Chapter 4 The If Then Statement

Transcription

1 The If Then Statement 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. Slide 1

2 Relational Operators Operator Meaning = equal to < less than <= less than or equal to > greater than >= greater than or equal to <> not equal to Slide 2

3 The If Then Else Statement 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. Slide 3

4 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 Slide 4 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.

5 The If Then ElseIf Statement 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. Slide 5

6 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 Slide 6

7 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 < 10 Me.lblMessage.Text = "Nice try." Case Is < 25 Me.lblMessage.Text = "Good." Case Is >= 25 Me.lblMessage.Text = "Great!" End Select Slide 7

8 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. Slide 8

9 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. Slide 9

10 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. Slide 10

11 Compound Boolean Expressions More than one Boolean expression in a single condition. Formed using the And, Or, or Not operators. Slide 11

12 And Truth Table And Exp1 Exp2 Result True True True True False False False True False False False False Slide 12

13 Or Truth Table Or Exp1 Exp2 Result True True True True False True False True True False False False Slide 13

14 Not Truth Table Exp True False Not Result False True Slide 14

15 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) Slide 15

16 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. Slide 16

17 Assignment Operators Operator Operation += addition and then assignment -= subtraction and then assignment Slide 17

18 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. Slide 18

19 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. Slide 19