1	Chapter 5 – Decisions 5.1 Relational and Logical Operators 5.2 If Blocks 5.3 Select Case Blocks
2	5.1 Relational and Logical Operators Condition is an expression involving relational or logical operators Result of the condition is Boolean – that is, True or False
3	Relational Operators in VB.Net < less than <= less than or equal to > greater than >= greater than or equal to = equal to <> not equal to
4	Example When a = 3, b = 4 (a + b) < 2 * a
5	Another Example a = 4 b = 3 c = "hello" d = "bye" ( c.Length – b ) = ( a / 2 )
6	Relational Operator Notes Relational operators are binary – they require an operand on both sides of the operator Result of a relational expression will always be Boolean They are evaluated from left to right with no order of operations
7	Logical Operators Used for joining Boolean expressions Not – makes a False condition True and vice versa And – will yield a True if and only if both expressions are True Or – will yield a True if one or the other or both expressions are True
8	Example To test if n falls between 2 and 5: (2 < n ) And ( n < 5 ) A complete relational expression must be on either side of the logical operators And and Or.
9	Syntax error The following is NOT a valid way to test if n falls between 2 and 5: (2 < n < 5 )
10	Example 5.3 n = 4, answ = “Y” Are the following conditions true or false? Not (n < 6) (answ = "Y") Or (answ = "y") (answ = "Y") And (answ = "y") Not(answ = "y")
11	Order of Operations The order of operations for evaluating Boolean expressions is: Arithmetic operators Relational operators Logical operators
12	Arithmetic Order of Operations Parenthesis Exponentiation Division and multiplication Addition and subtraction
13	Relational Order of Operations They all have the same precedence
14	Logical Order of Operations Not And Or
15	Common Error in Boolean Expressions A common error is to replace the condition Not ( 2 < 3 ) by the condition ( 2 > 3 ) The correct replacement is ( 2 >= 3 ) Because >= is the opposite of <, just as <= is the opposite of >
16	5.2 If Block The program will take a course of action based on whether a condition is true. If condition Then action1 Else action2 End If
17	Another example If block If condition Then action1 End If Statement2 Statement3
18	Pseudocode and flowchart for an If block
19	Example 1 Private Sub btnFindLarger_Click(...) _ Handles btnFindLarger.Click Dim num1, num2, largerNum As Double num1 = CDbl(txtFirstNum.Text) num2 = CDbl(txtSecondNum.Text) If num1 > num2 Then largerNum = num1 Else largerNum = num2 End If txtResult.Text = "The larger number is " largerNum End Sub
20	Example 2 If costs = revenue Then txtResult.Text = "Break even" Else If costs < revenue Then profit = revenue - costs txtResult.Text = "Profit is " & _ FormatCurrency(profit) Else loss = costs - revenue txtResult.Text = "Loss is " & _ FormatCurrency(loss) End If End If
21	Example 3 Private Sub btnEvaluate_Click(...) _ Handles btnEvaluate.Click Dim answer As Double answer = CDbl(txtAnswer.Text) If (answer >= 0.5) And (answer <= 1) Then txtSolution.Text = "Good, " Else txtSolution.Text = "No, " End If txtSolution.Text &= "it holds about 3/4 of" _ & " a gallon." End Sub
22	Example 4 Private Sub btnDisplay_Click(...) _ Handles btnDisplay.Click Dim message As String message = "Skittles is an old form of bowling in " _ & "which a wooden disk is used to knock down nine" _ & " pins arranged in a square." If txtAnswer.Text.ToUpper = "N" Then MsgBox(message, , "") End If txtQuote.Text = "Life ain't all beer and skittles." End Sub
23	ElseIf clause If condition1 Then action1 ElseIf condition2 Then action2 ElseIf condition3 Then action3 Else action4 End If
24	Example 5 Private Sub btnFindLarger_Click(...) _ Handles btnFindLarger.Click Dim num1, num2 As Double num1 = CDbl(txtFirstNum.Text) num2 = CDbl(txtSecondNum.Text) If (num1 > num2) Then txtResult.Text = "Larger number is " & num1 ElseIf (num2 > num1) Then txtResult.Text = "Larger number is " & num2 Else txtResult.Text = "The two are equal." End If End Sub
25	Example 6 Function CalculateFICA(ByVal ytdEarnings As Double, _ ByVal curEarnings As Double) _ As Double Dim socialSecurityBenTax, medicareTax As Double If (ytdEarnings + curEarnings) <= 84900 Then socialSecurityBenTax = 0.062 * curEarnings ElseIf ytdEarnings < 84900 Then socialSecurityBenTax = 0.062 * (84900 - ytdEarnings) End If medicareTax = 0.0145 * curEarnings Return socialSecurityBenTax + medicareTax End Function
26	Simplified Nested If Statement If cond1 Then If cond1 And cond2 Then If cond2 Then action action End If End If End If
27	Comments When one If block is contained inside another If block, the structure is referred to as nested If blocks. Care should be taken to make If blocks easy to understand.
28	More Comments Some programs call for selecting among many possibilities. Although such tasks can be accomplished with complicated nested If blocks, the Select Case block (discussed in the next section) is often a better alternative.
29	Select Case blocks A decision-making structure that simplifies choosing among several actions. Avoids complex nested If constructs. If blocks make decisions based on the truth value of a condition; Select Case choices are determined by the value of an expression called a selector.
30	Select Case Terminology Each of the possible actions is preceded by a clause of the form Case valueList where valueList itemizes the values of the selector for which the action should be taken.
31	Example 1 Private Sub btnEvaluate_Click(...) _ Handles btnEvaluate.Click Dim position As Integer 'selector position = CInt(txtPosition.Text) Select Case position Case 1 txtOutcome.Text = "Win" Case 2 txtOutcome.Text = "Place" Case 3 txtOutcome.Text = "Show" Case 4, 5 txtOutcome.Text = "You almost placed in the money." Case Else txtOutcome.Text = "Out of the money." End Select End Sub
32	Example 2 Private Sub btnDescribe_Click(...) Handles btnDescribe.Click Dim position As Integer position = CInt(txtPosition.Text) Select Case position Case 1 To 3 txtOutcome.Text = "In the money." Case Is >= 4 txtOutcome.Text = "Not in the money." End Select End Sub
33	Select Case Syntax The general form of the Select Case block is Select Case selector Case valueList1 action1 Case valueList2 action2 Case Else action of last resort End Select
34	Rules for Select Case Case Else (and its action) is optional Each value list contains one or more of the following types of items separated by commas: a literal; a variable; an expression; an inequality sign preceded by Is and followed by a literal, variable, or expression; a range expressed in the form a To b, where a and b are literals, variables, or expressions.
35	Flowchart for Select Case
36	Example 3 Select Case num Case y - x, x txtPhrase.Text = "Buckle my shoe." Case Is <= 4 txtPhrase.Text = "Shut the door." Case x + y To x * y txtPhrase.Text = "Pick up sticks." Case 7, 8 txtPhrase.Text = "Lay them straight." Case Else txtPhrase.Text = "Start all over again." End Select
37	Example 4 Select Case firstName Case "THOMAS" txtReply.Text = "Correct." Case "WOODROW" txtReply.Text = "Sorry, his name" _ & " was Thomas Woodrow Wilson." Case "PRESIDENT" txtReply.Text = "Are you for real?" Case Else txtReply.Text = "Nice try." End Select
38	Example 7 Function NumDays(ByVal season As String) _ As Integer Select Case season.ToUpper Case "WINTER" Return 87 Case "SPRING" Return 92 Case "SUMMER", "AUTUMN", "FALL" Return 93 End Select End Function
39	Comments In a Case clause of the form Case b To c, the value of b should be less than or equal to the value of c. The word Is should precede an inequality sign in a value list. If the word Is is accidentally omitted where required, the editor will automatically insert it when checking the line.
40	Data type comments The items in the value list must evaluate to a literal of the same data type as the selector. For instance, if the selector evaluated to a string value, as in Dim firstName As String firstName = txtBox.Text Select Case firstName then the clause Case firstName.Length would be meaningless.
41	Chapter 5 – Summary 5.1 Relational and Logical Operators 5.2 If Blocks 5.3 Select Case Block