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© 2006 Lawrenceville Press Slide 1 Chapter 5 The If…Then Statement (One-Way) Conditional control structure, also called a decision structure Executes a set of statements when a condition is true The condition is a Boolean expression For example, the statement If x = 5 Then y = 20 End If assigns the value 20 to y only if x is equal to 5.
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© 2006 Lawrenceville Press Slide 2 Chapter 5 Relational Operators OperatorMeaning = equal to < less than <= less than or equal to > greater than >= greater than or equal to <> not equal to
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© 2006 Lawrenceville Press Slide 3 Chapter 5 The If…Then…Else Statement (Two Way) Contains an Else clause that is executed when the If condition evaluates to false. For example, the statement If x = 5 Then y = 20 Else y = 10 End If assigns the value 20 to y if x is equal to 5 or the value 10 if x is not equal to 5.
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© 2006 Lawrenceville Press Slide 4 Chapter 5 Nested If…Then…Else Statements Should be indented to make the logic clear. Nested statement executed only when the branch it is in is executed. For example, the statement If x = 5 Then y = 20 Else If x > 5 Then y = 10 Else y = 0 End If End If evaluates the nested If…Then…Else only when x is not equal to 5.
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© 2006 Lawrenceville Press Slide 5 Chapter 5 The If…Then…ElseIf Statement (Multi Way) Used to decide among three or more actions. Conditions must be properly ordered for the statement to evaluate as expected. For example, the statement If x < 5 Then y = 20 ElseIf x < 10 Then y = 40 ElseIf x < 15 Then y = 80 End If would give very different results if the conditions were ordered differently.
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© 2006 Lawrenceville Press Slide 6 Chapter 5 The Select…Case Statement The result of an expression determines which statements to execute. The Case Else code is optional and is executed when none of the previous cases are met: Select Case numLegs Case 2 Me.lblMessage.Text = "human" Case 4 Me.lblMessage.Text = "beast" Case 8 Me.lblMessage.Text = "insect" Case Else Me.lblMessage.Text = "???" End Select
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© 2006 Lawrenceville Press Slide 7 Chapter 5 The Select…Case Is Statement Compares the result of an expression to a range of values to determine which statements to execute. For example: Select Case score Case Is = 25 Me.lblMessage.Text = "Great!" End Select
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© 2006 Lawrenceville Press Slide 8 Chapter 5 The Rnd() Function Uses a formula to generate a sequence of numbers that are each greater than 0 and less than 1 and then returns one number from the sequence. A random integer in a range is generated by using the formula: (highNum – lowNum + 1) * Rnd() + lowNum Random integers are produced by using the Int() function along with the Rnd() function: Int(21 * Rnd() + 10)'10 to 30 The Randomize() statement initializes the random number generator.
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© 2006 Lawrenceville Press Slide 9 Chapter 5 Algorithms A set of steps that outline how to solve a problem. Can be implemented in plain English or in a mix of English and program code called pseudocode. Algorithms allow a programmer to think through a program before actually typing code, which may reduce errors in logic.
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© 2006 Lawrenceville Press Slide 10 Chapter 5 Static Variables Declared with the keyword Static. Have a lifetime the duration of the program's running time. Used to extend the lifetime of local variables in a procedure. Should be explicitly initialized when declared. A better choice than a global variable because the scope of the variable can be limited.
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© 2006 Lawrenceville Press Slide 11 Chapter 5 Compound Boolean Expressions More than one Boolean expression in a single condition. Formed using the And, Or, or Not operators.
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© 2006 Lawrenceville Press Slide 12 Chapter 5 And Truth Table And Exp1Exp2Result True False TrueFalse
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© 2006 Lawrenceville Press Slide 13 Chapter 5 Or Truth Table Or Exp1Exp2Result True FalseTrue FalseTrue False
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© 2006 Lawrenceville Press Slide 14 Chapter 5 Not Truth Table Not ExpResult TrueFalse True
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© 2006 Lawrenceville Press Slide 15 Chapter 5 The MessageBox Class A predefined dialog box that displays a message to the user. Includes the Show() method for displaying the dialog box. For example: MessageBox.Show(message)
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© 2006 Lawrenceville Press Slide 16 Chapter 5 Counter Variables A variable that is incremented by a constant value. Used for counting guesses, the numbers of values entered, the number of times a button was clicked, and so on. The value of a counter is updated in a statement similar to: counter = counter + 1 Should be initialized when declared and updated by an unchanging amount.
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© 2006 Lawrenceville Press Slide 17 Chapter 5 Assignment Operators OperatorOperation += addition and then assignment example: counter += 1 -= subtraction and then assignment example: counter -= 1
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© 2006 Lawrenceville Press Slide 18 Chapter 5 The CheckBox Control (Name) should begin with chk. Text is the text displayed next to the box. Checked is set to True if the box should be displayed as checked. An If…Then statement is often used to determine if a check box is checked or cleared.
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© 2006 Lawrenceville Press Slide 19 Chapter 5 Line-Continuation Character The underscore character is the line-continuation character. There must be a space before and nothing after and cannot be within quotation marks. Used for dividing code to make it more readable.
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