CS 4 Intro to Programming using Visual Basic Do Loops Patchrawat Uthaisombut University of Pittsburgh 1 based on lecture notes by D. Schneider

Chapter 6 - VB 2005 by Schneider2 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. P. Uthaisombut3

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 P. Uthaisombut4

User Interface Design P. Uthaisombut5

Expected Results P. Uthaisombut6

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. ?) P. Uthaisombut7

Technical Issues How do we add exactly 12 lines into the List Box? P. Uthaisombut8

One solution Put 12 lstTable.Items.Add(…) in the code But there is a better solution … P. Uthaisombut9

Chapter 6 - VB 2005 by Schneider 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 Schneider11 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 Schneider12 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. P. Uthaisombut13

Chapter 6 - VB 2005 by Schneider14 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

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 P. Uthaisombut15

Chapter 6 - VB 2005 by Schneider16 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 Schneider17 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 Schneider18 Example: Repeat Request Until Proper Response is Given Do passWord = InputBox("What is the password?") passWord = passWord.ToUpper Loop Until passWord = "SHAZAM"

Chapter 6 - VB 2005 by Schneider19 Pseudocode and Flowchart for a Post-Test Loop

Chapter 6 - VB 2005 by Schneider20 Example 4: Form txtAmount txtWhen

Chapter 6 - VB 2005 by Schneider21 Example 4: Code Private Sub btnCalculate_Click(...) Handles btnCalculate.Click Dim balance As Double, numYears As Integer balance = CDbl(txtAmount.Text) Do While balance < balance += 0.06 * balance numYears += 1 Loop txtWhen.Text = "In " & numYears & _ " years you will have a million dollars." End Sub

Chapter 6 - VB 2005 by Schneider22 Example 4: Output

Chapter 6 - VB 2005 by Schneider23 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.