Programming with Microsoft Visual Basic 2010 5 th Edition CHAPTER FIVE MORE ON THE SELECTION...

Programming with Microsoft Visual Basic 2010 5th Edition

CHAPTER FIVE

MORE ON THE SELECTION STRUCTURE

Programming with Microsoft Visual Basic 2010, 5th Edition

Previewing the Math Practice Application

Open Math.exe file

The Math Practice application will be used by 1st and 2nd grade students

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Previewing the Math Practice Application (cont’d.)

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Figure 5-1 Math Practice application’s user interface


Lesson A ObjectivesAfter studying Lesson A, you should be able to: Include a nested selection structure in pseudocode and in a flowchartCode a nested selection structureDesk-check an algorithmRecognize common logic errors in selection structures

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Lesson A Objectives (cont’d.)

Include a multiple-alternative selection structure in pseudocode and in a flowchart

Code a multiple-alternative selection structure

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Nested Selection Structures

Selection structure◦ Used to make a decision and select the appropriate path

Nested selection structure◦ Selection structure that lies on true or false path of an outer selection structure

Two decisions are involved◦ Primary decision: Made by outer structure◦ Secondary decision: Made by inner structure

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Programming with Microsoft Visual Basic 2010, 5th Edition7

Figure 5-3 Selection structures you might use today


The Voter Eligibility Application

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Figure 5-4 Problem specification for the Voter Eligibility application


The Voter Eligibility Application (cont’d.)

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Figure 5-5 Sample run of the Voter Eligibility application


Figure 5-6 Flowchart showing the nested selection structure in the true path


Figure 5-7 Flowchart showing the nested selection structure in the false path


Figure 5-8 Code for the flowcharts in Figures 5-6 and 5-7 (continues)


Figure 5-8 Code for the flowcharts in

Figures 5-6 and 5-7 (cont’d.)


Logic Errors in Selection Structures

Common logic errors◦ Using compound condition instead of nested structure ◦ Reversing primary and secondary decisions◦ Using unnecessary nested selection structure

Algorithm◦ Set of step-by-step instructions for accomplishing a task

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Logic Errors in Selection Structures (cont’d.)

Desk-checking (or hand-tracing)◦ Using pencil and paper to follow algorithm’s steps

◦ Using sample data

◦ Goal is to verify that algorithm is correct

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Figure 5-9 A correct algorithm for the bonus procedure



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Figure 5-10 Results of desk-checking the correct algorithm


Using a Compound Condition Rather Than a Nested Selection Structure

Bonus scenario◦ Calculate bonus amount for all employee types◦ Give extra bonus to X contingent on sales amount ◦ Display the bonus amount

Problem when using compound condition◦ It does not indicate hierarchy in decision process

Nested selection structure does enforce hierarchy of decisions ◦ Extra bonus decision depends on sales type decision

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Figure 5-11 Correct algorithm and an incorrect algorithm containing the first logic error

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Figure 5-12 Results of desk-checking the incorrect algorithm shown in Figure 5-11

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Reversing the Primary and Secondary Decisions

Incorrect algorithm reverses primary and secondary decisions◦ All salespeople with sales >= 10000 get extra bonus◦ Amount of bonus depends on sales type

Extra bonus decision should depend on sales type decision first

Reminder: Desk-check your algorithm with sample data to verify its correctness

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Figure 5-13 Correct algorithm and an incorrect algorithm containing the second logic error

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Reversing the Primary and Secondary Decisions (cont’d.)

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Figure 5-14 Results of desk-checking the incorrect algorithm shown in Figure 5-13


Using an Unnecessary Nested Selection Structure

Problem with incorrect algorithm◦ Adds extra, redundant selection structure on inner false path

Extra code reduces efficiency and readability

The redundant selection structure:◦ Makes decision that was already decided in prior selection structure

Solution: Eliminate second nested structure

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Using an Unnecessary Nested Selection Structure (cont’d.)

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Figure 5-15 Correct algorithm and an incorrect algorithm containing the third logic error


Using an Unnecessary Nested Selection Structure (cont’d.)

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Figure 5-16 Results of desk-checking the inefficient algorithm shown in Figure 5-15


Multiple-Alternative Selection Structures

Multiple-alternative selection structures◦ Designed to choose from several alternatives

Condition in multiple-alternative selection structure does not require true/false answer

◦ Requires expression whose value determines which path is chosen

Example: Display message based on grades ranging from A to F

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Multiple-Alternative Selection Structures (cont’d.)

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Figure 5-17 Letter grades and messages


Figure 5-18 Pseudocode containing a multiple-alternative selection structure

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Figure 5-19 Flowchart containing a multiple-alternative selection structure

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Multiple-Alternative Selection Structures (cont’d.)

Figure 5-20◦ Shows two versions of code for btnDisplay control’s Click event procedure◦ Both versions use If…Then…Else statements◦ Second version contains more convenient way of writing a multiple-

alternative selection structure

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Figure 5-20 Two versions of the code containing a multiple-alternative selection structure (continues)

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Figure 5-20 Two versions of the code containing a multiple-alternative selection structure (cont’d.)

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The Select Case Statement

Simpler and clearer when there are many paths

Select Case statement◦ Begins with Select Case, ends with End Select◦ Each case represents a different instruction path◦ Optional Case Else clause handles all values not covered by other Case paths◦ selectorExpression is evaluated to determine path◦ Each case, except Case Else, has expressionList that is compared to selectorExpression

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Figure 5-22 Syntax and an example of the Select Case statement (continues)

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Figure 5-22 Syntax and an example of the Select Case statement (cont’d.)

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Specifying a Range of Values in a Case Clause

To specify range of minimum and maximum values◦ To keyword: Use if both upper and lower bounds are known ◦ Is keyword: Use if only upper or lower bound is known

Example with To: Case 1 To 5

Example with Is: Case Is > 10

Comparison operators used with Is = , < , <= , > , >= , <>

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Figure 5-23 Syntax and an example of specifying a range of values

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Lesson A Summary Nested selection structure lies on true or false path of outer selection structure

Desk-check◦ Validate algorithm by hand with sample data

Multiple-alternative selection structures◦ Use If/ElseIf/Else statement or Select Case statement

Use To and Is keywords to specify range of values in Case clause of Select Case statement

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Lesson B ObjectivesAfter studying Lesson B, you should be able to:

Include a group of radio buttons in an interface

Designate a default radio button

Include a check box in an interface

Create and call an independent Sub procedure

Generate random numbers

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Creating the Math Practice Application

Objective: Practice addition and subtraction

Specifications◦ 1st grade students use numbers 1 through 10◦ 2nd grade students use numbers 10 through 99◦ Should not allow problems whose answers will be negative numbers◦ Students should be able to check their answers ◦ Extra attempts allowed when answer is incorrect ◦ Application should track and display number of correct and incorrect

responses

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Adding a Radio Button to the Form

Radio Button◦ Limits user to one choice in group of options◦ Should be labeled so its purpose is understood ◦ Should have unique keyboard access key

RadioButton tool: Used to add radio button

Each group of radio buttons must be placed within container such as group box

Default radio button: Button initially selected ◦ Set Checked property to True

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Adding a Radio Button to the Form (cont’d.)

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Figure 5-29 Subtraction radio button added to the Operation group box


Adding a Check Box Control to the Form

Check box◦ Allows multiple selections among group of choices◦ Any number of check boxes can be selected at once◦ Does not limit choices like radio button control◦ Should be labeled to indicate its purpose

Checkbox tool: Used to add check box control

Lock controls, then set TabIndex after interface design is complete

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Adding a Check Box Control to the Form (cont’d.)

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Figure 5-30 Display summary check box added to the form


Coding the Math Practice Application

Procedures to code based on TOE chart ◦ Form’s load event procedure◦ Click event procedures for seven controls

One task will be coded with an independent Sub procedure◦ Generating and displaying two random numbers

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Creating an Independent Sub Procedure

Event procedure◦ Sub procedure associated with specific object and event◦ Processed when specific event occurs

Independent Sub procedure◦ Processed only when called (invoked) from code◦ Can be invoked from one or more places in an application◦ Not associated with specific control◦ Reduces amount of code; promotes modularity

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Figure 5-33 Independent Sub procedure syntax, example, and steps

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Figure 5-34 Pseudocode for the GenerateAndDisplayIntegers procedure

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Generating Random Integers

Pseudo-random number generator◦ Device that produces sequence of pseudo-random numbers ◦ Pseudo-random: Values statistically resemble randomness

To use pseudo-random number generator:◦ Create Random object within procedure◦ Generate random integers with Random.Next method

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Figure 5-35 Syntax and examples of generating random integers

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Figure 5-36 Random number generation code entered in the procedure

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Figure 5-37 Additional comments and code entered in the procedure

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Coding the Grade Radio Buttons’ Click Event Procedures

Radio button controls should invoke GenerateAndDisplayNumbers procedure

Call statement: Invokes independent procedure

Syntax Call procedureName([argumentList])◦ procedureName: Name of procedure to be invoked◦ argumentList: Optional list of arguments to pass

Keyword Call is optional◦ Can simply use procedure name

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Figure 5-38 Syntax and examples of the Call statement

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Coding the Operation Radio Buttons’ Click Event Procedures

Requirements for Click event procedures◦ Display appropriate mathematical operator (+ or -) ◦ Generate and display two random integers◦ Show addition problem by default when program starts

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Coding the Form’s Load Event ProcedureForm’s Load event procedure

Processed when application is started Form is not displayed until its instructions are processed

Two ways to initially display addition problem Call GenerateAndDisplayIntegers() radAddition.PerformClick()

Enter either statement in Load event procedure

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Lesson B Summary Radio button control

◦ Limits user to only one choice among group of choices

Check box control◦ Allows user to select multiple choices in a group

Independent Sub procedures◦ Not linked to controls and can be invoked anywhere in program

Use Random object to generate pseudo-random numbers

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Lesson B Summary (cont’d.)

Call statement◦ Used to call (invoke) independent Sub procedure

Use object.PerformClick() to invoke object’s Click event

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Lesson C ObjectivesAfter studying Lesson C, you should be able to: Code a check box’s Click event procedureShow and hide a control

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Coding the Check Answer Button’s Click Event Procedure

• Study procedure’s pseudocode to determine what variables and named constants are neededSix integer variables and one named constant are neededTwo variables have static storage type

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Figure 5-44 Pseudocode for the btnCheckAnswer control’s Click event procedure

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Figure 5-45 Memory locations used by the btnCheckAnswer control’s Click event procedure

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Coding the Display Summary Check Box’s Click Event Procedure

chkSummary control’s Click event procedure◦ Displays and hides grpSummary control◦ When user selects check box, event is triggered, showing group box and its

contents◦ When user deselects check box, event is triggered, hiding group box control

Checked property indicates whether check box is selected

Visible property indicates whether control is shown

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Lesson C Summary To show or hide control, set control’s Visible property

Use selection structure to determine if check box was either selected or deselected by user

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Programming with Microsoft Visual Basic 2010 5 th Edition CHAPTER FIVE MORE ON THE SELECTION...

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