Microsoft Visual Basic 2008: Reloaded Fourth Edition Chapter Four Making Decisions in a Program.

Microsoft Visual Basic 2008: Reloaded Fourth Edition

Chapter FourMaking Decisions in a Program

Microsoft Visual Basic 2010: Reloaded, Fourth Edition 2

Objectives

After studying this chapter, you should be able to:

• Include the selection structure in pseudocode and in a flowchart

• Explain the difference between single-alternative and dual-alternative selection structures

• Code a selection structure using the If…Then…Else statement

• Include comparison operators and logical operators in a selection structure’s condition

• Create a block-level variable

Objectives (cont'd.)

• Concatenate strings

• Use the ControlChars.NewLine constant

• Change the case of a string

• Include a check box in an interface

• Generate random numbers



The Selection Structure

• Selection structure (or decision structure):– Used to select a path to take based on the outcome

of a decision or comparison• Condition:

– The decision to be made– Results in a Boolean (True or False) answer

• Single-alternative selection structure: performs a set of tasks only when the condition is true

• True path: the tasks to perform when the condition is true


The Selection Structure (cont’d.)

• Dual-alternative selection structure: contains one set of tasks to perform when the condition is true and a different set of tasks to perform when the condition is false

• False path: the tasks to perform when the condition is false

• Pseudocode uses if…end if to denote a selection structure and else to denote the false path– Indent instructions within the selection structure


The Selection Structure (cont'd.)

Figure 4-1: Selection structures you might use today



Figure 4-1: Selection structures you might use today (cont’d.)


Figure 4-2: Problem specification for Mountain Biking


Figure 4-3: Interface for the Mountain Biking application



Figure 4-4: Pseudocode containing only the sequence structure



Figure 4-5: Modified problem specification and pseudocode containing a single-alternative selection structure


Figure 4-6: Single-alternative selection structure shown in a flowchart


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


Figure 4-8: Dual-alternative selection structure shown in a flowchart


Coding Single-Alternative and Dual-Alternative Selection Structures

• If…Then…Else statement: used to code single-alternative and dual-alternative selection structures– Else clause: an optional part of the If statement

• Only used for the dual-alternative selection structure

– Condition must be a Boolean expression that evaluates to either True or False

• Can contain variables, literal constants, named constants, properties, methods, arithmetic operators, comparison operators, and logical operators

• Statement block: set of statements in the true path or the false path


Figure 4-9: How to use the If…Then…Else statement


Figure 4-9: How to use the If…Then…Else statement (cont’d.)

Comparison Operators

• Comparison operators (or relational operators):– Used as part of the condition in an If…Then…Else

statement to compare two values

• Most commonly used comparison operators:– Equal to: =– Greater than: >– Greater than or equal to: >=– Less than: <– Less than or equal to: <=– Not equal to: <>



Figure 4-10: How to use comparison operators in a condition


Figure 4-10: How to use comparison operators in a condition (cont’d.)

Comparison Operators (cont’d.)

Comparison Operators (cont'd.)

• Comparison operators:– Have no order of precedence– Are evaluated from left to right in an expression– Are evaluated after any arithmetic operators in the

expression

• All expressions containing comparison operators evaluate to True or False only


Comparison Operators (cont'd.)


Figure 4-11: Evaluation steps for an expression containing arithmetic and comparison operators

Comparing Numeric Values

• Auction House application displays highest and lowest of two bids entered by the user


Figure 4-12: Sample run of the Auction House application

Comparing Numeric Values (cont'd.)


Figure 4-13: Pseudocode containing a single-alternative selection structure


Figure 4-14: Flowchart containing a single-alternative selection structure



Figure 4-15: Code entered in the Display button’s Click event procedure



Figure 4-15: Code entered in the Display button’s Click event procedure (cont’d.)



• Block-level variables: declared within a statement block and remain in memory until the procedure ends

• Block scope: A block-scope variable can only be used within the statement block in which it was declared

• Concatenation operator (&): connects or links two strings together

• ControlChars.NewLine constant:– Advances the insertion point to the next line



Figure 4-16: Illustration of the swapping concept



Figure 4-17: How to concatenate strings

Comparing Strings

• Addition and Subtraction Calculator application: displays the sum or difference of two numbers


Figure 4-18: Sample run of the Addition and Subtraction Calculator application

Comparing Strings (cont'd.)


Figure 4-19: Pseudocode containing a dual-alternative selection structure


Figure 4-20: Flowchart containing a dual-alternative selection structure

Comparing Strings (cont'd.)

• MaxLength property: text box property that specifies the maximum number of characters that can be entered

• CharacterCasing property: text box property that indicates if text should remain as typed or be converted to uppercase or lowercase



Figure 4-21: Calculate button’s Click event procedure


The ToUpper and ToLower Methods

• String comparisons in Visual Basic are case-sensitive

• ToUpper method: converts a string to uppercase• ToLower method: converts a string to lowercase• ToUpper and ToLower can be used to permanently

or temporarily convert a variable’s contents


Figure 4-22: How to use the ToUpper and ToLower methods

The ToUpper and ToLower Methods (cont’d.)


Figure 4-22: How to use the ToUpper and ToLower methods (cont'd.)



Figure 4-22: How to use the ToUpper and ToLower methods (cont'd.)



Figure 4-23: Examples of using the ToUpper method in the calcButton Click event procedure


Figure 4-23: Examples of using the ToUpper method in the calcButton Click event procedure (cont’d.)

Comparing Boolean Values

• Check boxes: used to offer the user one or more independent and nonexclusive items from which to choose


Figure 4-24: A different interface for the Addition and Subtraction Calculator application

Comparing Boolean Values (cont’d.)


Figure 4-25: Click event procedures for the subtractionCheckBox and calcButton

Comparing Boolean Values (cont’d.)


Figure 4-25: Click event procedures for the subtractionCheckBox and calcButton (cont’d.)


Logical Operators

• Logical operators (or Boolean operators):– Used to combine two or more conditions into one

compound condition

• Compound condition: a combination of conditions using logical operator(s)


Logical Operators (cont'd.)

Figure 4-26: How to use logical operators in a condition


Figure 4-26: How to use logical operators in a condition (cont'd.)



Figure 4-26: How to use logical operators in a condition (cont'd.)



• Truth tables: used to evaluate logical operators in an expression

• Short-circuit evaluation: an evaluation in which the second condition may not be evaluated

• AndAlso evaluates to True only when both sub-conditions are True

• OrElse evaluates to False only when both sub-conditions are False

• AndAlso and OrElse operations do not evaluate the second condition if the first condition is false



Figure 4-27: Truth tables for the AndAlso and OrElse logical operators


Using the Truth Tables

• Use And or AndAlso when both conditions must be true to give a true result

• Use Or or OrElse when one or both conditions must be true to give a true result

• Remember: logical operators are evaluated after arithmetic or comparison operators in an expression


The Carroll Company Application

• Data validation: – Process of verifying that the input data is within the

expected range

Figure 4-28: Two ways of writing the calcButton Click event procedure

Microsoft Visual Basic 2010: Reloaded, Fourth Edition 52Figure 4-28: Two ways of writing the calcButton Click event procedure (cont’d.)

The Carroll Company Application (cont'd.)


Figure 4-29: Sample run of the Carroll Company application using valid data

Figure 4-30: Sample run of the Carroll Company application using invalid data

Summary of Operators

Microsoft Visual Basic 2010: Reloaded, Fourth Edition 54Figure 4-31: Listing of arithmetic, concatenation, comparison, and logical operators

Summary of Operators (cont’d.)


Figure 4-31: Listing of arithmetic, concatenation, comparison, and logical operators (cont’d.)


Generating Random Integers

• Pseudo-random number generator: a device that produces a sequence of numbers that meets certain statistical requirements for randomness

• Random object: represents a pseudo-random number generator

• Random.Next method: – Generates a random integer– Can specify a minimum and maximum value


Figure 4-32: How to generate random integers

Generating Random Integers (cont'd.)


Figure 4-33: Sample run of the Random Integers application

Generating Random Integers (cont'd.)


Figure 4-34: Generate button’s Click event procedure

Programming Tutorial 1


Figure 4-36: MainForm for the Find the Mouse Game application

Programming Tutorial 2


Figure 4-44: MainForm for the Greenview Health Club application

Programming Example


Figure 4-50: MainForm in the Fat Calculator application


Summary

• Selection structure allows a procedure to make a decision and then take the appropriate action

• Three types of selection structures: single-alternative, dual-alternative, and multiple-alternative

• Diamond symbol represents a decision in a flowchart

• Expressions with comparison operators will result in an answer of True or False

• Comparison operators are evaluated from left to right in expressions, after arithmetic operators


Summary (cont'd.)

• Variables declared within a selection expression have block-level scope

• Concatenation: connecting or linking two strings together with the concatenation operator (&)

• ControlChars.Newline advances the insertion point to the next line in a control

• String comparisons are case-sensitive

• Use ToUpper and ToLower methods to temporarily convert the case of a string


Summary (cont'd.)

• Use check boxes to provide the user with one or more independent and nonexclusive choices

• Use logical operators to create compound conditions

• An expression containing a logical operator will evaluate to either True or False

• Logical operators have an order of precedence and are evaluated after arithmetic and comparison operators

• Use the pseudo-random number generator to generate random numbers

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Microsoft Visual Basic 2008: Reloaded Fourth Edition Chapter Four Making Decisions in a Program.

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