Microsoft Visual Basic 2008:
Reloaded
Fourth Edition
Chapter Four
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
• 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
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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
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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
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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
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The Selection Structure (cont'd.)
Figure 4-1: Selection structures you might use today
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The Selection Structure (cont'd.)
Figure 4-1: Selection structures you might use today (cont’d.)
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The Selection Structure (cont'd.)
Figure 4-2: Problem specification for Mountain Biking
Figure 4-3: Interface for the Mountain Biking application
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The Selection Structure (cont'd.)
Figure 4-4: Pseudocode containing only the sequence structure
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The Selection Structure (cont'd.)
Figure 4-5: Modified problem specification and pseudocode containing
a single-alternative selection structure
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Figure 4-6: Single-alternative selection structure shown in a flowchart
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Figure 4-7: Modified problem specification and pseudocode
containing a dual-alternative selection structure
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Figure 4-8: Dual-alternative selection structure shown in a flowchart
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Coding Single-Alternative and DualAlternative Selection Structures
• If…Then…Else statement: used to code singlealternative 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
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Figure 4-9: How to use the If…Then…Else statement
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Figure 4-9: How to use the If…Then…Else statement (cont’d.)
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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: <>
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Figure 4-10: How to use comparison operators in a condition
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Comparison Operators (cont’d.)
Figure 4-10: How to use comparison operators in a condition (cont’d.)
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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
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Comparison Operators (cont'd.)
Figure 4-11: Evaluation steps for an expression containing arithmetic and
comparison operators
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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
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Comparing Numeric Values (cont'd.)
Figure 4-13: Pseudocode containing a single-alternative selection structure
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Figure 4-14: Flowchart containing a single-alternative selection structure
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Comparing Numeric Values (cont'd.)
Figure 4-15: Code entered in the Display button’s Click event procedure
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Comparing Numeric Values (cont'd.)
Figure 4-15: Code entered in the Display button’s Click event procedure (cont’d.)
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Comparing Numeric Values (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
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Comparing Numeric Values (cont'd.)
Figure 4-16: Illustration of the swapping concept
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Comparing Numeric Values (cont'd.)
Figure 4-17: How to concatenate strings
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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
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Comparing Strings (cont'd.)
Figure 4-19: Pseudocode containing a dual-alternative selection structure
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Figure 4-20: Flowchart containing a dual-alternative selection structure
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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
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Figure 4-21: Calculate button’s Click event procedure
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The ToUpper and ToLower Methods
• String comparisons in Visual Basic are casesensitive
• 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
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The ToUpper and ToLower Methods
(cont’d.)
Figure 4-22: How to use the ToUpper and ToLower methods
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The ToUpper and ToLower Methods
(cont’d.)
Figure 4-22: How to use the ToUpper and ToLower methods (cont'd.)
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The ToUpper and ToLower Methods
(cont’d.)
Figure 4-22: How to use the ToUpper and ToLower methods
(cont'd.)
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Figure 4-23: Examples of using the ToUpper method in the calcButton Click
event procedure
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Figure 4-23: Examples of using the ToUpper method in the calcButton Click
event procedure (cont’d.)
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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
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Comparing Boolean Values (cont’d.)
Figure 4-25: Click event procedures for the subtractionCheckBox and
calcButton
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Comparing Boolean Values (cont’d.)
Figure 4-25: Click event procedures for the subtractionCheckBox and
calcButton (cont’d.)
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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)
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Logical Operators (cont'd.)
Figure 4-26: How to use logical operators in a condition
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Logical Operators (cont'd.)
Figure 4-26: How to use logical operators in a condition (cont'd.)
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Figure 4-26: How to use logical operators in a condition (cont'd.)
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Logical Operators (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 subconditions are True
• OrElse evaluates to False only when both subconditions are False
• AndAlso and OrElse operations do not evaluate the
second condition if the first condition is false
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Logical Operators (cont'd.)
Figure 4-27: Truth tables for the AndAlso and OrElse logical operators
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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
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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
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Figure 4-28: Two ways of writing the calcButton Click event procedure (cont’d.)
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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
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Summary of Operators
Figure 4-31: Listing of arithmetic, concatenation, comparison, and logical operators
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Summary of Operators (cont’d.)
Figure 4-31: Listing of arithmetic, concatenation, comparison, and logical operators
(cont’d.)
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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
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Figure 4-32: How to generate random integers
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Generating Random Integers (cont'd.)
Figure 4-33: Sample run of the Random Integers application
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Generating Random Integers (cont'd.)
Figure 4-34: Generate button’s Click event procedure
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Programming Tutorial 1
Figure 4-36: MainForm for the Find the Mouse Game application
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Programming Tutorial 2
Figure 4-44: MainForm for the Greenview Health Club application
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Programming Example
Figure 4-50: MainForm in the Fat Calculator application
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Summary
• Selection structure allows a procedure to make a
decision and then take the appropriate action
• Three types of selection structures: singlealternative, dual-alternative, and multiplealternative
• 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
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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
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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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