loop repeats a sequence of statements either as long as or until a certain condition is true.

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Do Loop Syntax

Do While condition

statement(s)

Loop

Condition is tested,If it is True,

the loop is run.If it is False,

the statements following the

Loop statementare executed.

These statements are inside the body of the loop and are run if the condition

above is True.

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Pseudocode and Flow Chart for a Do Loop

Simple Illustration of Do Loop

• When the user click the button, display 1, 2,…, 7 on the List Box.

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Example 1Private Sub btnDisplay_Click(...) _

Handles btnDisplay.Click

'Display the numbers from 1 to 7

Dim num As Integer = 1

Do While num <= 7

lstNumbers.Items.Add(num)

num += 1 'Add 1 to the value of num

Loop

End Sub

Back to our multiplication tablePublic Class Form1

Private Sub btnDisplay_Click(…) Handles btnDisplay.Click

Dim x As Integer

Dim y As Integer

x = CInt(txtInput.Text)

y = 1

Do While y <= 12

lstTable.Items.Add(x & " * " & y & " = " & x * y)

y += 1

Loop

End Sub

End Class

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Example: Repeat Request as Long as Response in Incorrect

Dim passWord As String = ""

Do While passWord <> "SHAZAM"

passWord = InputBox("What is the password?")

passWord = passWord.ToUpper

Loop

passWord is the loop control variable because the value stored in passWord is what is tested to determine if the loop should continue or stop.

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Post Test Loop

Do

statement(s)

Loop Until condition

Loop is executed once and then the conditionis tested. If it is False, the loop is run again.

If it is True, the statements following the Loop statement are executed.

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Example: Repeat Request Until Proper Response is Given

Do

passWord = InputBox("What is the password?")

passWord = passWord.ToUpper

Loop Until passWord = "SHAZAM"

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Pseudocode and Flowchart for a Post-Test Loop

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Example 4: Form

txtAmount

txtWhen

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Example 4: CodePrivate Sub btnCalculate_Click(...) Handles

btnCalculate.Click

Dim balance As Double, numYears As Integer

balance = CDbl(txtAmount.Text)

Do While balance < 1000000

balance += 0.06 * balance

numYears += 1

Loop

txtWhen.Text = "In " & numYears & _

" years you will have a million dollars."

End Sub

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Example 4: Output

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Comments

• Be careful to avoid infinite loops – loops that never end.

• Visual Basic allows for the use of either the While keyword or the Until keyword at the top or the bottom of a loop.

• This textbook will use only While at the top and only Until at the bottom.