Microsoft Visual Basic 2005: Reloaded Second Edition Chapter 4 Making Decisions in a Program

Objectives After studying this chapter, you should be able to: Include the selection structure in pseudocode and in a flowchart Write an If…Then…Else statement Write code that uses comparison operators and logical operators Create a variable having block-scope Concatenate strings Microsoft Visual Basic 2005: Reloaded, Second Edition

Objectives (continued) Use the ControlChars.NewLine constant Change the case of a string Determine whether a string contains data Display a message in a message box Include a nested selection structure in pseudocode, a flowchart, and code Microsoft Visual Basic 2005: Reloaded, Second Edition

Objectives (continued) Code an If/ElseIf/Else selection structure Include a Case selection structure in pseudocode, a flowchart, and code Generate random numbers Microsoft Visual Basic 2005: Reloaded, Second Edition

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 Four forms of selection structure: If If/Else If/ElseIf/Else Case Microsoft Visual Basic 2005: Reloaded, Second Edition

The Selection Structure (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Writing Pseudocode for the If and If/Else Selection Structures If selection structure: contains one set of instructions to process when the condition is true If/Else selection structure: Contains two sets of instructions One set is processed when the condition is true The other set is processed when the condition is false True path: path to follow when condition is true False path: path to follow when condition is false Microsoft Visual Basic 2005: Reloaded, Second Edition

Writing Pseudocode for the If and If/Else Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Flowcharting the If and If/Else Selection Structures Selection/repetition symbol: Diamond shape Represents both selection and repetition structures One flowline entering and two flowlines leaving Microsoft Visual Basic 2005: Reloaded, Second Edition

Flowcharting the If and If/Else Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Coding the If and If/Else Selection Structures Microsoft Visual Basic 2005: Reloaded, Second Edition

Coding the If and If/Else Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Coding the If and If/Else Selection Structures (continued) Else clause: an optional part of the If statement Statement block: set of statements terminated by an Else or End If Microsoft Visual Basic 2005: Reloaded, Second Edition

Comparison Operators Comparison operators (or relational operators): Used as part of the condition in an If statement Most commonly used comparison operators: Equal to: = Greater than: > Greater than or equal to: >= Less than: < Less than or equal to: <= Not equal to: <> Microsoft Visual Basic 2005: Reloaded, Second Edition

Comparison Operators (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Comparison Operators (continued) Have no order of precedence Are evaluated from left to right in an expression Microsoft Visual Basic 2005: Reloaded, Second Edition

Comparison Operators (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values Pseudocode for a procedure that displays highest and lowest of two numbers: Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values (continued) Block scope: the scope of a variable created within a block Block-scope variable: can only be used within the statement block in which it was declared Concatenation operator (&): links two strings ControlChars.NewLine constant: Advances the insertion point to the next line Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Swapping Numeric Values (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Example 2 Pseudocode for a procedure to allow the user to display the sum or difference of two numbers: Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Example 2 (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Example 2 (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Example 2 (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Comparison Operators – Example 2 (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using 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 Microsoft Visual Basic 2005: Reloaded, Second Edition

Using the ToUpper and ToLower Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using the ToUpper and ToLower Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using the ToUpper and ToLower Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Logical Operators Logical operators (or Boolean operators): Used to combine one or more conditions Compound condition: a combination of conditions using logical operator(s) Microsoft Visual Basic 2005: Reloaded, Second Edition

Logical Operators (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Logical Operators (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Logical Operators (continued) Truth tables: used to evaluate logical operators in an expression Short-circuit evaluation: an evaluation in which the second condition may not be evaluated And and Or operations always evaluate both conditions AndAlso and OrElse operations do not evaluate second condition if the first condition is false Microsoft Visual Basic 2005: Reloaded, Second Edition

Logical Operators (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Logical Operators (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

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 Use XOr when exactly one condition must be true to give a true result Logical operators are evaluated after arithmetic or comparison operators in an expression Microsoft Visual Basic 2005: Reloaded, Second Edition

Using the Truth Tables (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using the Truth Tables (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Logical Operators in an If…Then…Else Statement Data validation: Verifying that the input data is within the expected range Use an If…Then…Else statement to validate input data Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Logical Operators in an If…Then…Else Statement (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Logical Operators in an If…Then…Else Statement (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using Logical Operators in an If…Then…Else Statement (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The String.IsNullOrEmpty Method String.IsNullOrEmpty method: determine if a control’s Text property or String variable contains data Microsoft Visual Basic 2005: Reloaded, Second Edition

The String.IsNullOrEmpty Method (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Modifying the Skate-Away Sales Application Microsoft Visual Basic 2005: Reloaded, Second Edition

Modifying the Skate-Away Sales Application (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Modifying the Skate-Away Sales Application (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Modifying the Skate-Away Sales Application (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The MessageBox.Show Method Display message box with text, buttons and an icon When a message box is displayed, the program waits until the user selects a button MessageBox.Show returns an integer value indicating which button the user selected Microsoft Visual Basic 2005: Reloaded, Second Edition

The MessageBox.Show Method (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The MessageBox.Show Method (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The MessageBox.Show Method (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The MessageBox.Show Method (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The MessageBox.Show Method (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The MessageBox.Show Method (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures Nested selection structure: a selection structure that is completely contained within another selection structure Primary decision: decision made by the outer selection structure Secondary decision: decision made by the inner selection structure Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Nested Selection Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The If/ElseIf/Else Selection Structure Need a procedure to display a message based on a letter grade: Letter grade Message A Excellent B Above Average C Average D Below Average F Microsoft Visual Basic 2005: Reloaded, Second Edition

The If/ElseIf/Else Selection Structure (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The If/ElseIf/Else Selection Structure (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The Case Selection Structure Used when there are many paths from which to choose Simpler and clearer than using If/ElseIf/Else Microsoft Visual Basic 2005: Reloaded, Second Edition

The Case Selection Structure (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The Case Selection Structure (continued) Case selection structure in a flowchart: Uses the diamond symbol One flowline into the diamond, but many flowlines out of the diamond Case selection structure evaluates an expression to determine which path to take Case selection structure: Begins with Select Case Ends with End Select Has one Case clause for each possible value Microsoft Visual Basic 2005: Reloaded, Second Edition

The Case Selection Structure (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

The Case Selection Structure (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Using To and Is in an ExpressionList TO and IS keywords: specify a range of values in a Case clause’s expression list TO: When you know both the upper and lower bounds of the range IS: When you know only one end of the range Used with a comparison operator Microsoft Visual Basic 2005: Reloaded, Second Edition

Using To and Is in an Expression List (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Generating Random Integers Pseudo-random number generator: produces a sequence of numbers that meets certain statistical requirements for randomness Random.Next method: Generates a random integer Can specify a minimum and maximum value Microsoft Visual Basic 2005: Reloaded, Second Edition

Generating Random Integers (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Generating Random Integers (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Generating Random Integers (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

Programming Tutorial Microsoft Visual Basic 2005: Reloaded, Second Edition

Programming Example Microsoft Visual Basic 2005: Reloaded, Second Edition

Summary Selection structure is used for decisions Four forms of selection structures: If, If/Else, If/ElseIf/Else, and Case Diamond symbol represents a decision in a flowchart Expressions with comparison operators will result in an answer of True or False Variables declared within a selection expression have block-level scope Microsoft Visual Basic 2005: Reloaded, Second Edition

Summary (continued) Concatenation: linking two strings together Use logical operators to create compound conditions String.IsNullOrEmpty method will determine if a string contains data MessageBox.Show method returns an integer indicating which button was chosen Selection structures can be nested Microsoft Visual Basic 2005: Reloaded, Second Edition

Summary (continued) Use If/ElseIf/Else or Case structures when there are several possible alternative outcomes Use TO keyword to specify a range of valid values when both the lower and upper bound are known Use IS keyword with a comparison operator to specify a lower or upper bound but not both Use the pseudo-random number generator to generate random numbers Microsoft Visual Basic 2005: Reloaded, Second Edition