Chapter 6: The Repetition Structure

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Chapter 6 The Repetition Structure
Programming with Microsoft Visual Basic 2005,
Third Edition
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The Repetition Structure (Looping)Lesson A
Objectives

• Code the repetition structure using the
  For...Next and Do...Loop statements
• Include the repetition structure in pseudocode
• Include the repetition structure in a flowchart
• Initialize and update counters and accumulators

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Previewing the Completed Application

• Go to Run command on Windows Start menu
• Browse to the VB2005\Chap06 folder
• Open the Shoppers.exe file
• The Shoppers Haven user interface appears

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Previewing the Completed Application (continued)
Figure 6-1 Interface showing the discount and
discounted price amounts
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The Repetition Structure

• Repetition structure (loop)
• Repeatedly processes instructions until condition
  met
• Example calculate net pay for each employee
• Pretest loop
• Condition evaluated prior to instruction
  processing
• Posttest loop
• Condition evaluated after instruction processing

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The ForNext Statement

• ForNext statement
• Processes instructions a specific number of times
• Condition tested before processing (pretest loop)
  
• Also called a counter controlled loop
• Syntax and examples to follow

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The ForNext Statement (continued)
Figure 6-2 Syntax and examples of the For...Next
statement (continued)
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The ForNext Statement (continued)
Figure 6-2 Syntax and examples of the For...Next
statement
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The ForNext Statement (continued)

• Syntactic Elements of ForNext statement
• Begins with the For clause, ends with Next clause
• counter numeric variable tracking iterations
• startvalue and endvalue provide looping range
• stepvalue increments or decrements counter
• statements processed in the body of the loop
• Hexagon flowchart symbol representing ForNext

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The ForNext Statement (continued)
Figure 6-5 Pseudocode for the first example
shown in Figure 6-2
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The ForNext Statement (continued)
Figure 6-6 Flowchart for the first example shown
in Figure 6-2
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The Monthly Payment Calculator Application

• Task of xCalcButtons Click event procedure
• Calculate and display monthly car payments
• Use term of five years and rates from 5 10
• Basic structure of the ForNext statement
• Use a procedure level variable, rate, as a
  counter
• Set starting and ending values to 0.05 and 0.01
• Set the step value to 0.1
• Calculate monthly payment for current rate
• Display interest rate and corresponding payment

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The Monthly Payment Calculator Application
(continued)
Figure 6-8 Sample run of the application that
contains the procedure
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The DoLoop Statement

• DoLoop statement
• Codes both a pretest loop and a posttest loop
• Syntax and examples to follow
• Observe two variations of the syntax
• Variations correspond to pretest and posttest
  loop

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The DoLoop Statement (continued)
Figure 6-9 Syntax and examples of the Do...Loop
statement (continued)
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The DoLoop Statement (continued)
Figure 6-9 Syntax and examples of the Do...Loop
statement
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The DoLoop Statement (continued)

• Syntactic elements for the DoLoop statement
• Begins with Do clause, ends with the Loop clause
• Enter instructions to repeat between both clauses
• Use either While or Until keyword before
  condition
• Condition must evaluate to Boolean True or False
• Location of WhileUntil condition by syntax
  version
• Pretest loop appears in the Do clause
• Posttest loop appears in the Loop clause
• Diamond represents loop condition in a flowchart

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The DoLoop Statement (continued)
Figure 6-13 Pseudocode and flowchart for the
posttest loop example shown in Figure 6-9
(continued)
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The DoLoop Statement (continued)
Figure 6-13 Pseudocode and flowchart for the
posttest loop example shown in Figure 6-9
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Using Counters and Accumulators

• Used to calculate subtotals, totals, and averages
• Counter numeric variable used for counting
• Accumulator variable used to tally various
  amounts
• Initialize set initial value of counter or
  accumulator
• Updating (incrementing or decrementing)
• Changing value stored in counter or accumulator
• Update statement is used within a repetition
  structure

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The Sales Express Application

• Objective display average sales of company
• Structure of xCalcButton Click event procedure
• Use a pretest loop to retrieve each sales amount
• Update accumulator within loop to tally gross
  sales
• Update counter within loop to keep track of
  entries
• Exit loop after data entry has been completed
• If counter is gt 0, calculate average sales amount
• Display the average sales amount

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The Sales Express Application (continued)
Figure 6-15 Pseudocode for the xCalcButtons
Click event procedure
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Summary Lesson A

• Repetition structure (loop) repeats a set of
  instructions until some condition is met
• Use a For...Next statement to code pretest loops
• Use a Do...Loop statement to code pretest and
  posttest loops
• Counters and accumulators must be initialized
• Counters and accumulators are updated in a loop

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Nested Repetition StructuresLesson B Objectives

• Nest repetition structures

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Nesting Repetition Structures

• Nested repetition structure
• Inner loop placed entirely within outer loop
• Placement of inner loop is known as nesting
• Clocks use nested loops to keep track of the time
• Analogizing minute and second hands to loops
• Outer loop corresponds to the minute hands
• Inner loop corresponds to the second hand

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Nesting Repetition Structures (continued)
Figure 6-22 Nested loops used by a clock
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Monthly Payment Calculator ApplicationNested
For...Next Statements

• Objective calculate and display car payments
• Use nested loops in xCalcButtons Click event
• Role of the outer ForNext statement
• Control interest rates ranging from 5 - 10
• Increment rates at each iteration by 1
• Role of the inner ForNext statement
• Controls terms from 3 - 5 years

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Monthly Payment Calculator ApplicationNested
For...Next Statements (continued)
Figure 6-24 Monthly payments shown in the
interface
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Summary Lesson B

• To nest a repetition structure, place the entire
  inner loop within the outer loop

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Coding the Shoppers Haven ApplicationLesson C
Objectives

• Include a list box in an interface
• Select a list box item from code
• Determine the selected item in a list box

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Shoppers Haven

• Objective allow entry of price and discount rate
• Application requirements
• Discount rate range 10 through 30
• Discount rate should be incremented by 5
• Calculate discount amount and discounted price
• Display discount amount and discounted price

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Shoppers Haven (continued)
Figure 6-25 TOE chart for the Shoppers Haven
application
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Shoppers Haven (continued)
Figure 6-26 Partially completed user interface
for the Shoppers Haven application
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Including a List Box in an Interface

• List box
• Displays a list of choices
• User can select 0 or more items
• SelectionMode property
• Controls number of choices that can be selected
• Values None, One, MultiSimple, or MultiExtended
• ListBox tool used to add a list box to an
  interface
• List box can be made any size you want

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Adding Items to a List Box

• Collection group of objects treated as one unit
• Items collection
• Refers to group of items in a list box
• An index identifies each item in a collection
• Items collections Add method
• Specifies items you want displayed in a list box
• Implemented in forms Load event procedure
• Sorted property of a list box
• Displays list box items in dictionary order when
  true

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Adding Items to a List Box (continued)
Figure 6-28 Syntax and examples of the Add method
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Adding Items to a List Box (continued)
Figure 6-29 Items added to the list boxes
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The SelectedItem And SelectedIndex Properties

• SelectedItem property
• Contains value of item selected in a list box
• Value when no item is selected is the empty
  string
• SelectedIndex property
• Contains index of item selected in a list box
• Value when no item is selected is the number -1
• Default list box item
• Appears when the application is first loaded
• Can be chosen with SelectedItem and SelectedIndex

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The SelectedItem And SelectedIndex Properties
(continued)
Figure 6-34 Completed MainForms Load event
procedure
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The SelectedItem And SelectedIndex Properties
(continued)
Figure 6-35 First item selected in the
xRateListBox
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Coding the Text Boxs TextChanged Event Procedure

• Controls TextChanged event
• Occurs when contents of Text property change
• Additional requirement
• Clear controls when TextChanged event occurs in
  xOrigPriceTextBox

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Coding the Text Boxs TextChanged Event Procedure
(continued)
Figure 6-36 Completed ClearLabels procedure
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Coding the xCalcButtons Click Event Procedure

• xCalcButtons Click event procedure
• Last procedure to code in application

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Coding the xCalcButtons Click Event Procedure
(continued)
Figure 6-37 Pseudocode for the xCalcButtons
Click event procedure
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Coding the xCalcButtons Click Event Procedure
(continued)
Figure 6-39 Discount and discounted price
amounts shown in the interface
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Summary Lesson C

• A list box displays a list of items
• Use list boxs SelectionMode property to set
  number of items to select in a list box
• List box items can be sorted in dictionary order
• Use Item collections Add method to specify items
  that will display in the list box
• Text boxs TextChanged event occurs when a change
  is made to the controls contents