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Chapter 6 – Repetition

Chapter 6 – Repetition. 6.1 Do While and Do Until Loops 6.2 Processing Lists of Data with Do Loops 6.3 For...Next Loops. 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.

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Chapter 6 – Repetition

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  1. Chapter 6 – Repetition • 6.1 Do While and Do Until Loops • 6.2 Processing Lists of Data with Do Loops • 6.3 For...Next Loops Chapter 6 - VB 2005 by Schneider

  2. 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

  3. Do Loop Syntax Condition is tested, If it is True, the loop is run. If it is False, the statements following the Loop statement are executed. Do Whilecondition statement(s) Loop These statements are inside the body of the loop and are run if the condition above is True. Chapter 6 - VB 2005 by Schneider

  4. Pseudocode and Flow Chart for a Do Loop Chapter 6 - VB 2005 by Schneider

  5. 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 = num + 1 'Add 1 to the value of num Loop End Sub Chapter 6 - VB 2005 by Schneider

  6. Post Test Loop Do statement(s) Loop Untilcondition 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

  7. Example 2 Private Sub btnDisplay_Click(...) _ Handles btnDisplay.Click 'Display the numbers from 1 to 7 Dim num As Integer = 1 Do lstNumbers.Items.Add(num) num = num + 1 'Add 1 to the value of num Loop Until num > 7 End Sub Chapter 6 - VB 2005 by Schneider

  8. Pseudocode and Flowchart for a Post-Test Loop Chapter 6 - VB 2005 by Schneider

  9. 6.3 For…Next Loops • Declarations of Control Variables • Nested For … Next Loops Chapter 6 - VB 2005 by Schneider

  10. For…Next Loops • Used when we know how many times we want the loop to execute • A counter controlled loop Chapter 6 - VB 2005 by Schneider

  11. Sample Dim i As Integer For i = 1 To 5 lstTable.Items.Add(i & " " & i ^ 2) Next The loop control variable, i, is • Initialized to 1 • Tested against the stop value, 5 • Incremented by 1 at the Next statement Chapter 6 - VB 2005 by Schneider

  12. Do While equivalent i = 1 Do While i <= 5 lstTable.Items.Add(i & " " & i ^ 2) i += 1 Loop Chapter 6 - VB 2005 by Schneider

  13. For…Next Loop Syntax Chapter 6 - VB 2005 by Schneider

  14. Example 2 For index = 0 To n Step s lstValues.Items.Add(index) Next Start value Control variable Stop value Amount to add to index Chapter 6 - VB 2005 by Schneider

  15. Example with Negative Step Dim j As Integer For j = 10 To 1 Step -1 lstBox.Items.Add(j) Next lstBox.Items.Add("Blastoff") Chapter 6 - VB 2005 by Schneider

  16. Declaration Inside For Statement Dim i As Integer For i = 1 To 5 lstTable.Items.Add(i & " " & i ^ 2) Next For i As Integer = 1 To 5 lstTable.Items.Add(i & " " & i ^ 2) Next Chapter 6 - VB 2005 by Schneider

  17. Nested For…Next Loops Chapter 6 - VB 2005 by Schneider

  18. Example: Seats on an Airplane For i As Integer = 65 To 70 For j As Integer = 1 To 25 lstBox.Items.Add(Chr(i) & j) Next Next OUTPUT: A1 A2 A3 : Outer loop Inner loop Chapter 6 - VB 2005 by Schneider

  19. For and Next Pairs • For and Next statements must be paired. • If one is missing, the automatic syntax checker will complain with a wavy underline and a message such as “A ‘For’ must be paired with a ‘Next’.” Chapter 6 - VB 2005 by Schneider

  20. Start, Stop, and Step values • Consider a loop beginning with Fori As Integer = mTon Step s. • The loop will be executed exactly once if m equals n no matter what value s has. • The loop will not be executed at all if m is greater than n and s is positive, or if m is less than n and s is negative. Chapter 6 - VB 2005 by Schneider

  21. Altering the Control Variable • The value of the control variable should not be altered within the body of the loop. • Doing so might cause the loop to repeat indefinitely or have an unpredictable number of repetitions. Chapter 6 - VB 2005 by Schneider

  22. Non-integer Step Values • Can lead to round-off errors with the result that the loop is not executed the intended number of times. Chapter 6 - VB 2005 by Schneider

  23. Non-integer Step Example • A loop beginning with For i As Double = 1 To 2 Step 0.1 will be executed only 10 times instead of the intended 11 times. It should be replaced with For i As Double = _ 1 To 2.01 Step 0.1 Chapter 6 - VB 2005 by Schneider

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