Transcript 6.1 Do Loops 6.2 Processing Lists of Data with Do Loops

CS 4 Intro to Programming
using Visual Basic
Do Loops
Patchrawat Uthaisombut
University of Pittsburgh
based on lecture notes by D. Schneider
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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
Chapter 6 - VB 2005 by Schneider
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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.
Chapter 6 - VB 2005 by Schneider
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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 statement
are executed.
These statements are inside
the body of the loop and
are run if the condition
above is True.
Chapter 6 - VB 2005 by Schneider
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Pseudocode and Flow Chart
for a Do Loop
Chapter 6 - VB 2005 by Schneider
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Simple Illustration of Do Loop
• When the user click the button, display 1,
2,…, 7 on the List Box.
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Example 1
Private 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
Chapter 6 - VB 2005 by Schneider
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Back to our multiplication table
Public 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.
Chapter 6 - VB 2005 by Schneider
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Post Test Loop
Do
statement(s)
Loop Until condition
Loop is executed once and then the condition
is tested. If it is False, the loop is run again.
If it is True, the statements following the
Loop statement are executed.
Chapter 6 - VB 2005 by Schneider
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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
Chapter 6 - VB 2005 by Schneider
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Example 4: Form
txtAmount
txtWhen
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Example 4: Code
Private 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
Chapter 6 - VB 2005 by Schneider
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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.
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