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Chapter 6 – Repetition

Chapter 6 – Repetition. 6.1 Do Loops 6.2 For...Next Loops 6.3 List Boxes and Loops. 6.2 For…Next Loops. General Form of a For…Next Loop Nested For…Next Loops Local Type Inference. For…Next Loops. Used when we know how many times we want the loop to execute A counter controlled loop.

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Chapter 6 – Repetition

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  1. Chapter 6 – Repetition 6.1 Do Loops 6.2 For...Next Loops 6.3 List Boxes and Loops

  2. 6.2 For…Next Loops • General Form of a For…Next Loop • Nested For…Next Loops • Local Type Inference

  3. For…Next Loops • Used when we know how many times we want the loop to execute • A counter controlled loop

  4. For…Next Loop Syntax

  5. Simple For … Next Loop For i As Integer = 1 To 5 lstTable.Items.Add(i & " " & i ^ 2) Next The loop counter variable, i, is • initialized to 1 • tested against the stop value, 5 • 6 when the loop terminates … then dies! • incremented by 1 at the Next statement

  6. Comparing a While and For Loop Dim i As Integer = 1 Do While i <= 5 lstTable.Items.Add(i & " " & i ^ 2) i += 1 Loop COMPARED TO: For i As Integer = 1 To 5 lstTable.Items.Add(i & " " & i ^ 2) Next

  7. Population: Output Display a table showing the population of a city of 300,000 people that is expected to grow by 3% each year from 2010 to 2014.

  8. Population: Code Dim pop As Double = 300000 For yr As Integer = 2010 To 2014 lstTable.Items.Add(yr & " " & FormatNumber(pop, 0)) pop += 0.03 * pop Next yr could go from 1 to 5, however it increases the readability of the problem by using 2010 to 2014. After each pass, how much does the counter variable yr increment by?

  9. Step Clause • Normally after each pass the value of the counter variable increases by 1 • If the clause Step s is appended to the For statement, the value of s will be added to the counter variable after each pass. For i As Integer = 2 To 10 Step 2 lstTable.Items.Add(i & "" & i^2) Next Adds 2 to i at the end of each pass.

  10. Example with a Negative Step Value For j As Integer = 10 To 1 Step -1 lstBox.Items.Add(j) Next lstBox.Items.Add("Blastoff") To execute the statements within the loop, the initial value will be greater than the final value If the step value is a negative number, the value of the counter variable will decrease after each pass.

  11. Example: Nested For…Next Loops For i As Integer = 65 To 70 For j As Integer = 1 To 3 lstBox.Items.Add(Chr(i) & j) Next Next Output: A1 A2 A3 B1 B2 B3 : Outer loop Inner loop Remeber Chr of 65 is “A”

  12. 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’.”

  13. Start, Stop, and Step values • Consider a loop beginning with For i As Integer = m To n 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.

  14. Altering the Counter Variable • The value of the counter 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.

  15. Non-Integer Step Values • Can lead to round-off errors with the result that the loop is not executed the intended number of times. • We will most often use Integers for all values in the For header. For j As Double= 1.0 To 5.0 Step .1 lstBox.Items.Add(j) Next

  16. Local Type Inference • Often there is no need to have a variable exist after the for loop is complete. For j As Integer = 1 To 5 lstBox.Items.Add(j) Next lstBox.Items.Add(j) j is out of scope after the for loop

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