Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Six The Repetition Structure.

Programming with Microsoft Visual Basic 2008

Fourth Edition

Chapter Six

The Repetition Structure

Previewing the Shoppers Haven Application

• Open the Shoppers.exe file

• The Shoppers Haven application:– Allows user to enter an item’s original price and its

discount rate– Calculates and displays amount of discount and

discounted price

2Programming with Microsoft Visual Basic 2008, Fourth Edition

Previewing the Shoppers Haven Application (continued)

Figure 6-1: Discount and discounted price shown in the interface


Lesson A Objectives

After studying Lesson A, you should be able to:

• Code the repetition structure using the Do...Loop statement

• Include the repetition structure in pseudocode

• Include the repetition structure in a flowchart

• Initialize and update counters and accumulators

• Code the repetition structure using the For...Next statement

Programming with Microsoft Visual Basic 2008, Fourth Edition 4

The Repetition Structure

• Repetition structure (or loop): Repeatedly processes instructions until condition is met– Example: Calculate net pay for each employee

• Pretest loop: Evaluates condition prior to processing instructions

• Posttest loop: Evaluates condition after processing instructions


The Do…Loop Statement

• Do…Loop statement: Used to code both pretest loop and posttest loop

• Two variations of syntax: – One for pretest loop and one for posttest loop– While keyword: Indicates that instructions should

be processed while condition is true– Until keyword: Indicates that instructions should

be processed until condition becomes true


The Do…Loop Statement (continued)

• Do…Loop statement:– Begins with Do clause, ends with Loop clause– Instructions to be repeated are placed between Do

and Loop clauses– Use While or Until keyword before condition – Condition must evaluate to Boolean True or False

• Location of {While|Until} condition:– Pretest loop: Appears in Do clause– Posttest loop: Appears in Loop clause

• Diamond: Represents loop condition in flowchart


Figure 6-2: Syntax versions and examples of the Do...Loop statement



Figure 6-3: Processing steps for the pretest loop example from Figure 6-2




Figure 6-4: Processing steps for the posttest loop example from Figure 6-2


Figure 6-5: Pseudocode and flowchart for the pretest loop example shown in Figure 6-2




Figure 6-6: Pseudocode and flowchart for the posttest loop example shown in Figure 6-2


Counters and Accumulators

• Used to calculate subtotals, totals, and averages

• Counter: Numeric variable used for counting

• Accumulator: Variable used to tally various amounts

• Initializing: Assigning initial value to counter or accumulator

• Updating (incrementing or decrementing):– Changing value stored in counter or accumulator – Update statement must be within repetition

structure


The Sales Express Application

• Objective: Display average annual sales of company

• Application uses pretest loop

• Priming read: Use to prime (prepare or set up) loop by performing first action prior to entering loop


The Sales Express Application (continued)


Figure 6-7: Pseudocode for the bntCalc control’s Click event procedure

The For…Next Statement

• For…Next statement:– Processes instructions specific number of times– Condition tested before processing (pretest loop) – Also called counter controlled loop

• Must specify start value, end value, and step value– Start value and end value provide looping range– Step value increments or decrements counter

• Hexagon: Flowchart symbol


Figure 6-11: Pseudocode and flowchart for Example 1 in Figure 6-9


The For…Next Statement (continued)

The Monthly Payment Calculator Application

• Task of btnCalc control’s Click event procedure:– Calculate and display monthly car payments– Use term of five years and rates from 5 – 10%

• Basic structure of For…Next statement– Use procedure level variable, dblRate, as

counter– Set starting and ending values to 0.05 and 0.1– Set step value to 0.01– Calculate monthly payment for current rate– Display interest rate and corresponding payment


The Monthly Payment Calculator Application (continued)

Figure 6-11: Additional code entered in the btnCalc control’s Click event procedure


The Monthly Payment Calculator Application (continued)


Figure 6-12: Monthly payments shown in the interface

Lesson A Summary

• Repetition structure (loop): Repeats set of instructions until some condition is met

• Use Do...Loop statement to code pretest and posttest loops

• Counters are used to track count of something

• Accumulators are used to tally up an amount

• Counters and accumulators must be:– Initialized– Updated within loop


Lesson A Summary (continued)


• Use For...Next statement to code pretest loops

• Use hexagon to show a For…Next loop in flowchart

Lesson B Objectives

After studying Lesson B, you should be able to:

• Nest repetition structures

• Refresh the screen

• Delay program execution


Nested Repetition Structures

• Nested repetition structure: – Inner loop placed entirely within outer loop– Inner loop is referred to as nested loop

• Clocks use nested loops to keep track of time

• Minute and hour hands of clock can be compared to loops:– Outer loop corresponds to hour hand– Inner loop corresponds to minute hand


Nested Repetition Structures (continued)

Figure 6-16: Logic used by a clock’s hour and minute hands


The Refresh and Sleep Methods

• Refresh method: Ensures that any code appearing before it that affects interface’s appearance is processed

• Syntax: Me.Refresh()– Me refers to current form

• Sleep method: Delays program execution

• Syntax:System.Threading.Thread.Sleep(milliseconds)

– Millisecond: 1/1000 of second


The Refresh and Sleep Methods (continued)


Figure 6-18: Refresh and Sleep methods added to the procedure

Monthly Payment Calculator Application—Nested For...Next

Statements

• Objective: Calculate and display car payments

• Use nested loops in btnCalc control’s Click event

• Outer For…Next statement:– Control interest rates ranging from 5 - 10% – Increment rates at each iteration by 1%

• Inner For…Next statement:– Controls terms from 3 - 5 years


Monthly Payment Calculator Application—Nested For...Next Statements

(continued)

Figure 6-19: Outer and inner (nested) loops entered in the procedure


Monthly Payment Calculator Application—Nested For...Next Statements

(continued)

Figure 6-20: Monthly payments shown in the interface


Lesson B Summary

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

• To refresh interface, use Refresh method

• To pause program execution, use Sleep method


Lesson C Objectives

After studying Lesson C, you should be able to:

• Include a list box in an interface

• Select a list box item from code

• Determine the selected item in a list box


Coding the Shoppers Haven Application

• Objective: Allow entry of price and discount rate

• Application requirements:– Discount rates range from 10% through 30% – Discount rate should be incremented by 5%– Calculate discount amount and discounted price– Display discount amount and discounted price


Coding the Shoppers Haven Application (continued)

Figure 6-23: TOE chart for the Shoppers Haven application


Coding the Shoppers Haven Application (continued)

Figure 6-24: Partially completed user interface for the Shoppers Haven application


Including a List Box in an Interface

• List box: Displays list of choices from which 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 list box to interface

• List box can be made any size you want


Adding Items to a List Box

• Collection: Group of objects treated as one unit

• Items collection: Refers to group of items in list box

• Index: Unique number that identifies each item in collection – Is zero-relative (starts with 0)

• Items collection’s Add method: Used to add item to list box– Implemented in form’s Load event procedure to

prepare list box before form is displayed


Adding Items to a List Box (continued)

Figure 6-26: Syntax and examples of the Add method



Figure 6-27: Result of processing the code shown in Figure 6-26




• Sorted property of list box: Determines order in which items are displayed in list box– If true, newly added item is placed in its proper

position– If false, newly added item is placed at end of list– Uses dictionary sort order

The SelectedItem And SelectedIndex Properties

• SelectedItem property: Contains value of item selected in list box– Contains empty string when no item is selected

• SelectedIndex property: Contains index of item selected in list box– Contains -1 when no item is selected

• Default list box item: Appears when application is first loaded– Can be chosen by setting either SelectedItem or

SelectedIndex property


The SelectedItem And SelectedIndex Properties (continued)

Figure 6-30: Examples of the SelectedItem and SelectedIndex properties


The SelectedItem And SelectedIndex Properties (continued)

Figure 6-31: Default item selected in the list box


The SelectedValueChanged and SelectedIndexChanged Events


• SelectedValueChanged and SelectedIndexChanged events: Occur when user or code statement selects item in list box– Can use these events to process instructions

when selection is made

Coding the btnCalc Control’s Click Event Procedure


Figure 6-32: Pseudocode for the btnCalc control’s Click event procedure

Coding the btnCalc Control’s Click Event Procedure (continued)

Figure 6-33: Discount and discounted price shown in the interface


Lesson C Summary

• A list box displays list of items

• Use SelectionMode property to set number of items that can be selected in list box

• Use Items.Add method to add items to list box

• Use Sorted property to sort list box items in dictionary order

• Use SelectedItem or SelectedIndex property to determine which item was selected


Lesson C Summary (continued)


• Use SelectedValueChanged or SelectedIndexChanged events to perform tasks when an item in list box is selected

Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Six The Repetition Structure.

Documents

Transcript of Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Six The Repetition Structure.