Chapter 5: Control Structures: Iteration Visual Basic.NET Programming: From Problem Analysis to...

Chapter 5: Control Structures: Iteration

Visual Basic .NET Programming:

From Problem Analysis to Program Design

Visual Basic .NET Programming: From Problem Analysis to Program Design 2

Objectives

• Explore the iteration structure

• Implement iteration using the Do While and Do Until statements

• Implement iteration using the For Next statement

• Create nested structures


Exploring the Iteration Structure

• Iteration structure

– Execute one or more statements repeatedly

– Often called a loop

– Write a set of statements

• Repeatedly execute them until terminating condition is reached


Iteration Logic

• Basic iteration logic:

– First test to see whether loop should terminate

– If loop does not terminate

• Statement or statements in loop body executed

– Control returns to beginning



Iteration Logic (continued)

• Infinite loop

– Never ends

– Terminating condition never occurs

• Pre-test loop

– Terminating condition checked before body of loop executed

– Statements in loop may not be executed

• If terminating condition met


Iteration Logic (continued)

• Post-test loop

– Terminating condition checked after body of loop executed

– Statements in loop executed at least once

• Regardless of terminating condition



Controlling Iteration

• Counter control

– Employs Integer variable to count number of times loop has executed

– Variable called counter

– Update counter each time loop executes


Controlling Iteration (continued)

• Counter control (continued)

– When counter reaches predetermined value

• Loop terminates

– Can increment or decrement counter by any value that suits program logic



Controlling Iteration (continued)

• Sentinel-control logic

– Checks user input for specific value

– Terminates when value detected

– Value called sentinel

• Should be unique value



Implementing Iteration Using the Do While and Do Until

Statements• Iteration statements:

– Do

• Do While

• Do Until

– For


Writing Loops Using Do While

• Pre-test loop– Terminating condition is tested at beginning of

loop

• Syntax:Do While expression

Statements

Loop

• Loop continues to execute as long as expression is true


Example 5-3: Computing an exam average using a counter-

controlled Do While loop

1. ' define variables

2. Dim sum, average AsDouble

3. Dim numberOfExams As Integer = 5

4. Dim counter As Integer = 1

…



controlled Do While loop…

5. ' begin loop

6. Do While (counter <= numberOfExams)

7. Console.WriteLine(“Enter an Exam Score: “)

8. sum = sum + Convert.ToDouble(Console.ReadLine())

9. counter += 1 ' count the number of iterations

10. Loop

…



controlled Do While loop

…

11. ' compute & display the average

12. average = sum / numberOfExams

13. Console.WriteLine(“The average is: “ &

Math.Round(average, 1))

…



Writing Loops Using Do Until

• Executes until expression is true

– Contrast to Do While

• Syntax

Do Until expression

Statement(s)

Loop



Implementing Iteration Using the For Next Statement

• For Next loops provide only:

– Counter-controlled loop

– Pre-test loop

• Initializes counter variable

• Automatically increments counter

– Simplifies and shortens code




Example 5-9: Computing an Exam Average Using a For

Next Loop1. ' define variables

2. Dim sum, average As Double

3. Dim numberOfExams As Integer = 5

4. Dim counter As Integer

5. ' begin loop

6. For counter = 1 To 5 Step 1

7. Console.WriteLine("Enter an Exam Score: ")

…


Example 5-9: Computing an Exam Average Using a For

Next Loop (continued)…

8. sum = sum + Convert.ToDouble(Console.ReadLine())

9. Next

10. ' compute & display the average


12. Console.WriteLine("The average is: " & Math.Round(average, 1))


Creating Nested Structures

• Nested loop

– One iteration structure placed inside another

• Can place iteration inside selection

– And vice versa


Example 5-12: Computing an Exam Average Using a Nested

For Next Loop (excerpt)

For studentCounter = 1 To numberOfStudents

8. sum = 0 ' reinitialize sum

9. ' begin inner loop for exams

10. For examCounter = 1 To numberOfExams

11. Console.WriteLine (“Enter an Exam Score: “)

12. score = Convert.ToDouble (Console.ReadLine())


Example 5-12: Computing an Exam Average Using a Nested

For Next Loop (continued)

13. sum += score

14. Next ' end of inner loop


16. Console.WriteLine (“The average is: “ &

Math.Round(average, 1))

17. Next ' end of outer loop



Programming Example: Loan Amortization Computation

• Input

– Loan amount

– APR

– Loan duration expressed in number of months



(continued)

• Output

– Payment amount

– Monthly interest and principal paid

– Remaining loan balance

– Total interest paid



(continued)

• Program purpose:

– Compute and display amortization of a loan with monthly payments


Summary

• Use iteration structure to execute one or more statements repeatedly

– Also called a loop

• Loop logic types:

– Pre-test

– Post-test


Summary (continued)

• Control number of times a loop executes using:– Counter control

– Sentinel control

• Visual Basic .NET Loop structures:– Do While

– Do Until

– For

• Iteration and selection structures can be nested

Date post:	04-Jan-2016
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Chapter 5: Control Structures: Iteration Visual Basic.NET Programming: From Problem Analysis to...

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