Microsoft Visual Basic 2015: Reloaded

Transcription

1 Microsoft Visual Basic 2015: Reloaded Sixth Edition Chapter Five More on the Selection Structure

2 Objectives After studying this chapter, you should be able to: Determine whether a solution requires a nested selection structure Include a nested selection structure in pseudocode and in a flowchart Code a nested selection structure Determine whether a solution requires a multiplealternative selection structure Include a multiple-alternative selection structure in pseudocode and in a flowchart 2

3 Objectives (cont d.) Prevent the division by zero run time error Code a multiple-alternative selection structure Include radio buttons in an interface Display a message in a message box Use a message box s return value Prevent the entry of invalid characters in a text box 3

4 Making More than One Decision Some solutions require selection structures with only one decision Figure 5-1: A problem that requires a selection structure 4

5 Making More than One Decision (cont d.) The Nested Selection Structure Some solutions require selection structures with more than one decision When either a selection structure s True path or its False path contains another selection structure, the inner selection structure is referred to as a nested selection structure because it is contained (nested) within the outer selection structure 5

6 Making More than One Decision (cont d.) Figure 5-2: A problem that requires a nested selection structure 6

7 Making More than One Decision (cont d.) 7 Figure 5-3: A problem that requires two nested selection structures

8 Making More than One Decision (cont d.) The Cookie Palace Application The application calculates the total amount a customer owes, including a discount rate The discount rate depends on two items: Number of baskets ordered Club membership 8

9 Making More than One Decision (cont d.) The Cookie Palace Application If the number of baskets ordered is at least 3 The discount rate is 20% Else if the number of baskets ordered is less than 3 If the customer is a member of the Cookie Club, the discount rate is 15% If the customer is not a member of the Cookie Club, the discount rate is 5% 9

10 Making More than One Decision (cont d.) Figure 5-4: Problem specification and a solution for the Cookie Palace application (continues) 10

11 Making More than One Decision (cont d.) Figure 5-4: Problem specification and a solution for the Cookie Palace application 11

12 Making More than One Decision (cont d.) Figure 5-5: Another correct solution for the Cookie Palace application (continues) 12

13 Making More than One Decision (cont d.) Figure 5-5: Another correct solution for the Cookie Palace application 13

14 Making More than One Decision (cont d.) Figure 5-6: Code and sample run for the flowcharts in Figures 5-4 and 5-5 (continues) 14

15 Making More than One Decision (cont d.) Figure 5-6: Code and sample run for the flowcharts in Figures 5-4 and

16 Multiple-Alternative Selection Structures The Multiple-Alternative Selection Structure A selection structure that can choose from several alternatives Also referred to as extended selection structures 16

17 Multiple-Alternative Selection Structures (cont'd.) Figure 5-7: Problem specification and a solution for the Anderson Roofing application 17

18 Multiple-Alternative Selection Structures (cont'd.) Figure 5-8: Code versions and sample run for the Anderson Roofing solution in Figure

19 The Select Case Statement The Select Case statement is used when a multiple-alternative selection structure has many paths from which to choose Figure 5-9 How to use the Select Case statement 19

20 The Select Case Statement (cont d.) Specifying a Range of Values in a Case Clause Figure 5-10: How to specify a range of values in a Case clause 20

21 Using Radio Buttons in an Interface You create a Radio Button using the RadioButton tool in the Toolbox Radio buttons allow you to limit the user to only one choice from a group of two or more related but mutually exclusive choices Each radio button is labeled so the user knows the choice it represents Enter the label using sentence capitalization in the radio button s Text property 21

22 Using Radio Buttons in an Interface (cont d.) Add a group box to a form using the GroupBox tool Placing each group of radio buttons in a separate group box allows the user to select one button from each group Selecting a radio button deselects any others in the group The recommended maximum number of radio buttons in a group is seven Windows applications normally have one of the radio buttons in each group already selected as a default radio button 22 Figure 5-11: Interface for In a Flash application

23 Using Radio Buttons in an Interface (cont d.) 23 Figure 5-12 Price chart, displaybutton_click procedure, and a sample run of the In a Flash application

24 The MessageBox.Show Method Using a message box is one way that an application can communicate with the user during run time The MessageBox.Show method displays a message box The message box contains text, one or more buttons, and an icon 24

25 The MessageBox.Show Method (cont'd.) Figure 5-13 How to use the MessageBox.Show method 25

26 The MessageBox.Show Method (cont'd.) 26 Figure 5-14 Message boxes created by the code in Figure 5-13

27 The MessageBox.Show Method (cont'd.) Figure 5-15 How to use the MessageBox.Show method s return value 27

28 Using the KeyPress Event A control s KeyPress event occurs each time the user presses a key while the control has the focus The KeyPress event procedure has parameters, which appear within the parentheses in the procedure header A parameter represents information that is passed to the procedure when the event occurs. Figure 5-16: Code template for the basketstextbox_keypress procedure 28

29 Using the KeyPress Event The character corresponding to the pressed key is sent to the event s e parameter To prevent a text box from accepting an inappropriate character, you first use the e parameter s KeyChar property to determine the pressed key You then use the e parameter s Handled property to cancel the pressed key if it is an inappropriate one Cancel the key by setting the Handled property to True, like this: e.handled = True 29

30 Using the KeyPress Event (cont d.) You refer to the Backspace key on your keyboard using Visual Basic s ControlChars.Back constant The KeyPress event automatically allows the use of the Delete key for editing The line continuation character is used to split a long instruction into two or more physical lines in the Code Editor window The line continuation character is an underscore that is immediately preceded by a space and appears at the end of a physical line of code in the Code Editor window 30

31 Using the KeyPress Event (cont d.) 31 Figure 5-17: How to use the KeyPress event to control the characters accepted by a text box

32 Programming Tutorial 1 Figure 5-18 Rules and TOE chart for the Rock, Paper, Scissors Game application 32

33 Programming Tutorial 1 Figure 5-19 MainForm for the Rock, Paper, Scissors Game application Figure 5-23 Sample run of the rockpicturebox_click procedure 33

34 Programming Tutorial 2 Figure 5-27: Options and MainForm Figure 5-28: TOE chart for the Bubbles Car Wash application 34

35 Programming Example Figure 5-34: Price list and TOE Chart TOE chart 35 Figure 5-35: MainForm and tab order

36 Summary You can nest a selection structure in either the True or False path of another selection structure The primary decision is always made by an outer selection structure. The secondary decision is always made by a nested selection structure You can code a multiple-alternative selection structure using either If Then Else statements or the Select Case statement In a flowchart, a diamond is used to represent the condition in a multiple-alternative selection structure The diamond has several flowlines leading out of the symbol, with each flowline representing a possible path 36

37 Summary (cont'd.) In a Select Case statement, the data type of the expressions in the Case clauses should match the data type of the statement s selectorexpression A Case clause in a Select Case statement can contain more than one expression The selectorexpression needs to match only one of the expressions for the instructions in that Case to be processed In a Case clause s expressionlist: Use the keyword To when you know both the upper and lower values of the range you want to specify Use the keyword Is when you know only one end of the range 37

38 Summary (cont'd.) Radio buttons allow you to limit the user to only one choice from a group of two or more related but mutually exclusive choices If you need to include two groups of radio buttons in an interface, at least one of the groups must be placed within a container, such as a group box, panel, or table layout panel It is customary to have one radio button in each group of radio buttons selected when the interface first appears The selected radio button is the default radio button 38

39 Summary (cont'd.) If a radio button is selected, its Checked property contains the Boolean value True; otherwise, it contains the Boolean value False The MessageBox.Show method allows an application to communicate with the user while the application is running The MessageBox.Show method displays a message box that contains text, one or more buttons, and an icon Use sentence capitalization for the text argument in the MessageBox.Show method, but book title capitalization for the caption argument 39

40 Summary (cont'd.) You can code a text box s KeyPress event procedure to prevent the text box from accepting an inappropriate character The character is stored in the e parameter s KeyChar property To cancel the character, you set the e parameter s Handled property to True 40