Chapter FourThe Selection Structure

Programming with Microsoft Visual Basic 2010

5th Edition

Previewing the Monthly Payment Calculator Application

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The Monthly Payment Calculator application uses the selection structure

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Figure 4-2 Monthly payment amount

shown in the interface

Figure 4-1 Message box

Lesson A Objectives

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After studying Lesson A, you should be able to: Write pseudocode for the selection structureCreate a flowchart to help you plan an

application’s codeWrite an If...Then...Else statementInclude comparison operators and logical

operators in a selection structure’s conditionChange the case of a stringDetermine the success of the TryParse method

Making Decisions in a Program

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Three basic control structuresSequenceSelectionRepetition

All procedures in an application are written using one of more of these structuresProcedures in previous chapters used

sequence structure only

Making Decisions in a Program (cont’d.)

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Selection structure Chooses one of two paths based on conditionAlso called a decision structure

Example:If employee works over 40 hours, add

overtime payCondition

Decision expression evaluating to true or false

Making Decisions in a Program (cont’d.)

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Single-alternative selection structureTasks performed only when condition is true

Dual-alternative selection structureOne set of tasks performed if condition is true

Called true pathDifferent set of tasks performed if condition is

falseCalled false path

If and end if Denotes selection structure’s beginning and end

Else denotes beginning of false path

Making Decisions in a Program (cont’d.)

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Figure 4-3 Selection structures you might use today

Making Decisions in a Program (cont’d.)

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Example: Kanton Boutique

Figure 4-4 Problem specification for Kanton Boutique

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Figure 4-6 Pseudocode containing only the sequence structure

Figure 4-5 Interface for the Kanton Boutique application

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Figure 4-7 Modified problem specification and pseudocode containing a single-alternative selection structure

Making Decisions in a Program (cont’d.)

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Decision symbolDiamond shape in a flowchartRepresents the selection structure’s condition

Other symbolsOval: Start/stop symbolRectangle: Process symbolParallelogram: Input/output symbol

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Figure 4-8 Single-alternative selection structure shown in a flowchart

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Figure 4-9 Modified problem specification and pseudocode containing a dual-alternative selection structure

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Figure 4-10 Dual-alternative selection structure shown in a flowchart

Coding Single-Alternative and Dual-Alternative Selection Structures

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If…Then…Else statementUsed to code single and dual-alternative

selection structuresStatement block

Set of statements in each pathSyntax and examples shown in Figure 4-11

on next slide

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Figure 4-11 Syntax and examples of the If…Then…Else statement (continues)

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Figure 4-11 Syntax and examples of the If…Then…Else statement (cont’d.)

Comparison Operators

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Comparison operatorsUsed to compare two valuesAlways result in a True or False value

Rules for comparison operatorsThey do not have an order of precedenceThey are evaluated from left to rightThey are evaluated after any arithmetic

operators in the expression

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Figure 4-12 Listing and examples of commonly used comparison operators

Comparison Operators (cont’d.)

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Using comparison operators: Swapping numeric valuesSample application displays the lowest and

highest of two numbers entered by the user

Figure 4-14 Sample run of the Lowest and Highest

application

Comparison Operators (cont’d.)

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Figure 4-15 Pseudocode containing a single-alternative selection structure

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Figure 4-16 Flowchart containing a single-alternative selection structure

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Figure 4-17 Display button’s Click event procedure

Comparison Operators (cont’d.)

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Values input by the user are stored in variables with procedure scope

A temporary variable is used when values must be swappedDeclared within statement blockBlock-level variable

Block scopeRestricts use of variable to statement block

in which it is declared

Comparison Operators (cont’d.)

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Figure 4-18 Illustration of the swapping concept

Comparison Operators (cont’d.)

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Using comparison operators: Displaying the sum or differenceSample application displays the sum or

difference of two numbers entered by the user

Figure 4-19 Sample run of the Addition and Subtraction application

Comparison Operators (cont’d.)

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Figure 4-20 Pseudocode containing a dual-alternative selection structure

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Figure 4-21 Pseudocode containing a dual-alternative selection structure

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Figure 4-22 Calculate button’s Click event procedure

Logical Operators

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Logical operatorsUsed to create compound conditionsExpressions evaluate to a Boolean value

True or False

Six logical operators in Visual BasicNot, And, AndAlso, Or, OrElse, Xor

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Figure 4-23 Listing and examples of logical operators (continues)

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Figure 4-23 Listing and examples of logical operators (cont’d.)

Logical Operators (cont’d.)

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Truth tablesShow how logical operators are evaluated

Short-circuit evaluationBypasses evaluation of condition when

outcome can be determined without itOperators using technique: AndAlso, OrElse

Example: If state = "TN" AndAlso sales > $5000 Then…If state is not TN, no need to evaluate sales >

$5000

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Figure 4-24 Truth tables for the logical

operators

Logical Operators (cont’d.)

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Using the truth tablesScenario: Calculate a bonus for a salesperson

Bonus condition: “A” rating and sales > $9,000Appropriate operators: And, AndAlso (more efficient)Both conditions must be true to receive bonusSample code: strRating = "A” AndAlso dblSales > 9000

Logical Operators (cont’d.)

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Using logical operators: Calculating gross payScenario: Calculate and display employee gross

payRequirements for application

Verify hours are within range (>= 0.0 and <= 40.0)If data is valid, calculate and display gross payIf data is not valid, display error message

Can accomplish this using AndAlso or OrElseData validation

Verifying that input data is within expected range

Comparing Strings Containing Letters

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Scenario:Display “Pass” if ‘P’ is entered in txtLetter

control Display “Fail” if ‘F’ is entered in txtLetter

control Can use the OrElse or the AndAlso operatorNote that ‘P’ is not the same as ‘p’

They have different Unicode values

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Figure 4-28

Examples of using string comparisons in a

procedure

Converting a String to Uppercase or Lowercase

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String comparisons are case sensitiveCharacterCasing property:

Three case values: Normal (default), Upper, Lower

ToUpper method: Converts string to uppercase

ToLower method: Converts string to lowercase

Syntax and examples shown in Figure 4-29 on next two slides

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Figure 4-29 Syntax and examples of the ToUpper and ToLower methods (continues)

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Figure 4-29 Syntax and examples of the ToUpper and ToLower methods (cont’d.)

Converting a String to Uppercase or Lowercase (cont’d.)

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Using the ToUpper and ToLower Methods: Displaying a MessageProcedure requirements

Display message “We have a store in this state”Valid states: IL, IN, KYMust handle case variations in the user’s input

Can use ToLower or ToUpperCan assign a String variable to the input

text box’s value converted to uppercase

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Figure 4-29 Examples of using the ToUpper and ToLower

methods in a procedure

Comparing Boolean Values

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Boolean variable: Contains either True or False

Naming convention: “Is” denotes Boolean typeExample: blnIsInsured

When testing for a True value, it is not necessary to include the “= True”Examples in Figure 4-32 on next slide

Comparing Boolean Values (cont’d.)

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Figure 4-32 Examples of using Boolean values in a condition

Comparing Boolean Values (cont’d.)

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Comparing Boolean values: Determining whether a string can be converted to a numberTryParse method returns a numeric value

after converting the string, or 0 if it cannot be converted

TryParse also returns a Boolean value indicating success or failure of the conversion attempt

Use Boolean value returned by TryParse method in an If…Then…Else statement

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Figure 4-33 Syntax and example of using the Boolean value returned by the TryParse method

Summary of Operators

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Precedence of logical operatorsEvaluated after any arithmetic or comparison

operators in the expressionSummary listing of arithmetic,

concatenation, comparison, and logical operators in Figure 4-36 in text

Lesson A Summary

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Single and dual-alternative selection structuresUse If...Then...Else statement

Use comparison operators to compare two values

Use a temporary variable to swap values contained in two variables

Use logical operators to create a compound condition

Use text box’s CharacterCasing property to change text to upper- or lowercase

Lesson A Summary (cont’d.)

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Use ToUpper and ToLower to temporarily modify the case of input text

Use Boolean return value of TryParse method to determine whether string was successfully converted to numeric value

Arithmetic operators are evaluated first, then comparison operators, and finally logical operators

Lesson B Objectives

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After studying Lesson B, you should be able to:

Group objects using a GroupBox controlCalculate a periodic payment using the

Financial.Pmt methodCreate a message box using the

MessageBox.Show methodDetermine the value returned by a

message box

Creating the Monthly Payment Calculator Application

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Program requirementCalculate monthly payment on car loanApplication needs

The loan amount (principal)The annual percentage rate (APR) of interestThe life of the loan (term) in years

Creating the Monthly Payment Calculator Application (cont’d.)

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Adding a group box to the formGroup box: Container control for other controls

GroupBox tool: Used to add group box control to interface

Group box control provides:Visual separation of related controlsAbility to manage the grouped controls by

manipulating the group box controlLock controls to ensure that they are not movedBe sure to set TabIndex after placement of

controls

Coding the Monthly Payment Calculator Application

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Procedures required according to TOE chartClick event procedure code for the two

buttons Code for TextChanged, KeyPress, and Enter

events for text boxesProcedures that are already coded

btnExit Click event and TextChanged events for the text boxes

Procedure to code in Lesson BbtnCalc button’s Click event procedure

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Figure 4-42 TOE chart for the Monthly Payment Calculator application

Coding the Monthly Payment Calculator Application (cont’d.)

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Coding the btnCalc control’s Click event procedureCalculate monthly payment amountDisplay result in lblPayment control

Determine need for named constants and variables Constants: Items that do not change each

time procedure invokedVariables: Items will likely change each time

Coding the Monthly Payment Calculator Application (cont’d.)

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Figure 4-43 Pseudocode for the btnCalc control’s Click event procedure

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Figure 4-44 Partially completed Click event procedure

Using the Financial.Pmt Method

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Financial.Pmt methodCalculates periodic payment on loan or

investmentMust ensure that interest rate and number of

periods are expressed in same units (months or years)Convert annual interest rate to monthly rate by

dividing by 12Convert annual term to monthly term by multiplying

by 12

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Figure 4-45 Basic syntax and examples of the Financial.Pmt method

Using the Financial.Pmt Method (cont’d.)

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Figure 4-46 Selection structure’s true path coded in the procedure

The MessageBox.Show Method

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MessageBox.show methodDisplays message box with text message,

caption, button(s), and iconUse sentence capitalization for text

messageUse book title capitalization for captionIcons

Exclamation or question: Indicates user must make a decision before continuing

Information: Indicates informational messageStop: Indicates serious problem

The MessageBox.Show Method (cont’d.)

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Figure 4-50 Values returned by the MessageBox.Show method (continues)

The MessageBox.Show Method (cont’d.)

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Figure 4-50 Values returned by the MessageBox.Show method (cont’d.)

Lesson B Summary

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Group box: A container control that treats its contents as one unitUse GroupBox tool to add a group box

Use Financial.Pmt method to calculate loan or investment payments

MessageBox.Show method displays message box with text, one or more buttons, and icon

Lesson C Objectives

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After studying Lesson C, you should be able to:

Prevent the entry of unwanted characters in a text box

Select the existing text in a text box

Coding the KeyPress Event Procedures

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KeyPress eventOccurs when key is pressed while control has

focus Character corresponding to pressed key is sent

to KeyPress event’s e parameter KeyPress event can be used to prevent users

from entering inappropriate charactersUse e parameter’s KeyChar property to

determine pressed keyUse Handled property to cancel key if

needed

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Figure 4-57 Examples of using the KeyChar and Handled properties in the KeyPress event procedure

Coding the KeyPress Event Procedures (cont’d.)

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Figure 4-58 CancelKeys procedure

Coding the Enter Event Procedures

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Enter eventOccurs when text box receives focusIf text is selected, user can replace existing

text by pressing keyCan use Enter event to select all of text

SelectAll methodSelects all text contained in text boxAdd to each text box’s Enter event procedure

Coding the Enter Event Procedures (cont’d.)

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Figure 4-59 Syntax and an example of the SelectAll method

Lesson C Summary

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KeyPress event occurs when user presses key

Use KeyPress event to cancel unwanted key pressed by user

Use SelectAll method to select all contents of text box

Enter event occurs when text box receives focus

Use Enter event to process code when control receives focus