Lecture Set 5 Control Structures Part A - Decisions Structures

Lecture Set 5

Control Structures Part A - Decisions

Structures

Slide 2

Objectives

You have seen almost all of this before …

Use the Boolean data type in decision-making statements

Use If statements and Select Case statements to make decisions

Use logical operators to create complex conditional expressions

Learn how to write and evaluate conditional expressions (sometimes called Boolean expressions because they have True/False values)

Slide 3

Introduction to Decision-making - 1

Programming “simply involves the us of three basic control structures -- The sequence structure The decision structure The repetition structure

Recursion aside, these are the only three constructs you need to build algorithms This is it -- End of story!

Slide 4

Introduction to Decision-making - 2

This chapter discusses the decision-making structures provided by Visual Basic Statements execute conditionally based

on the outcome of a decision Three kinds of decision-making or

alternative path execution structures Single alternative decision structures (if-then)

Double alternative structures (if-then-else) Multiple alternative structures (including

the select) Much like Java and C++ - not much new

under the sun

Slide 5

Introduction to Boolean Data

Boolean data operates similarly to an on/off switch True signifies on False signifies off

Many properties are of the Boolean data type (can store only Boolean values) True or False Visible and Enabled for example

Slide 6

Declaring a Boolean Variable

Declare a Boolean variable Uninitialized Boolean variables have a

value of FalseDim Valid As Boolean

Declare and initialize a Boolean variableDim BrowseMode As Boolean = True

Declare multiple Boolean variablesDim Test1, Test2 As Boolean

Slide 7

Boolean Assignment Statements

The keywords True and False are used in Boolean assignment statements

Boolean expressions may also be used Example:

Dim Valid As BooleanValid = TrueValid = False

Slide 8

Decision-making Statements

Applications need the capability to execute one group of statements in certain circumstances and other statements in other circumstances

These statements are called decision-making statements or decision structures or alternative structures

The If statement is used to make decisions

Slide 9

Decision-making (Pseudocode)

If the input date is greater than the current date then

Display a message box indicating the input is invalid.

End of If statement

Slide 10

Flowchart of a Decision-making Statement

Slide 11

If Statements and Comparison Operators

A conditional statement executes one group of statements when a condition is True and another group of statements when a condition is False

Comparison operators are used in conditional statements

Conditional operations always produce a Boolean result

Slide 12

Comparison Operators

Equal to (=) Not equal to (<>) Less than (<) Greater than (>) Less than or equal to (<=) Greater than or equal to (>=)

Slide 13

Logical Operators

Name DescriptionAnd Returns True if both expressions are True. This

operator always evaluates both expressions.

Or Returns True if either expression is True. This operator always evaluates both expressions.

AndAlso Returns True if both expressions are True. This operator only evaluates the second

expression if necessary.OrElse Returns True if either expression is True. This operator only evaluates the second expression if necessary.Not Reverses the value of the expression.

Slide 14

Using Comparison Operators (Example)

Example:

Dim Result As BooleanDim Value1 As Integer = 3, Value2 As Integer = 5Result = Value1 < Value2 ' TrueResult = Value1 + 2 < Value2 – 1 ' False

Slide 15

Using Comparison Operators (Example, continued)

Parentheses can clarify the order of evaluation

The following two statements are equivalent:Result = Value1 + 2 < Value2 – 1Result = (Value1 + 2) < (Value2 – 1)

Slide 16

Evaluating a Condition

Slide 17

Comparison Operators and If Statements

Comparison operators are most commonly used with an If statement A group of statements executes only when

a condition is True This form of If statement is called a one-

way If statement The statements that execute as a result of

a condition are called a statement block

Slide 18

One-Way If Statement (Syntax)

AKA “Single Alternative Decision Structures”

If condition Then statements ‘a block of one or more statementsEnd IfNext sequential statement ‘(NSS)

condition must evaluate to a Boolean value If the condition is True, statements execute If the condition is False, statements do not

execute Execution continues at the statement

following the End If (the NSS) statements make up a statement block

Slide 19

One-Way If Statement (Example)

Dim CurrentValue As Boolean = True

If CurrentValue = True Then ' Statements that execute when ' CurrentValue is TrueEnd If

' statements

Slide 20

One-Way If Statement

Slide 21

Comparison Operations with Dates

Yep! We are a bit ahead of ourselves. But you will get a “taste” of using the date type

Comparison operations can be performed on dates

Dates in the past are less than dates in the future

Example:

Dim StartDate As DateTime = #3/22/2007#Dim EndDate As DateTime = #3/24/2007#If StartDate < EndDate = True Then EndDate = System.DateTime.TodayEnd If

Slide 22

Comparison Operations with Numeric Data Types

Comparison operations can be performed on numeric data

Example:Dim Value1 As Integer = 90If Value1 < 100 Then ' Statements execute when ' Value1 is less than 100End If' statements

Slide 23

Comparison Operations with Strings

Comparison operations can be performed with strings

Strings are compared character-by-character from left to right

String comparisons are performed in two ways Case sensitive (binary comparison)

A < B < E < Z < a < b < e < z Option Compare Binary

Case insensitive (text comparison) (A=a) < (B=b) < (E=e) < (Z=z) Option Compare Text

Slide 24

String Equality Using Text and Binary Comparison (aha – something NEW)

Slide 25

Binary and Text Comparisons (optional)

It’s all a matter of what you are used to … If you turn on Option Compare Binary in a module you get case sensitive comparisons

If you use Option Compare Text you get case insensitive comparisons

The default setting for a new project is Option Compare Binary You can change this setting for an entire

project by going to the Project Properties dialog box

But you probably will not want to do this

Slide 26

Introduction to Two-way If Statements

AKA “Double alternative decision structures” (if-then-else)

One statement block executes when a condition is True and another statement block executes when the condition is False

This form of If statement is commonly referred to as an If . . . Then . . . Else statement

Slide 27

Two-way If Statements (Syntax)

If condition Then statements(True)Else statements(False)End Ifstatements

Statements(True) execute if the condition is True

Statements(False) execute if the condition is False

Slide 28

Two-way If Statements (Example)

If Grade is greater than 75, set Pass to True. Otherwise, set Pass to False

Dim Pass As BooleanDim Grade As Integer = 80If Grade > 75 Then Pass = TrueElse Pass = FalseEnd If' statements

Slide 29

Two-way If Statement

Slide 30

Introduction to Multiway If Statements

AKA – “Multiple Alternative Decision Structures”

Multiway If statements have three or more possible outcomes

Slide 31

Multiway If Statements (Syntax)

If condition1 Then [statements][ElseIf condition2 Then [elseifStatements]][Else] [elseStatements]]End Ifstatements

Slide 32

Multiway If Statements (Dissection)

condition1 is first tested If True, then the first statement block

executes Execution continues as the statement

following the decision-making statement If False, condition2 is tested and then the

remaining conditions are tested If no conditions are True, then the

statements in the Else block execute The Else block is optional

Slide 33

Multiway If Statement

Slide 34

Multiway If Statement (Example)

Dim NumericGrade As Integer = 84Dim LetterGrade As StringIf NumericGrade >= 90 Then LetterGrade = "A"ElseIf NumericGrade >= 80 Then LetterGrade = "B"ElseIf NumericGrade >= 70 Then LetterGrade = "C"ElseIf NumericGrade >= 60 Then LetterGrade = "D"Else LetterGrade = "F"End If' statements

Slide 35

Notes About If Statements

If statements can be written in different ways Chose the If statement that is most readable This decision can be subjective

The Code Editor automatically indents blocks in an If statement

The Code Editor automatically inserts the End If

If statements can be nested One If statement can contain another If

statement

Slide 36

Introduction to Select Case Statements

Select Case statements are similar to multiway If statements

The same expression must be used in each condition

Select Case statements are faster than comparable multiway If statements

Select Case statements tend to be more readable than comparable multiway If statements

Slide 37

Select Case Statement (Syntax)

Select Case testExpression Case expressionList-1 statement-block1 [Case expressionList-2 statement-block2] [Case expressionList-n statement-blockn] [Case Else statements]End Select' statements

Slide 38

Select Case Statement (Dissection)

testExpression is evaluated once Each expressionList is then tested. If True, the

corresponding statement-block executes and the Select Case statement ends

Each expressionList is tested in order When an expressionList is found to be True, the

statement block executes and the Select Case statement ends

If no expessionList is True, then the statements in the Case Else block execute

Slide 39

Select Case Statement (Example)

Dim Quarter As Integer = 1Dim QuarterString As StringSelect Case Quarter Case 1 QuarterString = "First" Case 2 QuarterString = "Second" Case 3 QuarterString = "Third" Case 4 QuarterString = "Fourth" Case Else QuarterString = "Error"End Select' statements

Slide 40

Select Case Statement

Slide 41

Select Case Statement (Variations)

The To clause is used to test a range of values Case 90 to 100

The Is clause is used with comparison operators Case is > 90

A list of values can be created with a comma separated list Case 1, 3, 5

Slide 42

Logical Operators (Introduction)

Logical operators are used in conjunction with comparison and arithmetic operators

Logical operators perform the same task as a conjunction (and) or a disjunction (or) in English

The logical operators are And, Or, Not, Xor

See Table 7-2 for examples

Slide 43

Logical Operators (Precedence)

Logical operators (Not, And, Or, Xor) have an order of precedence Arithmetic operators are evaluated first Comparison operators are evaluated

second Logical operators are evaluated last (of all

the BINARY operators), from left to right in the following order:

Not, And, Or, Xor

Slide 44

Logical Operators (Example)

Evaluation of an expression (be sure you know how to do these):

Dim Result As BooleanResult = (3 + 4) > 6 And (4 + 1) < 6Result = 7 > 6 And 5 < 6Result = True And TrueResult = True

If I remove the parentheses in line 1, does it change the result? Why? How?

If I remove the parentheses in line 1, does it change the result? Why? How?

Slide 45

Logical Operators (Example, continued)

Evaluation of an expression using And and Xor

Dim Result As BooleanResult = (7 > 6) And (5 > 3) Xor (3 > 2)Result = True And True Xor TrueResult = True Xor TrueResult = False

Slide 46

The Not Operator

The Not operator is a unary operator Examples:

Result = Not (True) ' False Result = Not (False) ' True Result = Not (4 > 3) ' False

Slide 47

Using Logical Operators

Logical operators are typically combined with comparison and arithmetic operators in decision-making statements

Example:If Input >= CurrentMin And _ Input <= CurrentMax Then

Valid = TrueElse Valid = FalseEnd If' statements